a hypothesis stating that life began in the absence of life

• Introduction to Ecology; Major patterns in Earth’s climate
• Behavioral Ecology
• Population Ecology 1
• Population Ecology 2
• Community Ecology 1
• Community Ecology 2
• Ecosystems 1
• Ecosystems 2
• Strong Inference
• What is life?
• What is evolution?
• Evolution by Natural Selection
• Other Mechanisms of Evolution
• Population Genetics: the Hardy-Weinberg Principle
• Phylogenetic Trees
• Earth History and History of Life on Earth

Origin of Life on Earth

• Gene expression: DNA to protein
• Gene regulation
• Cell division: mitosis and meiosis
• Mendelian Genetics
• Chromosome theory of inheritance
• Patterns of inheritance
• Chemical context for biology: origin of life and chemical evolution
• Biological molecules
• Membranes and Transport
• Energy and enzymes
• Respiration, chemiosmosis and oxidative phosphorylation
• Oxidative pathways: electrons from food to electron carriers
• Fermentation, mitochondria and regulation
• Why are plants green, and how did chlorophyll take over the world? (Converting light energy into chemical energy)
• Carbon fixation
• Recombinant DNA
• Cloning and Stem Cells
• Adaptive Immunity
• Human evolution and adaptation

Learning Objectives

• Describe the requirements for the origin of life (carbon source, energy, segregate molecules from environment, hereditary mechanism)
• Describe the steps which led to the origin of life (organic molecules form, macromolecules polymerize, a hereditary mechanism develops, membrane-enclosed protocells form).
• Apply the principles of evolution by natural selection to pre-biotic scenarios.

The origin of life is a mystery, the ultimate chicken-and-egg conundrum ( R Service, 2015 ). When you and fellow students together discussed the defining characteristics of life , you probably included reproduction and hereditary information, transformation of energy, growth and response to the environment. You may also have said that, at least on Earth, all life is composed of cells, with membranes that form boundaries between the cell and its environment, and that cells were composed of organic molecules (composed of carbon, hydrogen, nitrogen, oxygen, phosphate, and sulfur – CHNOPS). The conundrum is that, on Earth today, all life comes from pre-existing life. Pasteur’s experiments disproved spontaneous generation of microbial life from boiled nutrient broth. No scientist has yet been able to create a living cell from organic molecules. So how could life have arisen on Earth, around 3.8 billion years ago ? (Keep in mind the scale of time we’re talking about here – the Earth is 4.6 billion years old, so it took almost a billion years for chemical evolution to result in biological life.) How can this question be addressed using the process of scientific inquiry?

Origin of life studies

Although scientists cannot directly address how life on Earth arose, they can formulate and test hypotheses about natural processes that could account for various intermediate steps, consistent with the geological evidence. In the 1920s, Alexander Oparin and J. B. S. Haldane independently proposed nearly identical hypotheses for how life originated on Earth. Their hypothesis is now called the Oparin-Haldane hypothesis, and the key steps are:

• formation of organic molecules, the building blocks of cells (e.g., amino acids, nucleotides, simple sugars)
• formation of polymers (longer chains) of organic molecules, that can function as enzymes to carry out metabolic reactions, encode hereditary information, and possibly replicate (e.g., proteins, RNA strands),
• formation of protocells; concentrations of organic molecules and polymers that carry out metabolic reactions within an enclosed system, separated from the environment by a semi-permeable membrane, such as a lipid bilayer membrane

The Oparin-Haldane hypothesis has been continually tested and revised, and any hypothesis about how life began must account for the 3 primary universal requirements for life: the ability to reproduce and replicate hereditary information; the enclosure in membranes to form cells; the use of energy to accomplish growth and reproduction.

1. How did organic molecules form on a pre-biotic Earth?

Miller-urey experiment.

Stanley Miller and Harold Urey tested the first step of the Oparin-Haldane hypothesis by investigating the formation of organic molecules from inorganic compounds. Their 1950s experiment produced a number of organic molecules, including amino acids, that are made and used by living cells to grow and replicate.

Miller-Urey experiment, Wikimedia Commons illustration by Adrian Hunter

Miller and Urey used an experimental setup to recreate what environmental conditions were believed to be like on early Earth. A gaseous chamber simulated an atmosphere with reducing compounds (electron donors) such as methane, ammonia and hydrogen. Electrical sparks simulated lightning to provide energy. In only about a week’s time, this simple apparatus caused chemical reactions that produced a variety of organic molecules, some of which are the basic building blocks of life, such as amino acids. Although scientists no longer believe that pre-biotic Earth had such a reducing atmosphere, such reducing environments may be found in deep-sea hydrothermal vents, which also have a source of energy in the form of the heat from the vents. In addition, more recent experiments – that used conditions that are thought to better reflect the conditions of early Earth – have also produced a variety of organic molecules including amino acids and nucleotides (the building blocks of RNA and DNA) ( McCollom, 2013 ).

The video below gives a nice overview of the rationale, setup, and findings from the Miller-Urey experiment (although it incorrectly overstates that Darwin showed that relatively simple creatures can gradually give rise to more complex creatures).

Organic molecules from meteors

Each day the Earth is bombarded with meteorites and dust from comets. Analyses of space dust and meteors that have landed on Earth have revealed that they contain many organic molecules. The in-fall of cometary dust and meteorites was far greater when the Earth was young (4 billion years ago). Many scientists believe that such extra-terrestrial organic matter contributed significantly to the organic molecules available at the time that life on Earth began. The figure below from Bernstein 2006 shows the 3 major sources of organic molecules on pre-life Earth: atmospheric synthesis by Miller-Urey chemistry, synthesis at deep-sea hydrothermal vents, and in-fall of organic molecules synthesized in outer space.

Fig 6 from M. Bernstein 2006, Prebiotic materials from on and off the early Earth

2. Formation of organic polymers

Given a high enough concentration of these basic organic molecules, under certain conditions these will link together to form polymers (chains of molecules covalently bonded together). For example, amino acids link together to form polypeptide chains, that fold to become protein molecules. Ribose, a 5-carbon sugar, can bond with a nitrogenous base and phosphate to a nucleotide. Nucleotides link together to form nucleic acids, like DNA and RNA. While this is accomplished now by enzymes in living cells, polymerization of organic molecules can also be catalyzed by certain types of clay or other types of mineral surfaces. Experiments testing this model have produced RNA molecules up to 50-units long, in only a 1-2 week period of time ( Ferris, 2006 ).

Enzymatic activity and hereditary information in one polymer: the RNA World hypothesis

The discovery by Thomas Cech that some RNA molecules can catalyze their own site-specific cleavage led to a Nobel prize (for Cech and Altman), the term “ ribozymes ” to denote catalytic RNA molecules, and the revival of a hypothesis that RNA molecules were the original hereditary molecules, pre-dating DNA. For origin-of-life researchers, here was the possibility that RNA molecules could both encode hereditary information, and catalyze their own replication. DNA as the first hereditary molecule posed real problems for origin-of-life researchers because DNA replication requires protein enzymes (DNA polymerases) and RNA primers (see page on DNA replication), so it’s difficult to envision how such a complex hereditary system could have evolved from scratch. With catalytic RNA molecules, a single molecule or family of similar molecules could potentially store genetic information and replicate themselves, with no proteins needed initially.

Populations of such catalytic RNA molecules would undergo a molecular evolution conceptually identical to biological evolution by natural selection. RNA molecules would make copies of each other, making mistakes and generating variants. The variants that were most successful at replicating themselves (recognize identical or very similar RNA molecules and most efficiently replicate them) would increase in frequency in the population of catalytic RNA molecules. The RNA world hypothesis envisions a stage in the origin of life where self-replicating RNA molecules eventually led to the evolution of a hereditary system in the first cells or proto-cells. A system of RNA molecules that encode codons to specify amino acids, and tRNA-like molecules conveying matching amino acids, and catalytic RNAs that create peptide bonds, would constitute a hereditary system much like today’s cells, without DNA.

At some point in the lineage leading to the Last Universal Common Ancestor, DNA became the preferred long-term storage molecule for genetic information. DNA molecules are more chemically stable than RNA (deoxyribose is more chemically inert than ribose). Having two complementary strands means that each strand of DNA can serve as a template for replication of its partner strand, providing some innate redundancy. These and possibly other traits gave cells with a DNA hereditary system a selective advantage so that all cellular life on Earth uses DNA to store and transmit genetic information.

Still, even today, ribozymes play universal and central roles in cellular information processing. The ribosome is a large complex of RNAs and proteins that reads the genetic information in a strand of RNA to synthesize proteins. The key catalytic activity, the formation of peptide bonds to link two amino acids together, is catalyzed by a ribosomal RNA molecule. The ribosome is a giant ribozyme. Since ribosomes are universal to all cells, such catalytic RNAs must have been present in the Last Universal Common Ancestor of all current life on Earth.

Visit the  http://exploringorigins.org/ribozymes.html page to view the first ribozyme from Tetrahymena, discovered by Tom Cech, and the structure of the ribosomal RNAs.

The  http://exploringorigins.org/nucleicacids.html page has videos of polymerization of RNA from nucleotides, template-directed RNA synthesis, and a model of RNA self-replication.

The video below explains the rationale behind the RNA world hypothesis and briefly describes some of the findings from different RNA world experiments.

3. Protocells: self-replicating and metabolic enzymes in a bag

All life on Earth is composed of cells. Cells have lipid membranes that separate their inner contents, the cytoplasm, from the environment. The lipid membranes allow cells to maintain high concentrations of molecules like nucleotides needed for self-replicating RNAs to function more efficiently. Cells also maintain large differences in concentration (concentration gradients) of ions across the membrane to drive transport processes and cellular energy metabolism.

Lipids are hydrophobic, and will spontaneously self-assemble in water to form either micelles or lipid bilayer vesicles. Vesicles that enclose self-replicating RNAs and other enzymes, take in reactants across the membrane, export products, grow by accretion of lipid micelles, and divide by fission of the vesicle, are called proto-cells or protobionts and may have been the precursors of cellular life.

See http://exploringorigins.org/protocells.html for video animations of proto-cells.

The video below explores the differences between chemical and biological evolution, and highlights proto-cells as an example of chemical evolution.

At what point would evolutionary processes, such as natural selection, begin to drive the origin of the first cells?

Biological evolution is restricted to living organisms. So once the first cells, complete with a hereditary system, were formed, they would be subject to evolutionary processes, and natural selection would drive adaptation to their local environments, and populations in different environments would undergo speciation as gene flow becomes restricted between isolated populations.

However, the RNA World Hypothesis envisions evolutionary processes driving populations of self-replicating RNA molecules or proto-cells containing such RNA molecules. RNA molecules that replicated imperfectly would produce daughter molecules with slightly different sequences. The ones that replicate better, or improve the growth replication of their host proto-cells, would have more progeny. Hence, molecular evolution of self-replicating RNA molecules or proto-cell populations containing self-replicating RNA molecules would favor the eventual formation of the first cells.

References and Resources

Article on HCN chemistry by Patel et al. 2015  with Science News article by R. Service .

Bernstein M 2006. Prebiotic materials from on and off the early Earth. Philos Trans R Soc Lond B Biol Sci. 361:1689-700; discussion 1700-2. PubMed PMID: 17008210 ; PubMed Central PMCID: PMC1664678 .

Exploring Life’s Origins:  http://exploringorigins.org/index.html

UN Sustainable Development Goal (SDG) 3: Good Health and Well-being –    The origin of life and the steps leading up to it are important topics of research in the field of astrobiology, which seeks to understand the conditions necessary for the emergence of life. Understanding the origin of life can provide insight into the fundamental processes that govern the development and evolution of living organisms, which can have implications for human health and well-being. For example, research into the origin of life and evolutionarily novel traits can lead to the development of new treatments for diseases, as well as the creation of new medicines and therapies.

4 Responses to Origin of Life on Earth

Here’s a link to a fun article on work by Georgia Tech’s Nick Hud using a $5 toaster oven for his origin-of-life research! http://nautil.us/issue/27/dark-matter/the-dawn-of-life-in-a-5-toaster-oven Thanks to Timothy for the find and the link

New paper reveals likely traits of the Last Universal Common Ancestor of all extant life: Weiss et al. 2016: The physiology and habitat of the last universal common ancestor, Nature Microbiology 1, Article number: 16116 doi:10.1038/nmicrobiol.2016.116 James McInerney wrote a brief commentary for the above article: http://www.nature.com/articles/nmicrobiol2016139

Fantastic (somewhat long) retelling of origin-of-life research, with the twist and turns, culminating with the current integrated hypotheses. http://www.bbc.com/earth/story/20161026-the-secret-of-how-life-on-earth-began

A new paper going beyond the Miller-Urey types of expts to explore what could emerge out of these prebiotic chemistries: Wolos et al 2020, Synthetic connectivity, emergence, and self-regeneration in the network of prebiotic chemistry https://science.sciencemag.org/content/369/6511/eaaw1955

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February 12, 2007

21 min read

A Simpler Origin for Life

The sudden appearance of a large self-copying molecule such as RNA was exceedingly improbable. Energy-driven networks of small molecules afford better odds as the initiators of life.

By Robert Shapiro

Extraordinary discoveries inspire extraordinary claims. Thus James Watson reported that, immediately after they had uncovered the structure of DNA, Francis Crick "winged into the Eagle (pub) to tell everyone within hearing that we had discovered the secret of life." Their structure--an elegant double helix--almost merited such enthusiasm. Its proportions permitted information storage in a language in which four chemicals, called bases, played the same role as twenty six letters do in the English language.

Further, the information was stored in two long chains, each of which specified the contents of its partner. This arrangement suggested a mechanism for reproduction, that was subsequently illustrated in many biochemistry texts, as well as on a tie that my wife bought for me at a crafts fair: The two strands of the DNA double helix parted company. As they did so, new DNA building blocks, called nucleotides, lined up along the separated strands and linked up. Two double helices now existed in place of one, each a replica of the original.

The Watson-Crick structure triggered an avalanche of discoveries about the way in which living cells function today. These insights also stimulated speculations about life's origins. Nobel Laureate H. J. Muller wrote that the gene material was "living material, the present-day representative of the first life," which Carl Sagan visualized as "a primitive free-living naked gene situated in a dilute solution of organic matter." In this context, "organic" specifies material containing bound carbon atoms. Organic chemistry, a subject sometimes feared by pre-medical students, is the chemistry of carbon compounds, both those present in life and those playing no part in life. Many different definitions of life have been proposed. Muller's remark would be in accord with what has been called the NASA definition of life: Life is a self-sustained chemical system capable of undergoing Darwinian evolution.

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Richard Dawkins elaborated on this image of the earliest living entity in his book The Selfish Gene : "At some point a particularly remarkable molecule was formed by accident. We will call it the Replicator . It may not have been the biggest or the most complex molecule around, but it had the extraordinary property of being able to create copies of itself." When Dawkins wrote these words 30 years ago, DNA was the most likely candidate for this role. As we shall see, several other replicators have now been suggested.

When RNA Ruled the World

Unfortunately, complications soon set in. DNA replication cannot proceed without the assistance of a number of proteins--members of a family of large molecules that are chemically very different from DNA. Proteins, like DNA, are constructed by linking subunits, amino acids in this case, together to form a long chain. Cells employ twenty of these building blocks in the proteins that they make, affording a variety of products capable of performing many different tasks--proteins are the handymen of the living cell. Their most famous subclass, the enzymes, act as expeditors, speeding up chemical processes that would otherwise take place too slowly to be of use to life.

The above account brings to mind the old riddle: Which came first, the chicken or the egg? DNA holds the recipe for protein construction. Yet that information cannot be retrieved or copied without the assistance of proteins. Which large molecule, then, appeared first in getting life started--proteins (the chicken) or DNA (the egg)?

A possible solution appeared when attention shifted to a new champion--RNA. This versatile class of molecule is, like DNA, assembled of nucleotide building blocks, but plays many roles in our cells. Certain RNAs ferry information from DNA to structures (which themselves are largely built of other kinds of RNA) that construct proteins. In carrying out its various duties, RNA can take on the form of a double helix that resembles DNA, or of a folded single strand, much like a protein. In 2006 the Nobel prizes in both chemistry and medicine were awarded for discoveries concerning the role of RNA in editing and censoring DNA instructions. Warren E. Leary could write in the New York Times that RNA "is swiftly emerging from the shadows of its better-known cousin DNA."

For many scientists in the origin-of-life field, those shadows had lifted two decades earlier with the discovery of ribozymes, enzyme-like substances made of RNA. A simple solution to the chicken-and-egg riddle now appeared to fall into place: Life began with the appearance of the first RNA molecule. In a germinal 1986 article, Nobel Laureate Walter Gilbert of Harvard University wrote in the journal Nature : "One can contemplate an RNA world, containing only RNA molecules that serve to catalyze the synthesis of themselves. & The first step of evolution proceeds then by RNA molecules performing the catalytic activities necessary to assemble themselves from a nucleotide soup." In this vision, the first self-replicating RNA that emerged from non-living matter carried out the functions now executed by RNA, DNA and proteins.

A number of additional clues seemed to support the idea that RNA appeared before proteins and DNA in the evolution of life. Many small molecules, called cofactors, play a necessary role in enzyme-catalyzed reactions. These cofactors often carry an attached RNA nucleotide with no obvious function. These structures have been considered "molecular fossils," relics descended from the time when RNA alone, without DNA or proteins, ruled the biochemical world. In addition, chemists have been able to synthesize new ribozymes that display a variety of enzyme-like activities. Many scientists found the idea of an organism that relied on ribozymes, rather than protein enzymes, very attractive.

The hypothesis that life began with RNA was presented as a likely reality, rather than a speculation, in journals, textbooks and the media. Yet the clues I have cited only support the weaker conclusion that RNA preceded DNA and proteins; they provide no information about the origin of life, which may have involved stages prior to the RNA world in which other living entities ruled supreme. Just the same, and despite the difficulties that I will discuss in the next section, perhaps two-thirds of scientists publishing in the origin-of life field (as judged by a count of papers published in 2006 in the journal Origins of Life and Evolution of the Biosphere ) still support the idea that life began with the spontaneous formation of RNA or a related self-copying molecule. Confusingly, researchers use the term "RNA World" to refer to both the strong and the weak claims about RNA's role prior to DNA and proteins. Here, I will use the term "RNA first" for the strong claim that RNA was involved in the origin of life.

The Soup Kettle is Empty

The attractive features of RNA World prompted Gerald Joyce of the Scripps Research Institute and Leslie Orgel of the Salk Institute to picture it as "the molecular biologist's dream" within a volume devoted to that topic. They also used the term "the prebiotic chemist's nightmare" to describe another part of the picture: How did that first self-replicating RNA arise? Enormous obstacles block Gilbert's picture of the origin of life, sufficient to provoke another Nobelist, Christian De Duve of Rockefeller University, to ask rhetorically, "Did God make RNA?"

RNA's building blocks, nucleotides, are complex substances as organic molecules go. They each contain a sugar, a phosphate and one of four nitrogen-containing bases as sub-subunits. Thus, each RNA nucleotide contains 9 or 10 carbon atoms, numerous nitrogen and oxygen atoms and the phosphate group, all connected in a precise three-dimensional pattern. Many alternative ways exist for making those connections, yielding thousands of plausible nucleotides that could readily join in place of the standard ones but that are not represented in RNA. That number is itself dwarfed by the hundreds of thousands to millions of stable organic molecules of similar size that are not nucleotides.

The RNA nucleotides are familiar to chemists because of their abundance in life and their resulting commercial availability. In a form of molecular vitalism, some scientists have presumed that nature has an innate tendency to produce life's building blocks preferentially, rather than the hordes of other molecules that can also be derived from the rules of organic chemistry. This idea drew inspiration from a well known experiment published in 1953 by Stanley Miller. He applied a spark discharge to a mixture of simple gases that were then thought to represent the atmosphere of the early Earth. Two amino acids of the set of 20 used to construct proteins were formed in significant quantities, with others from that set present in small amounts. (A description of the Miller experiment and the chemical structures of an amino acid and a nucleotide can be found in "The Origin of Life on the Earth," by L. E. Orgel; Scientific American, October 1994.) In addition, more than 80 different amino acids, some present and others absent from living systems, have been identified as components of the Murchison meteorite, which fell in Australia in 1969. Nature has apparently been generous in providing a supply of these particular building blocks. By extrapolation of these results, some writers have presumed that all of life's building could be formed with ease in Miller-type experiments and were present in meteorites and other extraterrestrial bodies. This is not the case.

A careful examination of the results of the analysis of several meteorites led the scientists who conducted the work to a different conclusion: inanimate nature has a bias toward the formation of molecules made of fewer rather than greater numbers of carbon atoms, and thus shows no partiality in favor of creating the building blocks of our kind of life. (When larger carbon-containing molecules are produced, they tend to be insoluble, hydrogen-poor substances that organic chemists call tars.) I have observed a similar pattern in the results of many spark discharge experiments.

Amino acids, such as those produced or found in these experiments, are far less complex than nucleotides. Their defining features are an amino group (a nitrogen and two hydrogens) and a carboxylic acid group (a carbon, two oxygens and a hydrogen) both attached to the same carbon. The simplest of the 20 used to build natural proteins contains only two carbon atoms. Seventeen of the set contain six or fewer carbons. The amino acids and other substances that were prominent in the Miller experiment contained two and three carbon atoms. By contrast, no nucleotides of any kind have been reported as products of spark discharge experiments or in studies of meteorites, nor have the smaller units (nucleosides) that contain a sugar and base but lack the phosphate.

To rescue the RNA-first concept from this otherwise lethal defect, its advocates have created a discipline called prebiotic synthesis. They have attempted to show that RNA and its components can be prepared in their laboratories in a sequence of carefully controlled reactions, normally carried out in water at temperatures observed on Earth. Such a sequence would start usually with compounds of carbon that had been produced in spark discharge experiments or found in meteorites. The observation of a specific organic chemical in any quantity (even as part of a complex mixture) in one of the above sources would justify its classification as "prebiotic," a substance that supposedly had been proved to be present on the early Earth. Once awarded this distinction, the chemical could then be used in pure form, in any quantity, in another prebiotic reaction. The products of such a reaction would also be considered "prebiotic" and employed in the next step in the sequence.

The use of reaction sequences of this type (without any reference to the origin of life) has long been an honored practice in the traditional field of synthetic organic chemistry. My own PhD thesis advisor, Robert B. Woodward, was awarded the Nobel Prize for his brilliant syntheses of quinine, cholesterol, chlorophyll and many other substances. It mattered little if kilograms of starting material were required to produce milligrams of product. The point was the demonstration that humans could produce, however inefficiently, substances found in nature. Unfortunately, neither chemists nor laboratories were present on the early Earth to produce RNA.

I will cite one example of prebiotic synthesis, published in 1995 by Nature and featured in the New York Times . The RNA base cytosine was prepared in high yield by heating two purified chemicals in a sealed glass tube at 100 degrees Celsius for about a day. One of the reagents, cyanoacetaldehyde, is a reactive substance capable of combining with a number of common chemicals that may have been present on the early Earth. These competitors were excluded. An extremely high concentration was needed to coax the other participant, urea, to react at a sufficient rate for the reaction to succeed. The product, cytosine, can self-destruct by simple reaction with water. When the urea concentration was lowered, or the reaction allowed to continue too long, any cytosine that was produced was subsequently destroyed. This destructive reaction had been discovered in my laboratory, as part of my continuing research on environmental damage to DNA. Our own cells deal with it by maintaining a suite of enzymes that specialize in DNA repair.

The exceptionally high urea concentration was rationalized in the Nature paper by invoking a vision of drying lagoons on the early Earth. In a published rebuttal, I calculated that a large lagoon would have to be evaporated to the size of a puddle, without loss of its contents, to achieve that concentration. No such feature exists on Earth today.

The drying lagoon claim is not unique. In a similar spirit, other prebiotic chemists have invoked freezing glacial lakes, mountainside freshwater ponds, flowing streams, beaches, dry deserts, volcanic aquifers and the entire global ocean (frozen or warm as needed) to support their requirement that the "nucleotide soup" necessary for RNA synthesis would somehow have come into existence on the early Earth.

The analogy that comes to mind is that of a golfer, who having played a golf ball through an 18-hole course, then assumed that the ball could also play itself around the course in his absence. He had demonstrated the possibility of the event; it was only necessary to presume that some combination of natural forces (earthquakes, winds, tornadoes and floods, for example) could produce the same result, given enough time. No physical law need be broken for spontaneous RNA formation to happen, but the chances against it are so immense, that the suggestion implies that the non-living world had an innate desire to generate RNA. The majority of origin-of-life scientists who still support the RNA-first theory either accept this concept (implicitly, if not explicitly) or feel that the immensely unfavorable odds were simply overcome by good luck.

A Simpler Replicator?

Many chemists, confronted with these difficulties, have fled the RNA-first hypothesis as if it were a building on fire. One group, however, still captured by the vision of the self-copying molecule, has opted for an exit that leads to similar hazards. In these revised theories, a simpler replicator arose first and governed life in a "pre-RNA world." Variations have been proposed in which the bases, the sugar or the entire backbone of RNA have been replaced by simpler substances, more accessible to prebiotic syntheses. Presumably, this first replicator would also have the catalytic capabilities of RNA. Because no trace of this hypothetical primal replicator and catalyst has been recognized so far in modern biology, RNA must have completely taken over all of its functions at some point following its emergence.

Further, the spontaneous appearance of any such replicator without the assistance of a chemist faces implausibilities that dwarf those involved in the preparation of a mere nucleotide soup. Let us presume that a soup enriched in the building blocks of all of these proposed replicators has somehow been assembled, under conditions that favor their connection into chains. They would be accompanied by hordes of defective building blocks, the inclusion of which would ruin the ability of the chain to act as a replicator. The simplest flawed unit would be a terminator, a component that had only one "arm" available for connection, rather than the two needed to support further growth of the chain.

There is no reason to presume than an indifferent nature would not combine units at random, producing an immense variety of hybrid short, terminated chains, rather than the much longer one of uniform backbone geometry needed to support replicator and catalytic functions. Probability calculations could be made, but I prefer a variation on a much-used analogy. Picture a gorilla (very long arms are needed) at an immense keyboard connected to a word processor. The keyboard contains not only the symbols used in English and European languages but also a huge excess drawn from every other known language and all of the symbol sets stored in a typical computer. The chances for the spontaneous assembly of a replicator in the pool I described above can be compared to those of the gorilla composing, in English, a coherent recipe for the preparation of chili con carne. With similar considerations in mind Gerald F. Joyce of the Scripps Research Institute and Leslie Orgel of the Salk Institute concluded that the spontaneous appearance of RNA chains on the lifeless Earth "would have been a near miracle." I would extend this conclusion to all of the proposed RNA substitutes that I mentioned above.

Life With Small Molecules

Nobel Laureate Christian de Duve has called for "a rejection of improbabilities so incommensurably high that they can only be called miracles, phenomena that fall outside the scope of scientific inquiry." DNA, RNA, proteins and other elaborate large molecules must then be set aside as participants in the origin of life. Inanimate nature provides us with a variety of mixtures of small molecules, whose behavior is governed by scientific laws, rather than by human intervention.

Fortunately, an alternative group of theories that can employ these materials has existed for decades. The theories employ a thermodynamic rather than a genetic definition of life, under a scheme put forth by Carl Sagan in the Encyclopedia Britannica: A localized region which increases in order (decreases in entropy) through cycles driven by an energy flow would be considered alive. This small-molecule approach is rooted in the ideas of the Soviet biologist Alexander Oparin, and current notable spokesmen include de Duve, Freeman Dyson of the Institute for Advanced Study, Stuart Kauffman of the Santa Fe Institute, Doron Lancet of the Weizmann Institute, Harold Morowitz of George Mason University and the independent researcher G¿nter W¿chtersh¿user. I estimate that about a third of the chemists involved in the study of the origin of life subscribe to theories based on this idea. Origin-of-life proposals of this type differ in specific details; here I will try to list five common requirements (and add some ideas of my own).

(1) A boundary is needed to separate life from non-life. Life is distinguished by its great degree of organization, yet the second law of thermodynamics requires that the universe move in a direction in which disorder, or entropy, increases. A loophole, however, allows entropy to decrease in a limited area, provided that a greater increase occurs outside the area. When living cells grow and multiply, they convert chemical energy or radiation to heat at the same time. The released heat increases the entropy of the environment, compensating for the decrease in living systems. The boundary maintains this division of the world into pockets of life and the nonliving environment in which they must sustain themselves.

Today, sophisticated double-layered cell membranes, made of chemicals classified as lipids, separate living cells from their environment. When life began, some natural feature probably served the same purpose. David W. Deamer of the University of California, Santa Cruz, has observed membrane-like structures in meteorites. Other proposals have suggested natural boundaries not used by life today, such as iron sulfide membranes, mineral surfaces (in which electrostatic interactions segregate selected molecules from their environment), small ponds and aerosols.

(2) An energy source is needed to drive the organization process. We consume carbohydrates and fats, and combine them with oxygen that we inhale, to keep ourselves alive. Microorganisms are more versatile, and can use minerals in place of the food or the oxygen. In either case, the transformations that are involved are called redox reactions. They involve the transfer of electrons from an electron rich (or reduced) substance to an electron poor (or oxidized) one. Plants can capture solar energy directly, and adapt it for the functions of life. Other forms of energy are used by cells in specialized circumstances--for example, differences in acidity on opposite sides of a membrane. Yet others, such as radioactivity and abrupt temperature differences, might be used by life elsewhere in the universe. Here I will consider redox reactions as the energy source.

(3) A coupling mechanism must link the release of energy to the organization process that produces and sustains life. The release of energy does not necessarily produce a useful result. Chemical energy is released when gasoline is burned within the cylinders of my automobile, but the vehicle will not move unless that energy is used to turn the wheels. A mechanical connection, or coupling, is required. Each day, in our own cells, each of us degrades pounds of a nucleotide called ATP. The energy released by this favorable reaction serves to drive processes that are less favorable but necessary for our biochemistry. Linkage is achieved when the reactions share a common intermediate, and the process is speeded up by the intervention of an enzyme. One assumption of the small-molecule approach is that coupled reactions and primitive catalysts sufficient to get life started exist in nature.

(4) A chemical network must be formed, to permit adaptation and evolution. We come now to the heart of the matter. Imagine for example that an energetically favorable redox reaction of a naturally-occurring mineral is linked to the conversion of an organic chemical A to another one B within a compartment. The favorable, energy releasing, entropy-increasing reaction of the mineral drives the A-to-B transformation. I call this key transformation a driver reaction, for it serves as the engine that mobilizes the organization process. If B simply reconverts back to A or escapes from the compartment, we would not be on a path that leads to increased organization. By contrast, if a multi-step chemical pathway--say, B to C to D to A--reconverts B to A, then the steps in that circular process (or cycle) would be favored because they replenish the supply of A, allowing the continuing discharge of energy by the mineral reaction.

If we visualize the cycle as a circular railway line, the energy source keeps the trains traveling around it one way. Each station may also be the hub for a number of branch lines, such as one connecting station D to another station, E. Trains could travel in either direction along that branch, depleting or augmenting the cycle's traffic. Thanks to the continual depletion of A, however, material is drawn from D to A. The resulting depletion of D in turn tends to draw material from E to D. In this way, material is "pulled" along the branch lines into the central cycle, maximizing the energy release that accompanies the driver reaction.

The cycle could also adapt to changing circumstances. As a child, I was fascinated by the way in which water, released from a leaky hydrant, would find a path downhill to the nearest sewer. If falling leaves or dropped refuse blocked that path, the water would back up until another route was found around the obstacle. In the same way, if a change in the acidity or in some other environmental circumstance should hinder a step in the pathway from B to A, material would back up until another route was found. Additional changes of this type would convert the original cycle into a network. This trial-and-error exploration of the chemical "landscape" might also turn up compounds that could catalyze important steps in the cycle, increasing the efficiency with which the network utilized the energy source.

(5) The network must grow and reproduce. To survive and grow, the network must gain material at a rate that compensates for the paths that remove it. Diffusion of network materials out of the compartment into the external world is favored by entropy and will occur to some extent, especially at the start of life when the boundary is a crude one established by the environment rather than one of the highly effective cell membranes available today after billions of years of evolution. Some side reactions may produce gases, which escape, or form tars, which will drop out of solution. If these processes together should exceed the rate at which the network gains material, then it would be extinguished. Exhaustion of the external fuel would have the same effect. We can imagine, on the early Earth, a situation where many startups of this type occur, involving many alternative driver reactions and external energy sources. Finally, a particularly hardy one would take root and sustain itself.

A system of reproduction must eventually develop. If our network is housed in a lipid membrane, then physical forces may split it, after it has grown enough. (Freeman Dyson has described such a system as a "garbage-bag world" in contrast to the "neat and beautiful scene" of the RNA world.) A system that functions in a compartment within a mineral may overflow into adjacent compartments. Whatever the mechanism may be, this dispersal into separated units protects the system from total extinction by a localized destructive event. Once independent units were established, they could evolve in different ways and compete with one another for raw materials; we would have made the transition from life that emerges from nonliving matter through the action of an available energy source to life that adapts to its environment by Darwinian evolution.

Changing the Paradigm

Systems of the type I have described usually have been classified under the heading "metabolism first," which implies that they do not contain a mechanism for heredity. In other words, they contain no obvious molecule or structure that allows the information stored in them (their heredity) to be duplicated and passed on to their descendants. However a collection of small items holds the same information as a list that describes the items. For example, my wife gives me a shopping list for the supermarket; the collection of grocery items that I return with contains the same information as the list. Doron Lancet has given the name "compositional genome" to heredity stored in small molecules, rather than a list such as DNA or RNA.

The small molecule approach to the origin of life makes several demands upon nature (a compartment, an external energy supply, a driver reaction coupled to that supply, and the existence of a chemical network that contains that reaction). These requirements are general in nature, however, and are immensely more probable than the elaborate multi-step pathways needed to form a molecule that can function as a replicator.

Over the years, many theoretical papers have advanced particular metabolism first schemes, but relatively little experimental work has been presented in support of them. In those cases where experiments have been published, they have usually served to demonstrate the plausibility of individual steps in a proposed cycle. The greatest amount of new data has perhaps come from G¿nter W¿chtersh¿user and his colleagues at the Technische Universit¿t M¿nchen. They have demonstrated portions of a cycle involving the combination and separation of amino acids, in the presence of metal sulfide catalysts. The energetic driving force for the transformations is supplied by the oxidation of carbon monoxide to carbon dioxide. They have not yet demonstrated the operation of a complete cycle or its ability to sustain itself and undergo further evolution. A "smoking gun" experiment displaying those three features is needed to establish the validity of the small molecule approach.

The principal initial task is the identification of candidate driver reactions--small molecule transformations (A to B in the example before) that are coupled to an abundant external energy source (such as the oxidation of carbon monoxide or a mineral). Once a plausible driver reaction has been identified, there should be no need to specify the rest of the system in advance. The selected components (including the energy source) plus a mixture of other small molecules normally produced by natural processes (and likely to have been abundant on the early Earth) could be combined in a suitable reaction vessel. If an evolving network were established, we would expect the concentration of the participants in the network to increase and alter with time. New catalysts that increased the rate of key reactions might appear, while irrelevant materials would decrease in quantity. The reactor would need an input device to allow replenishment of the energy supply and raw materials, and an outlet to permit the removal of waste products and chemicals that were not part of the network.

In such experiments, failures would be easily identified. The energy might be dissipated without producing any significant changes in the concentrations of the other chemicals or the chemicals might simply be converted to a tar, which would clog the apparatus. A success might demonstrate the initial steps on the road to life. These steps need not duplicate those that took place on the early Earth. It is more important that the general principle be demonstrated and made available for further investigation. Many potential paths to life may exist, with the choice dictated by the local environment.

An understanding of the initial steps leading to life would not reveal the specific events that led to the familiar DNA-RNA-protein-based organisms of today. However, because we know that evolution does not anticipate future events, we can presume that nucleotides first appeared in metabolism to serve some other purpose, perhaps as catalysts or as containers for the storage of chemical energy (the nucleotide ATP still serves this function today). Some chance event or circumstance may have led to the connection of nucleotides to form RNA. The most obvious function of RNA today is to serve as a structural element that assists in the formation of bonds between amino acids in the synthesis of proteins. The first RNAs may have served the same purpose, but without any preference for specific amino acids. Many further steps in evolution would be needed to "invent" the elaborate mechanisms for replication and specific protein synthesis that we observe in life today.

If the general small-molecule paradigm were confirmed, then our expectations of the place of life in the universe would change. A highly implausible start for life, as in the RNA-first scenario, implies a universe in which we are alone. In the words of the late Jacques Monod, "The universe was not pregnant with life nor the biosphere with man. Our number came up in the Monte Carlo game." The small-molecule alternative, however, is in harmony with the views of biologist Stuart Kauffman: "If this is all true, life is vastly more probable than we have supposed. Not only are we at home in the universe, but we are far more likely to share it with unknown companions."

ROBERT SHAPIRO is professor emeritus of chemistry and senior research scientist at New York University. He is author or co-author of over 125 publications, primarily in the area of DNA chemistry. In particular, he and his co-workers have studied the ways in which environmental chemicals can damage our hereditary material, causing changes that can lead to mutations and cancer. In 2004, he was awarded the Trotter Prize in Information, Complexity and Inference. Shapiro has written four books for the general public: Life Beyond Earth (with Gerald Feinberg); Origins, a Skeptic's Guide to the Creation of Life on Earth ; The Human Blueprint (on the effort to read the human genome); and Planetary Dreams (on the search for life in our Solar System). When he is not involved in research, lecturing or writing, he enjoys running, hiking, wine-tastings, theater and travel. He is married and has a 35-year-old son.

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3.1: Spontaneous Generation

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Skills to Develop

• Explain the theory of spontaneous generation and why people once accepted it as an explanation for the existence of certain types of organisms
• Explain how certain individuals (van Helmont, Redi, Needham, Spallanzani, and Pasteur) tried to prove or disprove spontaneous generation

Clinical Focus - PART 1

Barbara is a 19-year-old college student living in the dormitory. In January, she came down with a sore throat, headache, mild fever, chills, and a violent but unproductive (i.e., no mucus) cough. To treat these symptoms, Barbara began taking an over-the-counter cold medication, which did not seem to work. In fact, over the next few days, while some of Barbara’s symptoms began to resolve, her cough and fever persisted, and she felt very tired and weak.

Exercise \(\PageIndex{1}\)

What types of respiratory disease may be responsible?

Humans have been asking for millennia: Where does new life come from? Religion, philosophy, and science have all wrestled with this question. One of the oldest explanations was the theory of spontaneous generation, which can be traced back to the ancient Greeks and was widely accepted through the Middle Ages.

The Theory of Spontaneous Generation

The Greek philosopher Aristotle (384–322 BC) was one of the earliest recorded scholars to articulate the theory of spontaneous generation, the notion that life can arise from nonliving matter. Aristotle proposed that life arose from nonliving material if the material contained pneuma (“vital heat”). As evidence, he noted several instances of the appearance of animals from environments previously devoid of such animals, such as the seemingly sudden appearance of fish in a new puddle of water. 1

This theory persisted into the 17 th century, when scientists undertook additional experimentation to support or disprove it. By this time, the proponents of the theory cited how frogs simply seem to appear along the muddy banks of the Nile River in Egypt during the annual flooding. Others observed that mice simply appeared among grain stored in barns with thatched roofs. When the roof leaked and the grain molded, mice appeared. Jan Baptista van Helmont, a 17 th century Flemish scientist, proposed that mice could arise from rags and wheat kernels left in an open container for 3 weeks. In reality, such habitats provided ideal food sources and shelter for mouse populations to flourish.

However, one of van Helmont’s contemporaries, Italian physician Francesco Redi (1626–1697), performed an experiment in 1668 that was one of the first to refute the idea that maggots (the larvae of flies) spontaneously generate on meat left out in the open air. He predicted that preventing flies from having direct contact with the meat would also prevent the appearance of maggots. Redi left meat in each of six containers (Figure \(\PageIndex{1}\)). Two were open to the air, two were covered with gauze, and two were tightly sealed. His hypothesis was supported when maggots developed in the uncovered jars, but no maggots appeared in either the gauze-covered or the tightly sealed jars. He concluded that maggots could only form when flies were allowed to lay eggs in the meat, and that the maggots were the offspring of flies, not the product of spontaneous generation.

Figure \(\PageIndex{1}\): Francesco Redi’s experimental setup consisted of an open container, a container sealed with a cork top, and a container covered in mesh that let in air but not flies. Maggots only appeared on the meat in the open container. However, maggots were also found on the gauze of the gauze-covered container.

In 1745, John Needham (1713–1781) published a report of his own experiments, in which he briefly boiled broth infused with plant or animal matter, hoping to kill all preexisting microbes. 2 He then sealed the flasks. After a few days, Needham observed that the broth had become cloudy and a single drop contained numerous microscopic creatures. He argued that the new microbes must have arisen spontaneously. In reality, however, he likely did not boil the broth enough to kill all preexisting microbes.

Lazzaro Spallanzani (1729–1799) did not agree with Needham’s conclusions, however, and performed hundreds of carefully executed experiments using heated broth. 3 As in Needham’s experiment, broth in sealed jars and unsealed jars was infused with plant and animal matter. Spallanzani’s results contradicted the findings of Needham: Heated but sealed flasks remained clear, without any signs of spontaneous growth, unless the flasks were subsequently opened to the air. This suggested that microbes were introduced into these flasks from the air. In response to Spallanzani’s findings, Needham argued that life originates from a “life force” that was destroyed during Spallanzani’s extended boiling. Any subsequent sealing of the flasks then prevented new life force from entering and causing spontaneous generation (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): (a) Francesco Redi, who demonstrated that maggots were the offspring of flies, not products of spontaneous generation. (b) John Needham, who argued that microbes arose spontaneously in broth from a “life force.” (c) Lazzaro Spallanzani, whose experiments with broth aimed to disprove those of Needham.

Exercise \(\PageIndex{2}\)

• Describe the theory of spontaneous generation and some of the arguments used to support it.
• Explain how the experiments of Redi and Spallanzani challenged the theory of spontaneous generation.

Disproving Spontaneous Generation

The debate over spontaneous generation continued well into the 19 th century, with scientists serving as proponents of both sides. To settle the debate, the Paris Academy of Sciences offered a prize for resolution of the problem. Louis Pasteur, a prominent French chemist who had been studying microbial fermentation and the causes of wine spoilage, accepted the challenge. In 1858, Pasteur filtered air through a gun-cotton filter and, upon microscopic examination of the cotton, found it full of microorganisms, suggesting that the exposure of a broth to air was not introducing a “life force” to the broth but rather airborne microorganisms.

Later, Pasteur made a series of flasks with long, twisted necks (“swan-neck” flasks), in which he boiled broth to sterilize it (Figure \(\PageIndex{3}\)). His design allowed air inside the flasks to be exchanged with air from the outside, but prevented the introduction of any airborne microorganisms, which would get caught in the twists and bends of the flasks’ necks. If a life force besides the airborne microorganisms were responsible for microbial growth within the sterilized flasks, it would have access to the broth, whereas the microorganisms would not. He correctly predicted that sterilized broth in his swan-neck flasks would remain sterile as long as the swan necks remained intact. However, should the necks be broken, microorganisms would be introduced, contaminating the flasks and allowing microbial growth within the broth.

Pasteur’s set of experiments irrefutably disproved the theory of spontaneous generation and earned him the prestigious Alhumbert Prize from the Paris Academy of Sciences in 1862. In a subsequent lecture in 1864, Pasteur articulated “ Omne vivum ex vivo ” (“Life only comes from life”). In this lecture, Pasteur recounted his famous swan-neck flask experiment, stating that “…life is a germ and a germ is life. Never will the doctrine of spontaneous generation recover from the mortal blow of this simple experiment.” 4 To Pasteur’s credit, it never has.

Figure \(\PageIndex{3}\): (a) French scientist Louis Pasteur, who definitively refuted the long-disputed theory of spontaneous generation. (b) The unique swan-neck feature of the flasks used in Pasteur’s experiment allowed air to enter the flask but prevented the entry of bacterial and fungal spores. (c) Pasteur’s experiment consisted of two parts. In the first part, the broth in the flask was boiled to sterilize it. When this broth was cooled, it remained free of contamination. In the second part of the experiment, the flask was boiled and then the neck was broken off. The broth in this flask became contaminated. (credit b: modification of work by “Wellcome Images”/Wikimedia Commons)

Exercise \(\PageIndex{3}\)

• How did Pasteur’s experimental design allow air, but not microbes, to enter, and why was this important?
• What was the control group in Pasteur’s experiment and what did it show?
• The theory of spontaneous generation states that life arose from nonliving matter. It was a long-held belief dating back to Aristotle and the ancient Greeks.
• Experimentation by Francesco Redi in the 17th century presented the first significant evidence refuting spontaneous generation by showing that flies must have access to meat for maggots to develop on the meat. Prominent scientists designed experiments and argued both in support of (John Needham) and against (Lazzaro Spallanzani) spontaneous generation.
• Louis Pasteur is credited with conclusively disproving the theory of spontaneous generation with his famous swan-neck flask experiment. He subsequently proposed that “life only comes from life.”
• 1 K. Zwier. “Aristotle on Spontaneous Generation.” http://www.sju.edu/int/academics/cas...R.%20Zwier.pdf
• 2 E. Capanna. “Lazzaro Spallanzani: At the Roots of Modern Biology.” Journal of Experimental Zoology 285 no. 3 (1999):178–196.
• 3 R. Mancini, M. Nigro, G. Ippolito. “Lazzaro Spallanzani and His Refutation of the Theory of Spontaneous Generation.” Le Infezioni in Medicina 15 no. 3 (2007):199–206.
• 4 R. Vallery-Radot. The Life of Pasteur , trans. R.L. Devonshire. New York: McClure, Phillips and Co, 1902, 1:142.

Contributor

Nina Parker, (Shenandoah University), Mark Schneegurt (Wichita State University), Anh-Hue Thi Tu (Georgia Southwestern State University), Philip Lister (Central New Mexico Community College), and Brian M. Forster (Saint Joseph’s University) with many contributing authors. Original content via Openstax (CC BY 4.0; Access for free at  https://openstax.org/books/microbiology/pages/1-introduction )

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1.4: Prebiotic Earth and the origin of life

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• Gerald Bergtrom
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Thinking about Life's Origins- A Short Summary of a Long History

By all accounts, the earth must have been a very unpleasant place soon after its formation! Volcanic eruptions, meteorites, thunder, lighting, dust, fires, etc. For that reason, the period from 4.8 to 4.0 billion years ago is called the Hadean Eon, after Hades, the hell of the ancient Greeks! Until recently, geological, geochemical and fossil evidence suggested that life arose between 3.8 and 4.1 billion years ago. In fact, questions about life’s origins are probably as old as the ancient Greeks! We only have records of human notions of life’s origins dating from biblical accounts and, just a bit later, from Aristotle’s musings. While Aristotle did not suggest that life began in hell, he and other ancient Greeks did speculate about life’s origins by  spontaneous   generation , in the sense of  abiogenesis  ( life  originating from  non-life ). 

Later, the dominant theological accounts of creation in Europe in the middle ages muted any notions of origins and evolution. While a few medieval voices ran counter to strict biblical readings of the creation stories, it was not until the Renaissance in the 14th -17th century that an appreciation of ancient Greek  humanism  was reawakened, and with it, scientific curiosity and the ability to engage in rational questioning and research.

Charles Darwin suggested in 1859 that life might have begun in a " warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, &c., present, that a protein compound was chemically formed ready to undergo still more complex changes. " He even realized that these chemical constituents would not have survived in the atmosphere and waters of his day, but must have done so in a prebiotic world. In  On the Origin of Species , he referred to life having been ‘created’. There, Darwin was not referring to a biblical basis of creation; he clearly meant that life originated “ by some wholly unknown process " at a time before which there was no life. 

Among Darwin’s friends and contemporaries were Charles Lyell and Roderick Murchison, both geologists who understood much about the slow geological changes that shaped the earth. Darwin was therefore familiar with the concept of extended periods of geological time, amounts of time he believed was necessary for the natural selection of traits leading to species divergence.

Fast-forward to the 1920s when J.H.B.S. Haldane and A. Oparin offered an hypothesis about the life’s origins based on notions of the chemistry and physical conditions that might have existed on a  prebiotic earth . Their proposal assumed that the earth’s atmosphere was hot, hellish and reducing (i.e., filled with inorganic molecules able to give up electrons and hydrogens). There are more than a few hypotheses for which chemicals were already present on earth, or that formed when the planet formed about 4.8 billion years ago. We’ll start our exploration with Oparin and Haldane’s  reducing atmosphere . Then we look at possibility that life began under  non-reducing conditions  .

From Inorganic to Organic molecules, and to Life

A prerequisite to the prebiotic chemical experimentation is a source of organic molecules. Just as life requires energy (to do anything and everything!), converting inorganic molecules into organic molecules requires an input of free energy . Today, most living things get free energy by oxidizing nutrients or directly from the sun by photosynthesis. Recall that in fact all the chemical energy sustaining life today ultimately comes from the sun. But, before there were cells, how did organic molecules form from inorganic precursors? Oparin and Haldane hypothesized a reducing atmosphere on the prebiotic earth (there was no oxygen before photosynthesis was possible), rich in inorganic molecules with reducing power (like H 2 , NH 3 , CH 4 , and H 2 S) as well as CO 2 to serve as a carbon source. The predicted physical conditions on this prebiotic earth were:

• lots of water (oceans).
• hot (no free O 2 ).
• lots ionizing (e.g., X, \(\gamma \)) radiation from space, (no protective ozone layer).
• frequent ionizing (electrical) storms generated in an unstable atmosphere.
• volcanic and thermal vent activity.

Origins of Organic Molecules and a Primordial Soup

Oparin suggested that abundant sources of free energy fueled the reductive synthesis of the first organic molecules to create what he called a “ primeval soup ”. No doubt, he called this primeval concoction a “soup” because it would have been rich in chemical (nutrient) free energy. The Oparin/Haldane proposal received strong support from the experiments of Stanley Miller and Harold Urey (Urey had already won the 1934 Nobel Prize in Chemistry for discovering deuterium). Miller and Urey tested the prediction that, under Haldane and Oparin’s prebiotic earth conditions, inorganic molecules could produce the organic molecules in what came known as the primordial soup . Their famous experiment, in which they provided energy to a mixture of inorganic molecules with reducing power, is illustrated below.

Miller’s earliest published data indicated the presence of several organic molecules in their ocean flask, including a few familiar metabolic organic acids (lactate, acetate, several amino acids…) as well as several highly reactive aldehydes and nitriles . The latter can interact in spontaneous chemical reactions to form organic compounds. Later analyses further revealed purines, carbohydrates and fatty acids in the flask. Later still, 50 years after Miller’s experiments (and a few years after his death), some un-analyzed sample collection tubes from those early experiments were discovered.

When the contents of these tubes were analyzed with newer, more sensitive detection techniques, they were shown to contain additional organic molecules not originally reported, including 23 amino acids.

Clearly, the thermodynamic and chemical conditions proposed by Oparin and Haldane could support the reductive synthesis of organic molecules. At some point, Oparin and Haldane’s evolving chemistries would have to have been internalized inside of semipermeable aggregates (or boundaries) destined to become cells. Examples of such structures are discussed below. A nutrient-rich primordial soup would likely have favored the genesis of heterotrophic cells that could use environmental nutrients for energy and growth, implying an early evolution of fermentative pathways similar to glycolysis. But, these first cells would quickly consume the nutrients in the soup, quickly ending the earth’s new vitality!

So, one must propose an early evolution of least small populations of cells that could capture free energy from inorganic molecules ( chemoautotrophs ) or even sunlight ( photoautotrophs ). As energy-rich organic nutrients in the ‘soup’ declined, autotrophs (notably photoautotrophs that could split water using solar energy) would be selected. Photoautotrophs would fix CO 2 , reducing it with H - ions from water. Photoautotrophy ( photosynthesis ) would thus replenish carbohydrates and other nutrients in the oceans and add O 2 to the atmosphere.

Oxygen would have been toxic to most cells, but a few already had the ability to survive oxygen. Presumably these spread, evolving into cells that could respire , i.e., use oxygen to burn environmental nutrients. Respiratory metabolism must have followed hard on the heels of the spread of photosynthesis.

Photosynthesis began between 3.5 and 2.5 billion years ago (the Archaean Eon). Eventually, photosynthetic and aerobic cells and organisms achieved a natural balance to become the dominant species in our oxygen-rich world.

The Tidal Pool Scenario for an Origin of Polymers and Replicating Chemistries

In this scenario, prebiotic organic monomers would concentrate in tidal pools in the heat of a primordial day, followed by polymerization by dehydration synthesis. The formation of polymer linkages is an ‘uphill’ reaction requiring free energy. Very high temperatures (the heat of baking ) can link monomers by dehydration synthesis in the laboratory, and may have done so in tidal pool sediments to form random polymers. This scenario further assumes that the dispersal of these polymers from the tidal pools with the ebb and flow of high tides. The tidal pool scenario is illustrated below.

The concentration of putative organic monomers at the bottom of tidal pools may have offered opportunities to catalyze polymerization, even in the absence of very high heat. Many metals (nickel, platinum, silver, even hydrogen) are inorganic catalysts, able to speed up many chemical reactions. The heavier metals were likely to exist in the earth’s crust as well as in the sediments of primordial oceans, as they do today. Such mineral aggregates in soils and clays have been shown to possess catalytic properties. Furthermore, metals (e.g., magnesium, manganese…) are now an integral part of many enzymes, consistent with an origin of biological catalysts in simpler aggregated mineral catalysts in ocean sediments.

Before life, the micro-surfaces of mineral-enriched sediment, if undisturbed, could have been able to catalyze the same or at least similar reactions repeatedly, leading to related sets of polymers. Consider the possibilities for RNA monomers and polymers, based on the assumption that life began in an RNA world. The possibilities are illustrated below.

The result predicted here is the formation not only of RNA polymers (perhaps only short ones at first), but of H-bonded double-stranded RNA molecules that might effectively replicate at each cycle of concentration, polymerization and dispersal. Heat and the free energy released by these same reactions could have supported polymerization, while catalysis would have enhanced the fidelity of RNA replication.

Of course, in the tidal pool scenario, repeated high heat or other physical or chemical attack might also degrade newly formed polymers. But what if some RNA double strands were more resistant to destruction. Such early RNA duplexes would accumulate at the expense of the weaker, more susceptible ones. Only the fittest replicated molecules would be selected and persist in the environment! The environmental accumulation of structurally related, replicable and stable polymers reflects a prebiotic chemical homeostasis (one of those properties of life!)

Overall, this scenario hangs together nicely, and has done so for many decades. However, there are now challenging questions about the premise of a prebiotic reducing environment. Newer evidence points to an earth atmosphere that was not at all reducing, casting doubt on the idea that the first cells on the planet were heterotrophs. Recent proposals posit alternative sources of prebiotic free energy and organic molecules that look quite different from those assumed by Oparin, Haldane, Urey and Miller.

Gerald Bergtrom. Formation of Organic Molecules in an Earthly Reducing Atmosphere. (2021, January 3). Retrieved April 21, 2021, from https://bio.libretexts.org/@go/page/16543

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From soup to cells: The origin of life

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How did life originate?

Living things (even ancient organisms like bacteria) are enormously complex. However, all this complexity did not leap fully-formed from the primordial soup. Instead life almost certainly originated in a series of small steps, each building upon the complexity that evolved previously:

1. Simple organic molecules were formed. Simple organic molecules, similar to the nucleotide shown below, are the building blocks of life and must have been involved in its origin. Experiments suggest that organic molecules could have been synthesized in the atmosphere of early Earth and rained down into the oceans. RNA and DNA molecules — the genetic material for all life — are just long chains of simple nucleotides.

2. Replicating molecules evolved and began to undergo natural selection. All living things reproduce, copying their genetic material and passing it on to their offspring. Thus, the ability to copy the molecules that encode genetic information is a key step in the origin of life — without it, life could not exist. This ability probably first evolved in the form of an RNA self-replicator — an RNA molecule that could copy itself.

Many biologists hypothesize that this step led to an “RNA world” in which RNA did many jobs, storing genetic information, copying itself, and performing basic metabolic functions. Today, these jobs are performed by many different sorts of molecules (DNA, RNA, and proteins , mostly), but in the RNA world, RNA did it all.

Self-replication opened the door for natural selection . Once a self-replicating molecule formed, some variants of these early replicators would have done a better job of copying themselves than others, producing more “offspring.” These super-replicators would have become more common — that is, until one of them was accidentally built in a way that allowed it to be a super-super-replicator — and then, that variant would take over. Through this process of continuous natural selection, small changes in replicating molecules eventually accumulated until a stable, efficient replicating system evolved.

3. Replicating molecules became enclosed within a cell membrane. The evolution of a membrane surrounding the genetic material provided two huge advantages: the products of the genetic material could be kept close by and the internal environment of this proto-cell could be different than the external environment. Cell membranes must have been so advantageous that these encased replicators quickly out-competed “naked” replicators. This breakthrough would have given rise to an organism much like a modern bacterium.

4. Some cells began to evolve modern metabolic processes and out-competed those with older forms of metabolism. Up until this point, life had probably relied on RNA for most jobs (as described in Step 2 above). But everything changed when some cell or group of cells evolved to use different types of molecules for different functions: DNA (which is more stable than RNA) became the genetic material, proteins (which are often more efficient promoters of chemical reactions than RNA) became responsible for basic metabolic reactions in the cell, and RNA was demoted to the role of messenger, carrying information from the DNA to protein-building centers in the cell. Cells incorporating these innovations would have easily out-competed “old-fashioned” cells with RNA-based metabolisms, hailing the end of the RNA world.

5. Multicellularity evolved. As early as two billion years ago, some cells stopped going their separate ways after replicating and evolved specialized functions. They gave rise to Earth’s first lineage of multicellular organisms, such as the 1.2 billion year old fossilized red algae in the photo below.

Where did life originate?

Studying the origin of life

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The Origin of Life

• First Online: 08 June 2021

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• Kenji Ikehara   ORCID: orcid.org/0000-0002-2111-8054 2  

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The riddle of the origin of life remains unsolved irrespective of strenuous efforts by many researchers. What is the major difficulty, which has become the obstacle against solving the origin of life? The reason would be as follows. All kinds of extant organisms on the present Earth are living under the core life system composed of three biopolymers; DNA/RNA carrying genetic information, tRNA realizing genetic code and protein with catalytic function. Nowadays, all the biopolymers with ordered sequence are synthesized under the genetic system, as DNA is synthesized by self-replication, both tRNA and protein are produced by expression of the respective genes. However, all the first three biopolymers, as a matter of course, must be synthesized through random processes, because the sequence of every biopolymer with ordered sequence never be designed in advance. Therefore, the greatest obstacle, which makes it difficult to solve the riddle of the origin of life, would be to explain the way how synthesis of mere polymers with random sequence could be converted to production of biopolymers with ordered sequence during the repeated random processes. For the purpose, several hypotheses including RNA world hypothesis have been proposed by other researchers. However, the origin of life has been unfortunately unsolved still now. Then, first, those hypotheses proposed thus far have been introduced and discussed to clarify the problems encompassed in the hypotheses in the first part of this chapter. Thereafter, I introduce my idea how biopolymers with ordered sequence, as protein, tRNA and gene, could be formed through random processes on the primitive Earth. Recently, I proposed anticodon stem loop hypothesis on the origin of tRNA, suggesting that the first tRNA could be produced through random processes as a small and stable hairpin loop RNA composed of only 17 nucleotides including anticodon in the loop, based on analysis of 5′ anticodon stem sequences of Pseudomonas aeruginosa PAO1 tRNAs. Taking it into consideration together with protein 0 th -order structure for producing an immature water-soluble globular protein by random joining of [GADV]-amino acids and with the first single-stranded RNA created by random joining of GNC anticodons carried by the most primitive tRNAs, it was understood that the “three keys” for formation of the respective three biopolymers with ordered sequence could be obtained. In other words, it has become possible to explain main steps to the emergence of life with the three keys for the first time. The establishment process of the fundamental life system composed of the six members containing three other members or cell structure, metabolism and genetic code, in addition to the three keys or protein, tRNA and gene, is explained to give a solution to the serious problem of the origin of life.

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Ikehara, K. (2021). The Origin of Life. In: Towards Revealing the Origin of Life. Springer, Cham. https://doi.org/10.1007/978-3-030-71087-3_9

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Factoring Origin of Life Hypotheses into the Search for Life in the Solar System and Beyond

1 National Aeronautics and Space Administration Headquarters, Washington, DC 20546, USA

2 Department of Geology, The University of North Carolina, Chapel Hill, NC 27599, USA

Bruce Damer

3 Department of Biomolecular Engineering, University of California, Santa Cruz, CA 95064, USA or

4 Digital Space Research, Boulder Creek, CA 95006, USA

Two widely-cited alternative hypotheses propose geological localities and biochemical mechanisms for life’s origins. The first states that chemical energy available in submarine hydrothermal vents supported the formation of organic compounds and initiated primitive metabolic pathways which became incorporated in the earliest cells; the second proposes that protocells self-assembled from exogenous and geothermally-delivered monomers in freshwater hot springs. These alternative hypotheses are relevant to the fossil record of early life on Earth, and can be factored into the search for life elsewhere in the Solar System. This review summarizes the evidence supporting and challenging these hypotheses, and considers their implications for the search for life on various habitable worlds. It will discuss the relative probability that life could have emerged in environments on early Mars, on the icy moons of Jupiter and Saturn, and also the degree to which prebiotic chemistry could have advanced on Titan. These environments will be compared to ancient and modern terrestrial analogs to assess their habitability and biopreservation potential. Origins of life approaches can guide the biosignature detection strategies of the next generation of planetary science missions, which could in turn advance one or both of the leading alternative abiogenesis hypotheses.

1. Introduction

Over the past five decades, David Deamer has been a leader in origins of life research [ 1 ]. His contributions have encompassed membrane formation, meteoritic organics, and nanopore sequencing, among other topics. Throughout his career, he has also maintained an interest in the rapidly-evolving field of astrobiology. Both of the authors collaborated extensively with Deamer on a landing site proposal for the National Aeronautics and Space Administration’s (NASA’s) Perseverance rover. His significant contributions to the landing site selection process inspired us to further contemplate the value of origins of life research as an integral component of space exploration.

As the works of David Deamer and others demonstrate, cross-disciplinary research between the fields of biochemistry, astrobiology, and planetary geology has the potential to yield exceptional dividends in the search for extraterrestrial life, both within our Solar System and beyond. Testable hypotheses for how prebiotic chemical processes can lead to living microbial communities on a habitable world can guide our thinking about which worlds could harbor life today, and on which worlds life might never arise. While Earth possesses a wide diversity of habitable environments and stable conditions which have persisted for much of its history, most other worlds in the Solar System lack one or more essential elements for a thriving biosphere, such as an atmosphere, a temperature regime narrow enough to support liquid water, a magnetosphere providing protection from solar radiation, or an inventory of carbonaceous compounds. To identify currently- or once-habitable sites with a high degree of confidence, the astrobiology community should consider three guiding questions:

• When was a planetary body most habitable, and how long did these conditions last?
• Where might life have appeared and thrived on this world as it gained and then lost some aspects of habitability?
• What biosignatures should missions search for in each of the above environments?

The goal of this review paper is to consider these three questions from the perspective of origins of life research. The habitable worlds of the Solar System will be viewed from the perspectives of origins of life hypotheses developed by the astrobiology community, primarily origins scenarios centered around hydrothermal vents submerged in oceans or hot spring fields on exposed volcanic landmasses. This article will synthesize new ideas linking planetary science and the search for extraterrestrial biosignatures with the origins of life and the study of ancient and modern terrestrial microbial communities. These ideas will then be used to pose questions and define parameters that could help guide the astrobiology missions of the coming decades.

2. Background and Framing of the Hypotheses

Life is postulated to have arisen in the late Hadean or early Archaean periods, between 4.5 and 3.7 billion years ago [ 1 ]; however, the identity of the environment where abiogenesis took place remains a topic of debate. Submarine hydrothermal vents (e.g., [ 2 ]) and terrestrial hydrothermal fields (e.g., [ 3 ]) present two alternative locations for an origin of life on Earth. Although numerous other hypotheses have been presented, current evidence suggests that only hydrothermal sites can provide the full array of thermodynamic and environmental conditions required for abiogenesis (e.g., [ 4 ]).

While the two hypotheses have often been described as being in competition (an “apples to apples” comparison), they may actually be dissimilar and independent theories (an “apples to oranges” comparison) [ 5 , 6 ]. Each hypothesis concentrates on a different portion of the process required for an origin of life. The following framing statements might help to clarify these differences. The first statement describes where each hypothesis focuses and has achieved some success experimentally, the second statement illustrates where each hypothesis places much of its conjecture, and the third statement delineates a pathway to falsification for each hypothesis.

Framing of the Submarine Hydrothermal Vent Hypothesis:

The current experimental focus of the Submarine Hydrothermal Vent Hypothesis is to utilize energy gradients for the synthesis and metabolic engagement of small organic molecules and monomers, which are precursors to biochemical processes.

The conjectural edge of the Submarine Hydrothermal Vent Hypothesis is that the vent environment can support the continuous synthesis of large populations of monomers, encapsulating them in compartments which can permit the formation of polymers of catalytic length.

The Submarine Hydrothermal Vent Hypothesis can be falsified by determining that it is thermodynamically implausible to fix carbon into sufficient concentrations of key reactants to support further prebiotic reactions utilizing the available compounds and energy in actual vent environments.

Framing of the Hot Spring Hypothesis:

The current experimental focus of the Hot Spring Hypothesis is to demonstrate the self-assembly and evolution through combinatorial selection of protocells: membrane-bounded collections of interacting polymers whose source monomers and their compartments are built up of exogenously-delivered organic compounds.

The conjectural edge of the Hot Spring Hypothesis is that protocell populations undergoing selection within fluctuating pools experiencing wet-dry cycling can select and evolve structural, catalytic, and informational polymers supporting the eventual emergence of living microbial communities.

The Hot Spring Hypothesis can be falsified by showing that, when concentrated in hot spring pools, organic molecules supplied by meteoritic, atmospheric, and geothermal sources cannot form protocells which undergo combinatorial selection of functional polymers.

To a considerable extent, the two hypotheses do not overlap. Both utilize the heat, chemical energy, and thermal and chemical gradients available in hydrothermal environments. However, the vent hypothesis focusses on the synthesis of organic precursors, whereas the hot spring hypothesis focusses on polymer-vesicle protocell self-assembly and selection. The main difference between the two environments is that submarine vents occur in a saline, submerged seafloor setting, while hot springs are comprised of interconnected fluctuating pools exposed to the atmosphere. Chemically and environmentally, the differences between these locations vastly outweigh their similarities; in many ways, they might be considered entirely unrelated settings. The submarine hydrothermal vents which may operate beneath the ice shell of Enceladus are quite distinct from the subaerial hot springs exposed to the early Martian atmosphere.

It may well be that if we transplanted the organic synthesis pathways of submarine vents to a hot spring, they would fail to operate. Similarly, if we attempted to introduce simple polymer-encapsulating membranous protocells to a submarine vent, they would likely be disrupted by the saline environment and shear forces. As hot springs are not present on the icy moons of the giant planets and hydrothermal vents have not yet been found in Mars’ hypothetical northern ocean, the hypotheses are truly site-specific when they are applied beyond Earth. Despite their differences, we hope to show how the future success or failure of each hypothesis carries implications for how and where life can start on any habitable world. We also hope that the reader can keep all of this in mind as we present the models, experimental bases, conjectures, and applications of both hypotheses to the search for biosignatures.

2.1. The Submarine Hydrothermal Vent Origins of Life Hypothesis

The submarine vent origins of life hypothesis hinges on the theory that strong thermal and chemical gradients present near undersea vents can synthesize biologically relevant organic molecules from initial reactants such as carbon dioxide, hydrogen sulfide, and molecular hydrogen. Submarine hydrothermal vents were discovered in 1977, and typically appear along mid-ocean ridges [ 7 ]; two classes of vents have been discovered to date. “Black smoker” vents form when seawater circulating in the subsurface contacts a magma chamber, is heated, and rises through the seafloor [ 8 ]. Dissolved sulfides precipitate from the solution upon contact with low-temperature seawater, gradually building up porous hydrothermal chimneys. The seawater percolating through black smoker vents is typically sulfur-rich and high-temperature (360–400 degrees Celsius). “White smoker” vents form tens of kilometers off-axis from mid-ocean ridges, and vent lower-temperature hydrothermal fluids (40–90 degrees Celsius) [ 9 ]. These solutions are highly alkaline, with pHs between 9 and 11, and they trigger serpentinization reactions when they contact the olivine-rich seafloor. As in black smokers, these reactions gradually build up a porous chimney, in this case made predominantly of carbonate. Black smokers [ 10 , 11 ] and alkaline vents [ 2 ] have been proposed as potential sites for an origin of life. Of the two systems, white smokers are generally considered to present more promising thermal and pH conditions for biochemical reactions [ 12 ].

The first step towards abiogenesis is the accumulation and concentration of organic compounds in a “primordial soup” [ 13 ]. The two most widespread sources of organics on the Hadean Earth were chondritic meteorites [ 14 ] and atmospheric photochemistry [ 15 ]. Carbon compounds from these sources become highly diluted if they fall into the ocean and, therefore, would have been insufficiently abundant to participate in prebiotic chemistry at hydrothermal vents. An alternative in situ source of organics could have been serpentinization reactions at the vents. These provide the reducing power and catalytic minerals needed to synthesize molecules outgassed by submarine vents into organic compounds, particularly the constituent monomers of biologically-relevant polymers [ 16 ]. The sources for these reactions are contained within outgassed hydrothermal fluids and include dissolved carbon dioxide, hydrogen sulfide, nitrogen, and hydrogen, which, together with water, contain five of the six elements essential for biochemistry (carbon, hydrogen, nitrogen, oxygen, and sulfur). The sixth, phosphorus, is supplied by continental weathering, albeit in lower concentrations during the Hadean period [ 17 , 18 ]. Serpentinization reactions can transform these molecules into the basic compounds necessary for prebiotic chemistry, such as nitrous oxide, ammonia, methane, and pyruvate [ 19 , 20 , 21 ].

The reactants produced by serpentinization may be concentrated in pores within the vent chimney [ 2 ]. Each hydrothermal vent contains dozens of pores, which could act as natural experimental chambers for monomer assembly and complexification [ 22 ]. Submarine vent surfaces contain minerals such as sphalerite and pyrite, which can act as catalysts for the accumulation and synthesis of organic compounds out of the molecules produced by serpentinization reactions [ 23 ]. These minerals significantly lower the activation energy required to create moderately complex organics, including amino acids—the primary ingredients of proteins [ 24 , 25 ]. After these building blocks are present, the next stage in any origins of life scenario is the assembly of catalytic and informational polymers out of activated monomers. This is perhaps the largest remaining knowledge gap in the timeline for an origin of life in hydrothermal vents, as condensation reactions are thermodynamically challenging in saline water [ 26 ]. Although several potential polymerization processes have been proposed, the most promising may be the assembly of macromolecules on the surfaces of hydrothermal sediments [ 16 ]. Hydrothermal vent pores are coated in mineral gels, which can act as concentration sinks for monomers [ 27 ]. Despite being submerged in water, silica-rich gels have been proposed to facilitate the concentrating conditions necessary for polymer synthesis [ 28 ]. Condensation reactions in hydrothermal vents remain an active area of research, which will likely see significant developments in the coming years. Once polymers are present in a vent system, the thermal gradient across a pore opening might select for long replicating oligonucleotides [ 29 ]. This process could progressively build the complex polymers required for cellular stability, metabolism, and replication ( Figure 1 ).

( a ) An overview of the submarine hydrothermal vent origins of life hypothesis. The space and time gradients flow from bottom to top. Simple molecules such as carbon dioxide, hydrogen sulfide, and molecular hydrogen bind to pyrite and sphalerite and are assembled into larger monomers via condensation reactions within mineral gels. Circulation through pore openings select for the longest oligonucleotides. These are encapsulated into amphiphilic lipid membranes that self-assemble in basic solutions. As the protocells are transported vertically up the vent chimney, they are stressed by decreasing thermal energy and decreasing pH, which promote the development of increasingly resilient and robust populations. ( b ) Image of an oceanic hydrothermal vent (credit: NOAA). Adapted from Westall et al. [ 30 ]. Reproduced with permission from Mary Ann Liebert, Inc.; New Rochelle, NY.

The formation of protocells requires the encapsulation of polymers in lipid membranes. Until recently, this has been an obstacle for an origin of life in submarine vents. Amphiphilic lipids naturally self-assemble into membranous vesicles in freshwater, but it was thought that high concentrations of ions preclude this process in seawater [ 31 ]. Recent research demonstrates that longer amphiphiles can self-assemble via Fischer–Tropsch synthesis, given the presence of alkaline water and moderately high temperatures [ 32 ]. These conditions are present in the chimneys of “white smoker” hydrothermal vents, so Fischer–Tropsch synthesis could represent a feasible path towards the creation of protocellular membranes in deep-sea hydrothermal systems. Complex polymers concentrated on catalytic surfaces might then be encapsulated within these membranes, forming the first protocells [ 2 ].

A plausible origins of life hypothesis must naturally select for stable protocells which are resilient to environmental variations. The chimney of a hydrothermal vent presents strong thermal, pH, and chemical gradients, as well as shear forces, that can stress protocells as they move from the seafloor towards the open ocean [ 2 , 10 ]. The low-temperature, turbulent, saline waters of the deep ocean might select for protocells with membrane stability proteins that can gradually evolve into a cytoskeleton, and active ion-pumping proteins to remove the potassium and sodium ions which are present in high concentrations in seawater. Early microbes would most likely have been methanogens, reducing and fixing carbon dioxide using the abundant free hydrogen in hydrothermal seawater [ 33 , 34 ]. This process is a precursor to the Wood-Ljungdahl pathway, which reduces carbon dioxide in modern archaea.

The crucial step in the transition from precellular life to early microbes was the development of a hereditary molecule capable of transferring information across multiple generations of cells. A leading hypothesis states that RNA, rather than DNA, was the first self-replicating molecule in early microbes [ 35 , 36 ]. As gene expression in modern cells depends upon the transfer of information from DNA to proteins via an mRNA intermediary, it is plausible that early microbial life in a submarine vent would have first used RNA for information storage. Under certain environmental conditions, laboratory experiments have synthesized the RNA nucleotides adenine, thymine, guanine, and uracil in simulated black smoker hydrothermal vents [ 37 ]. Like other complex polymers [ 29 ], these bases are oligomerized in the presence of catalytic mineral surfaces. Chains of up to four nucleotides have been synthesized under these conditions, which could be evidence for a plausible pathway to RNA formation in submarine vents [ 37 ].

A plausible model for the origins of life must also explain how early microbes attained a widespread distribution. The first protocells would have been restricted to the immediate vicinity of a hydrothermal vent, as they probably would have depended on the vent chimney for thermal energy and redox gradients [ 2 ]. Gradually, they could have developed the ability to pump sodium through cell membranes via electron bifurcation [ 38 ]. Active ion transport would have been a highly advantageous development for protocells, as it would have allowed them to survive indefinitely in the saline environment of the open ocean. This process could have been powered by acetyl phosphate (AcP), an energy storage molecule similar to adenosine triphosphate (ATP) which is readily synthesized in seawater [ 39 ]. An early form of the acetyl-CoA pathway for carbon dioxide reduction would have allowed methanogens and acetogens to produce energy in the open ocean, and spread beyond hydrothermal vent sites [ 40 , 41 ]. One of the merits of the submarine vent origins of life hypothesis is that the seafloor would have been insulated from the frequent cataclysmic impacts of the Late Heavy Bombardment [ 42 ]. Following the conclusion of this period and the development of photosynthesis, microbial life would have been free to colonize the continents.

The first submarine hydrothermal vents were discovered in 1977 [ 7 ]. Since then, hundreds of vents have been found along almost every mid-ocean ridge [ 43 ]; thermal anomalies suggest that many more remain undiscovered [ 12 ]. The best-studied black smoker and white smoker hydrothermal systems are the Faulty Towers Complex [ 8 ] and the Lost City Complex [ 9 ], respectively. Despite the absence of solar energy, both are teeming with diverse forms of life, from methanogenic archaea up through tubeworms. The seafloor underneath the Lost City Complex is similar in composition to the oceanic crust of the Hadean Earth [ 44 ], which distinguish it as a realistic analog for ancient hydrothermal vents. The alkaline water and proton gradients required for an origin of life in marine vents exist naturally in this system. The oldest known marine hydrothermal sediments on Earth are the Nuvvuagittuq Belt Formation in Quebec, Canada, which have been dated to between 3.77 and 4.28 Ga. Dodd et al. [ 45 ] discovered microstructures in these rocks which were interpreted as haematite tubes and filaments similar to those produced by microflora in modern submarine vents. Subsequent experimentation demonstrated that abiotic chemical gardening can also produce similar features [ 46 ]. Confirmed Hadean microfossils would strongly support the submarine vent origins of life hypothesis; however, additional evidence will be required to determine whether or not microbial life was present in the Nuvvuagittuq Belt before the Late Heavy Bombardment.

Although the submarine vent origins of life hypothesis is supported by an emerging foundation of theory and laboratory experimentation, several key issues remain unresolved. One crucial step towards biogenesis is the formation of complex organic polymers. Although these molecules might be elongated and replicated across hydrothermal vent pores [ 29 ], the initial synthesis of catalytic-length polymers from monomers in a porous chimney has not yet been demonstrated. Hydrothermal chimneys could concentrate simple reactants and products such as carbon dioxide and molecular hydrogen in pores, but many of these molecules are lost to the open ocean via circulation through vent openings. Monomers and polymers in the vicinity of hydrothermal vents would be rapidly broken down via hydrolysis, and would be unable to support further reactions [ 31 ]. Therefore, it is unclear whether or not long biologically-relevant polymers can survive long enough to be incorporated into lipid vesicles. Thus far, experimental support for the submarine vent origins of life hypothesis is purely laboratory-based, as no experiments have been conducted in situ at a vent site. While they are productive and controlled, laboratory experiments can occasionally be undermined by unrealistic reagents and chamber-induced artifacts [ 47 ]. Additional studies will be needed to help resolve these issues.

2.2. The Hot Spring Origins of Life Hypothesis

Although much of the past three decades of research on life’s origins has been focused on submarine hydrothermal vents, terrestrial hot springs have recently emerged as a plausible alternative. Mulkidjanian [ 48 ] speculated that cellular life may have begun in anoxic freshwater hot springs rather than in seawater, and that key chemical pathways to ribonucleotides could be facilitated by ultraviolet (UV) radiation exposure in a subaerial landscape. In recent years, Damer and Deamer have synthesized many previous findings with a model of combinatorial selection, which proposes an “end-to-end” pathway from simple, self-assembled protocells through an intermediate stage called the “progenote” to the emergence of the first primitive microbial communities [ 3 , 49 ]. The crux of the hot spring origins of life hypothesis is that cycles of hydration and dehydration in hydrothermal fields could synthesize biologically relevant polymers from monomers delivered to subaerial landscapes on the early Earth, and encapsulate those polymers into membrane-bounded compartments to form protocells. These protocells could act as natural experiments, and could be subjected to a form of combinatorial selection that amplifies populations of functional polymers encapsulated within increasingly robust protocells. According to the hot spring hypothesis, this selection represented the initial step toward Darwinian evolution that ultimately gave rise to much more complex, living cells ( Figure 2 ).

An overview of the hot spring origins of life hypothesis. The space and time gradients flow from top to bottom. Organic molecules synthesized in the atmosphere and contained in chondritic meteorites are collected in hot spring pools located on volcanic landmasses. Wet-dry cycling, depicted in the inset figure, synthesizes these molecules into polymers of increasing length. These polymers are encapsulated in lipid membranes which naturally self-assemble in freshwater. Protocells are stressed and combinatorially selected by conditions of varying pH, shear forces, and temperature. Ultimately, protocells could evolve to withstand conditions beyond the hot spring, develop a form of photosynthesis, and colonize rivers, lakes, and ocean margins. Credit: Damer and Deamer [ 3 ].

Two plausible sources could have provided organic material to feed an origin of life in hot springs. Chondritic asteroids and meteorites produced by accretion, impacts, and photochemical processing within the asteroid belt contain high concentrations of complex organics. The amino acid glycine was detected in Stardust mission samples from the comet Wild-2 [ 50 ], and sugars such as ribose are present in carbon-rich meteorites [ 51 ]. The influx of meteorites and interplanetary dust particles carrying organic compounds would have been thousands of times greater in the Hadean than it is today [ 14 , 52 ], and the organics delivered by these bodies could have survived on the surface of the early Earth for long periods of time due to a lack of atmospheric oxygen. These meteoritic organics falling onto land would be blown or washed into pools, which can concentrate them sufficiently to support chemical reactions. In addition, compounds containing amino acids produce pyrroles when they come in contact with water; these compounds could then be transported to hot spring pools and become oligomerized [ 53 ].

Hot springs and geochemical reactions in the underlying magma are also a potential source of organic compounds. For instance, Archean hot springs concentrated the primary CHNOPS biogenic elements in abundance [ 54 ]. These have the potential to combine into simple organic molecules analogous to those produced in laboratory settings [ 15 , 55 ]. Furthermore, hydrocarbon derivatives, such as long chain monocarboxylic acids and alcohols, can be abiotically produced in hot spring settings by Fischer–Tropsch synthesis [ 56 ]. One caveat to these conclusions is that several promising hot spring experiments (e.g., [ 31 , 55 ]) used water sourced from the Yellowstone National Park hydrothermal system, which is dominated by rhyolitic magmas. This chemistry was most likely absent during the Hadean and Archaean periods [ 57 ]; it remains to be shown that andesitic and basaltic hot springs can synthesize the same organic compounds.

The exterior surfaces of sinter outcrops are ideal sheltered faces where organic molecules can accumulate and undergo diverse reactions as they become concentrated by evaporation [ 54 ]. Mineral surfaces at the edge of hot spring pools, such as silica nodule faces, can support wet-dry cycling and, therefore, become kinetic traps where the rate of condensation of organic molecules exceeds the rate of hydrolysis [ 49 ]. Milshteyn et al. [ 31 ] recently demonstrated that lipid vesicles readily assemble in water samples from the hot springs of Yellowstone National Park. Outcrop faces can also serve as surfaces where amphiphilic lipids can self-assemble into multilamellar membranous structures during wet-dry cycles.

Regular cycling between wet and dry conditions has been proposed as a crucial driving factor in the origin of life, and it occurs ubiquitously in hot springs [ 49 ]. Highly periodic fluctuating water levels are produced by the varying activity of hydrothermal springs, which ensures that mineral surfaces undergo numerous wet-dry cycles. Evaporation concentrates any organic compounds dissolved in the hydrothermal fluids into a thin film which coats mineral surfaces. The chemical energy made available by concentrating and organizing potential reactants promotes condensation reactions. For instance, if nucleotides are present, ester bonds link hydroxyl and phosphate groups into nucleic acid polymers [ 58 , 59 ]. These can be encapsulated in lipid vesicles budding off from the outer layers to form protocells during the rehydration of the pool [ 55 ]. During the interim phase of dehydration, protocells and other pool contents, including solutes, are concentrated in a moist “gel” phase. As the water level recedes, the aggregated protocells fuse together, forming multilamellar structures and depositing, or “coupling,” their cargoes of polymers back into the dry phase, where they can be re-synthesized, elongated, or possibly copied by templating processes.

Just as wet-dry cycles are able to produce populations of protocells through budding from dried films, they also are capable of disrupting them [ 49 ]. Most of the vesicles produced in each cycle would be stressed and decomposed by wet-dry cycles, but a few might happen to contain polymers that stabilize their membranes, much as the cytoskeleton does in cells today. The stabilizing effect was probably the first function of polymers encapsulated in vesicles [ 3 ]. Dehydration might also incorporate additional amphiphilic compounds and polymers into surviving vesicles, which could create increasingly complex and robust protocells. Like the submarine vent hypothesis, the hot spring origins of life hypothesis could support the proposal that RNA was the first hereditary molecule in early microbes [ 3 ]. The synthesis of RNA nucleobases has not yet been demonstrated in hot spring conditions; however, Becker et al. [ 60 ] has proposed a chemical pathway that might produce adenine, thymine, guanine, and uracil during wet-dry cycling. Photochemical reactions driven by high levels of ultraviolet radiation might have incorporated these nucleobases into nucleotides [ 61 ]; radiation might then have linked nucleotides into longer chains of RNA [ 62 ]. Given hereditary material, sets of interacting polymers could then be selected for the expression of primitive proteins and move beyond providing simple stability. Populations of protocells that survive initial wet-dry cycling may accumulate on mineral surfaces during the dehydration of hot spring pools, forming moist aggregates [ 3 ]. Within these aggregates, increasingly concentrated solutes within the evaporating pool volume could enter the protocells and participate in metabolic reactions. These reactions have been theorized to enable the sharing of products across the aggregate, creating an interacting network effect. This process would gradually make the aggregate more robust; it would become capable of growth and evolution as it forms and re-forms during wet-dry cycles. Damer [ 63 ] proposed that this aggregate would constitute a progenote , an ancestor of prokaryotic cells actively developing the relationship between genotype and phenotype [ 64 ]. According to the hot spring hypothesis, the progenote is the key unit of selection and operation which enables the transition from simple protocells to living microbial communities.

A typical Hadean hot spring hosting a population of protocells would have been located on an elevated volcanic island, so a hydrothermal discharge channel could have carried self-assembled protocells and progenote aggregates downhill into increasingly saline bodies of water such as rivers, lakes, and eventually oceans [ 61 ]. The downhill transport of protocells would subject these populations to an adaptation gradient, selecting for cells with stable membranes and the ability to transport nutrient solutes across membrane boundaries. In addition, protocell populations might develop a primitive form of photosynthesis to replace the chemical energy available in the hot spring environment but lacking in more dilute aqueous settings. These adaptations, and the continuous cross-distribution of innovations across landscapes, could create an evolutionary “network effect” which could drive the transition from protocellular pre-life to early microbes. Increasingly robust microbial communities would be able to colonize the sea shores and the bulk of the open ocean, and ultimately would obtain a global distribution.

Like submarine hydrothermal vents, hot springs on Earth host diverse microbial communities, including thermophilic archaea, photosynthetic bacteria, and diatoms (e.g., [ 65 , 66 ]). While hydrothermal fields have a global distribution, three of the most-studied systems are Rotorua in New Zealand, Dallol in Ethiopia, and El Tatio in Chile. Rotorua is one of several geothermal fields located in New Zealand’s Taupo volcanic zone, which gives rise to wide thermal, compositional, and pH variability in hot spring pools [ 67 ]. Deamer et al. [ 55 ] demonstrated the polymerization and encapsulation of RNA monomers during wet-dry cycling at the Hells Gate hydrothermal area in Rotorua. El Tatio and Dallol represent boundary cases for habitability; they are the world’s highest-elevation [ 66 ] and most acidic [ 68 ] hot springs, respectively. All three of these hot springs host thriving microbial ecosystems. The oldest known evidence for microbial communities occupying a hot spring environment is located in the Dresser Formation in the Pilbara, Australia [ 69 ]. The opaline silica outcrops of the Dresser Formation are 3.48 billion years old, and they were originally deposited in a volcanic caldera fed by hydrothermal fluids. This ancient environment possessed the key characteristics of modern hot springs, including fluctuating water levels, compositional variability in pools, and a complete inventory of bioessential elements [ 54 ]. Djokic et al. [ 70 ] detected fossilized spherical gas bubbles within Dresser Formation sinter outcrops, and interpreted them to have been trapped by biofilms produced by archaea. This hypothesis was subsequently confirmed by deep drilling experiments, which discovered preserved biogenic organics within the protected lower layers of the formation [ 71 ]. As of this writing, these biosignatures are among the oldest evidence of life on land. Although the Dresser Formation is the only known Archaean hot spring, hot springs are relatively common features associated with volcanic regions. Therefore, it is conceivable that ancient analogs could have been present on the rare volcanic land masses and islands which may have emerged from the global ocean on the early Earth [ 72 , 73 ].

Although the hot spring origins of life hypothesis can account for processes such as condensation reactions and membrane self-assembly which can be challenging in submarine vents, it has several key limitations of its own. The most serious may be what is known as “the phosphate problem” [ 74 ]. While phosphorylation is an essential biochemical process, phosphorus has a low solubility in water. One potential solution to this quandary could be redox reactions powered by dissolved hydrogen and iron, but the reduction of phosphorus requires concentrations of these elements which are unrealistic for hot springs. Whereas life in a marine vent would most likely generate energy using a primitive form of the Wood–Ljungdahl pathway [ 34 , 41 ], reactions that lead to metabolism in a hot spring have not yet been demonstrated. Clay minerals are common in and near hydrothermal fields, and they may adsorb organic reactants [ 75 ]; however, these reactants may be released in basic fluids. Most hot springs also lack the abundant trace metals found in submarine vents. The Hadean Earth may have been covered by a global ocean [ 76 ]; in this case, hot springs would have been restricted to rare volcanic islands [ 72 , 73 ], significantly limiting the number of locations available for an origin of life on land. Finally, the prospects for surface habitability during the Hadean are uncertain at best due to frequent impacts [ 42 ] and high levels of solar radiation [ 77 ]. An origin of life on land may only have occurred after the cessation of frequent impacts 3.9 billion years ago. As with the submarine vent origins of life hypothesis, future research on the hot spring hypothesis is needed to resolve these obstacles.

2.3. Other Origins of Life Hypotheses

Hydrothermal environments such as marine vents and hot springs are currently favored for an origin of life due to their ability to supply microbes with thermal energy and concentrate prebiotic reactants. However, this review would be incomplete without mentioning other prominent origins of life hypotheses. Darwin [ 78 ] formulated the first origins of life hypothesis, which stated that life began in a “warm little pond” on land. An updated version of this theory predicts an origin of life in carbonate-rich lakes [ 79 ] or in tidal pools on Hadean beaches [ 13 , 80 ], which would concentrate water and organic molecules produced by photochemistry. However, the energy available in these locations may have been insufficient to power the prebiotic reactions necessary for the formation of complex polymers [ 14 ]. Ebisuzaki and Maruyama [ 81 ] proposed that life began in a natural nuclear fission reactor (a geyser powered by an underground source of uranium-235), where ionizing radiation can promote chemical reactions and where wet-dry cycling can also take place. One issue with this scenario is that the geologic record contains evidence of only one natural reactor, which is in Oklo, Gabon. Although additional nuclear geysers may have been active in the Hadean, no evidence for these environments exists. Dobson et al. [ 82 ] suggested that atmospheric aerosols may have been precursors to life, as they concentrate organic material. Finally, some researchers have embraced the theory of Panspermia, which states that microbial life originated on either Mars or an exoplanet and travelled to Earth inside a piece of impact ejecta (e.g., [ 83 , 84 ]). This hypothesis is dependent upon life’s ability to survive an impact event and an interplanetary transit lasting tens of thousands of years. Although several of these hypotheses present intriguing conclusions and merit further research, they do not have the same extensive experimental support shared by the submarine vent and hot spring models. Therefore, they will not be considered in further detail in this paper.

3. Evaluation of Habitable Worlds Using the Hypotheses

Over a dozen planets, dwarf planets, and moons in our Solar System are confirmed or hypothesized to have harbored liquid water at some point in their histories [ 85 ]. Each potentially habitable world presents a unique set of conditions which may or may not be favorable to an origin of life [ 86 , 87 , 88 ]; these conditions will be described and assessed in this section.

Although Mars has lacked permanent bodies of liquid water for the past three billion years, its early history resembled that of the Earth [ 89 ]. Deuterium-hydrogen ratios in carbonaceous chondrites suggest that water was delivered to both Earth and Mars early in their respective histories, so Mars could have been habitable as early as 4.6 billion years ago [ 90 ]. Widespread deposits of hydrated minerals such as phyllosilicates, carbonates, sulfates, and chlorides formed during the Noachian period (4.6–3.7 billion years ago) in at least 10 classes of aqueous environments [ 91 ]. Three habitable ancient environments have been ground-truthed and characterized by the Curiosity, Opportunity, and Spirit rovers from this timeframe [ 92 , 93 , 94 ]. During the Hesperian period, Mars gradually left surface habitability; the atmosphere was stripped by the solar wind [ 95 ], water on the surface became acidic [ 89 ], and catastrophic deluges carved transient lakes and rivers [ 96 , 97 ]. By the end of the Hesperian, the surface of Mars was no longer permanently habitable. Therefore, an origin of life on Mars would most likely have occurred between the end of the Late Heavy Bombardment (3.8 Ga) and the end of the Hesperian period (3.0 Ga).

Noachian Mars met all of the conditions required for an origin of life in marine and terrestrial hydrothermal systems ( Figure 3 ). Marine hydrothermal sediments have been detected on Mars, albeit in one location to date. The Eridania region is an interconnected network of five deep basins, which held an ancient sea 3.8 billion years ago during the late Noachian [ 98 ]. The floors of these depressions are covered in massive blocks of chaos terrain around 400 m tall, which contain numerous alteration minerals including phyllosilicates, carbonates, serpentine, and talc. Michalski et al. [ 98 ] concluded that these sediments were most likely produced in a deep-sea hydrothermal system. As each basin’s hydrothermal sediments cover a roughly circular area 100 km in diameter, the Eridania sea would have provided a vast expanse of catalytic surfaces and energy sources to support prebiotic reactions. In addition to seas such as Eridania, Mars may have held a large ocean in its northern hemisphere during the early Hesperian period [ 99 ]. Isotopic analysis lends support to this theory [ 100 ], but no deep-sea hydrothermal sediments have been detected in Vastitas Borealis as of this writing.

Candidate hydrothermal systems on Mars. ( a ) Mars Reconnaissance Orbiter (MRO) image of submarine hydrothermal sediments in the Eridania Basin [ 98 ]. ( b ) Mars Exploration Rover (MER) Spirit image of nodular digitate structures in the Columbia Hills, which have been interpreted as silica sinter deposits [ 94 ]. ( c ) Perspective view of Nili Tholus generated using MRO stereo images [ 106 ]. The light-toned outcrops surrounding the volcanic cone are composed of amorphous silica. ( d ) Hydrated silica deposits in the Jezero Crater delta, which were most likely sourced from the NE Syrtis Major volcanic region [ 107 ]. All images are credit of NASA.

Candidate Noachian and Hesperian hot springs have been discovered in multiple locations on Mars. The best-studied of these systems is located in the Columbia Hills in Gusev Crater, and it was discovered by the Mars Exploration Rover (MER) Spirit in 2007 [ 101 ]. The rover found nodular clasts, which are covered in digitate structures and composed of 85% opaline silica, adjacent to a volcanic tephra deposit named Home Plate. Gertrude Weise, a patch of soil containing more than 90% Opal A, was exhumed adjacent to these rocks. The particular mineral assemblages found by the rover could have formed in either silica-precipitating hot springs or volcanic fumaroles [ 102 ]. Several subsequent findings, including a stratiform distribution of silica and the inability of sandblasting to produce digitate structures, provide strong support for the hot spring interpretation [ 94 ]. MER Spirit remotely detected silica at Pioneer Mound, a one-meter-tall structure northwest of Home Plate. Pioneer Mound has a profile which resembles extinct hot spring mounds such as those at Puchuldiza, Chile [ 94 ]. If this is indeed the origin of Pioneer Mound, it could have powered wet-dry cycling conducive to an origin of life in a hot spring. Lithostratigraphic analysis indicates that the candidate silica sinter in the Columbia Hills forms a discontinuous layer sandwiched between two volcanic units [ 94 ]. Therefore, the deposits are most likely late Noachian or early Hesperian in age. Analyses of archival MER data, coupled with the exploration of analog environments, has suggested that the centimeter-scale digitate structures which protrude out of the nodular clasts could be biomediated microstromatolites [ 103 ]. Due to their high biosignature preservation potential, these silica deposits have been identified as high-value targets for future exploration [ 104 , 105 ].

In addition to the ground-truthed ancient hydrothermal system in the Columbia Hills, multiple candidate Martian hot springs have been observed from orbit. Perhaps the most convincing example is located in Nili Patera, a late Hesperian volcanic caldera [ 108 ]. Multiple opaline silica deposits have been documented on the flanks and in the surroundings of Nili Tholus, a 520-meter volcanic cone within the main caldera [ 106 ]. Unlike the silica cobbles at Home Plate, the Nili Patera silica deposits are amorphous rather than opaline. However, these minerals could have easily been dehydrated through diagenetic alteration by the Martian atmosphere or by surficial iron. The formation of hydrated silica requires aqueous weathering, and it most often occurs in hydrothermal systems. Given their former volcanic setting, the deposits in Nili Patera are most likely products of hot springs or fumaroles [ 106 ]. The low sulfur abundance of the Syrtis Major region suggests the former, but the latter remains a non-trivial possibility in the absence of fine-scale mineralogy data. A second occurrence of candidate hydrothermal silica has been detected in southwest Melas Chasma, Vallis Marineris [ 109 ]. The silica occurs in mounds 100–200 m in diameter, which fill low-lying depressions within the extensive regional lakebeds. It is difficult to explain their distribution, as hydrothermal and volcanic features are rare throughout Vallis Marineris [ 110 ]. The hydrothermal origins hypothesis for the Melas Chasma silica has not yet explained how lava percolated to the surface in this select location. The silica deposits in Nili Patera and SW Melas Chasma have both been dated to the late Hesperian period [ 106 , 109 ], when Mars was transitioning out of surface habitability; however, conditions could potentially have supported hardy microbes. Finally, hydrated silica has been detected in the Jezero Crater delta, near the landing site for the Mars 2020 rover [ 107 ]. It was likely sourced from one of the silica deposits in the surrounding Northeast Syrtis Major volcanic region. As it was discovered recently, further research will be necessary to determine whether or not this silica was produced by hydrothermal processes. Given the large number of former volcanic complexes on Mars, other similar hydrothermal sites could await detection.

Although organics-rich solutions in large, stable Noachian lakes may be too dilute to promote the rapid abiotic reactions necessary for an origin of life, they remain subjects of interest to the astrobiology community. Paleolakes formed relatively early on both Earth and Mars, and they present thermally and chemically stable environments in which life can persist for long periods of time [ 111 ]. Terrestrial lacustrine environments concentrate decaying organic matter on their floors, which increases the likelihood that Martian lakebeds could preserve Noachian biosignatures [ 112 ]. For these reasons, paleolakes have been prioritized as targets for multiple Mars rovers [ 111 ]. Mars Exploration Rover Spirit explored Gusev Crater [ 113 ], Mars Science Laboratory Curiosity explored Gale Crater [ 114 ], and Mars 2020 Perseverance will explore Jezero Crater [ 105 ]. However, for lakes and seas to be relevant in the search for biosignatures, life must have spread there from the hydrothermal site where it originated. The likelihood of microbial transport taking place depended on the climate of Noachian Mars. Early Mars may have been “warm and wet,” with frequent precipitation and clement surface temperatures [ 115 ]. This hypothesis is supported by geomorphologic features indicative of an extensive Martian hydrosphere. If it is correct, microbes that developed the ability to thrive beyond their host hydrothermal system could have been distributed across hundreds of kilometers by runoff, impact events, groundwater flow, and/or atmospheric transport [ 116 ]. Under a warm climate, lacustrine systems could have been promising sites for microbial life and biosignature preservation. Alternatively, multiple climate models suggest that Noachian Mars was “cold and icy,” with a contiguous ice sheet covering the southern highlands [ 117 , 118 ]. Southern hemisphere seas such as the Eridania Basin could have been covered by this ice sheet, isolating deep-sea hydrothermal systems from the rest of the planet [ 98 ]. Microbial transport from hot springs to lakes would also be challenging under such conditions, as precipitation would be infrequent and the areas beyond the hydrothermal site would be parched and irradiated [ 119 ].

The surface of Venus is inhospitable to life as we know it, with a constant surface temperature of 750 degrees Kelvin and an atmospheric pressure of 9.3 × 10 6 pascals. The leading theory on Venus’ climatic history is that it left surface habitability early in its history during a runaway greenhouse period [ 120 , 121 , 122 ]. However, some recent climate models suggest that Venus may have harbored liquid water on its surface for up to three billion years after the formation of the Solar System [ 123 , 124 ]. In these scenarios, the onset of a strong greenhouse effect was delayed by Venus’ slow rotational period [ 123 ]. Like Earth, Venus has highlands and lowlands; these could be potential analogs to submarine and continental crust [ 125 ]. Way et al. [ 123 ] predicted that 60% of the surface of Venus was once covered by a liquid water ocean 300 m deep. Venus also has an extensive volcanic rock record, including some evidence for modern activity [ 126 ]. As Venus is similar in mass, diameter, and composition to Earth, it is reasonable to propose that both planets had similar levels of volcanic activity during the early history of the Solar System. In fact, surface water and volcanic activity could have persisted longer on Venus than they did on Mars. If extensive oceans and volcanic provinces were present on ancient Venus, the planet could have supported an origin of life in submarine hydrothermal vents. Likewise, life could have originated in hot springs located on highstanding continental crust. However, current radar and spectroscopic datasets are insufficient to detect evidence of any ancient Venusian hydrothermal systems.

3.3. Ocean Worlds

One of the most surprising discoveries of NASA’s planetary science program is the sheer abundance of water in the outer Solar System [ 127 ]. Subsurface oceans have been confirmed on Jupiter’s moons Europa and Ganymede and on Saturn’s moons Enceladus and Titan. Over a dozen candidate ocean worlds orbit Jupiter, Saturn, Uranus, and Neptune, or independently orbit the Sun as dwarf planets ( Figure 4 ) [ 85 ]. Europa and Enceladus in particular have been denoted as astrobiologically-relevant environments. Europa’s surface is crisscrossed by hundreds of intersecting linear ridges, which likely formed when its thin water-ice crust was fractured by tidal forces. The uneven distribution of these features can only be explained by a liquid water ocean separating its crust from its core [ 128 ]. Volumetric calculations suggest that Europa’s global ocean contains twice as much water as all of Earth’s oceans combined [ 129 ]. Enceladus’ south polar region features hundreds of plumes which vent out water-ice crystals; these jets are also fed by a global subsurface ocean [ 130 , 131 ].

Distinctive features of moons and dwarf planets in the outer Solar System with potential subsurface oceans. ( a ) Europa: linear ridges are created as water upwells through tidal fractures in the moon’s icy crust. ( b ) Enceladus: geysers eject large quantities of saline water into space. ( c ) Ganymede: high-albedo, grooved regions have experienced recent tectonic activity. ( d ) Triton: dark deposits on the south polar cap are emplaced by eruptions of sublimating nitrogen ice. ( e ) Ceres: salt mounds form when water percolates to the surface and evaporates. ( f ) Pluto: Sputnik Planum, a basin filled with nitrogen ice, is aligned with the dwarf planet’s tidal axis. All images are credit of NASA.

To produce a deep-sea hydrothermal system, a heated planetary core or mantle must be in contact with an ocean [ 132 ]. Many large outer Solar System moons, such as Jupiter’s Callisto, have oceans sandwiched between two layers of ice [ 127 ]. These moons therefore lack traditional submarine vents, although hydrothermal fluids might still percolate into their oceans via magma plume-generated conduits [ 133 ]. The oceans of Europa and Enceladus, in contrast, are likely in contact with cores heated by tidal forces [ 134 ]. Indirect evidence suggests that hydrothermal vents on both worlds are actively producing the compounds necessary for microbial life. Sodium chloride on the surface of Europa could have been produced in submarine vents [ 135 ], while molecular hydrogen detected in the plumes of Enceladus was produced by inferred marine hydrothermal systems [ 136 ]. Given the presence of such ecosystems, the oceans of Europa, Enceladus, and other ocean worlds could be conducive to an origin of life in hydrothermal vents. Marine hydrothermal systems in the outer Solar System could be broadly similar to those on Earth; compositional analyses of surface of Europa and the plumes of Enceladus suggest that the seawater on both moons carries the same salts. Giant planets provide a ready source of heat for their moons in the form of tidal forces. Therefore, stable conditions in the oceans of Europa and Enceladus could have persisted for hundreds of millions, if not billions, of years. If polymers can be synthesized in the chimneys of hydrothermal vents, as the submarine vent hypothesis states, then microbial life could be thriving on the seafloors of multiple ocean worlds.

However, if life can only begin on land in hot springs, the ocean worlds of the outer Solar System might be habitable but lifeless [ 137 ]. Hydrothermal systems on ocean worlds are constantly immersed in water, and therefore must take the form of submarine vents without wet-dry cycling. Unlike hot springs or submarine vents on Earth, a hydrothermal system deep within the ocean of Europa would be completely isolated from meteoritic infall. Therefore, it would need to continuously form the necessary organic building blocks of life from simpler starting reactants such as CO 2 . Phosphorus availability would also be significantly reduced on ocean worlds [ 138 ]. On Earth, phosphorus enters the hydrosphere through the fluvial erosion of continental crust; runoff makes this key element for biochemistry available to both submarine vents and hot springs. Without landmasses to supply them, hydrothermal vents on Europa or Enceladus might have access to several orders of magnitude less phosphorus than similar systems on Earth. Enceladus has a moderately saline ocean with dissolved sodium, chlorine, and carbonate ions [ 139 , 140 ], which could potentially inhibit the assembly of protocells if bilayer lipid membranes are unable to self-assemble in seawater. Finally, it is important to note that the high ocean floor pressures on the planet-sized moons Europa, Ganymede, and Titan could inhibit faulting, which is a prerequisite for the formation of hydrothermal systems [ 141 ]. While Mars could have supported an origin of life in submarine vents or hot springs, the prospects for life on ocean worlds are highly dependent on which hypothesis proves to be correct.

Titan is the only planetary body in the Solar System besides the Earth with large bodies of liquid on its surface [ 142 ]. Its polar regions feature lakes of liquid methane and ethane which are over 100 m deep and cover a total surface area of 1.6 million square kilometers. The surface temperature of Titan is approximately equal to the triple point of methane, and its lakes are sustained by a complex hydrologic cycle involving all three phases of matter [ 143 ]. Seasonal precipitation of liquid methane in the equatorial regions sculpts canyons which empty into low-lying areas [ 144 ]. The longest of these canyons, Vid Flumina, is 570 m deep and 412 km long. In addition to its methane hydrosphere, Titan also has a subsurface ocean of liquid water [ 145 ], which could facilitate an origin of life in deep-sea hydrothermal vents (see Section 3.3 ).

Life on the surface of Titan would depend on methane-based cryochemistry rather than liquid water [ 146 ]. Therefore, any discussion of life on Titan must first consider whether or not complex organic compounds can form cellular components under such conditions. Laboratory experiments have identified azotosomes, nitrogen-based equivalents of lipids, as potential components of membranes in a cryogenic environment such as the surface of Titan [ 147 ]. The organelles and structural components of cells on Earth are composed of proteins, and one can imagine that various carbon and nitrogen compounds could fulfill these functions on Titan [ 146 ]. Multiple hereditary molecules have been create under controlled conditions, so four-base DNA is not a prerequisite for the transmission of genetic information [ 148 ]. The ingredients for a methane-based biochemistry are certainly present on Titan, although they have only been predicted in theory rather than proven through experimentation [ 146 ].

The fluctuating hot springs proposed for an origin of life on land have no direct equivalent on Titan due to the moon’s frigid climate and thick crust of ice. It is notable that several regions near Titan’s equator display a bright absorption feature at 5 microns [ 149 ], which could be indicative of methane-based evaporitic deposits. These same regions are inundated by methane floods during the spring season on Titan [ 150 ]. One mechanism for the formation of these sediments is that they could accumulate gradually through the evaporation of ephemeral equatorial lakes. Although the cyclic dissipation of these lakes could conceivably concentrate complex organics and encourage protocell self-assembly, the extremely low temperatures and lack of activation energy make it doubtful that a methane-based biochemistry on Titan could be established.

Multiple cryovolcanic complexes have been identified with Cassini radar data [ 151 ], and it is therefore possible that Titan could harbor cryogenic hot springs in its volcanic regions ( Figure 5 ). A fluctuating water level due to varying rates of cryovolcanic activity could enable wet-dry cycling in these low-temperature geothermal springs. Cryogenic hydrothermal fields on Titan would have access to abundant complex organics synthesized by photochemistry in the moon’s atmosphere [ 152 ]. One obstacle facing an origin of life in these environments is the salinity of the water. As the cryovolcanoes on Titan are most likely fed by a subsurface ocean [ 151 ], any water would be highly saline, similar to the water underneath the crust of Europa. Without freshwater or catalytic structures such as the pores in hydrothermal vent chimneys, it is unclear whether complex polymers or bilayered lipid membranes could be synthesized.

Potential sites for an origin of life on Titan. ( a ) Evaporites in Atacama Lacuna display a strong 5-micron absorption feature in a Cassini Visible/Near Infrared Mapping Spectrometer image. ( b ) Cassini radar image of Selk Crater, the landing site for the National Aeronautics and Space Administration’s (NASA) Dragonfly mission. ( c ) Cassini digital elevation model of Sotra Facula, a putative cryovolcano in Titan’s equatorial region. All images are credit of NASA.

3.5. Exoplanets

To date, more than 4000 planets have been discovered orbiting stars other than the Sun [ 153 ]. The majority of these were detected by the Kepler Space Telescope, which confirmed 2327 exoplanets—including about 30 potentially habitable worlds [ 154 ]. These numbers are likely to rise precipitously with the advent of next-generation instruments such as the Transiting Exoplanet Survey Satellite (TESS) [ 153 ]. The James Webb Space Telescope (JWST) will have the required sensitivity to detect trace gases and biosignatures, including methane, water vapor, carbon dioxide, and molecular oxygen, in planetary atmospheres [ 155 ]; the Hubble Space Telescope has already discovered water vapor in the atmosphere of a super-earth-sized planet in its star’s habitable zone [ 156 ]. However, creating a complete atmospheric profile for Proxima Centauri b, the nearest potentially habitable exoplanet, will take upwards of 60 hours [ 157 ]; as JWST observing time will be distributed among multiple scientific disciplines, the telescope will only be able to analyze a small subset of exoplanet atmospheres. Origins of life hypotheses can therefore be employed to optimize the use of JWST observing time by predicting which exoplanets are most likely to be inhabited based on their properties.

If a habitable zone exoplanet has a substantial atmosphere, moderate temperatures and pressures will allow liquid water to persist on its surface. However, the amount of water can vary based on the planet’s distance from its star at formation. Terrestrial exoplanets that accrete in their stars’ habitable zones most likely form with continents and oceans, similar to those of Earth, Mars, and Venus. Earth-sized planets hypothesized to have landmasses include Kepler 458b [ 158 ] and Kepler 62f [ 159 ]. However, planets that form beyond the outer edge of the habitable zone would contain large quantities of water ice; if they migrate inwards, this ice would melt into a global ocean of liquid water [ 160 , 161 ]. Infrared observations reveal that the three habitable planets of the TRAPPIST-1 system, for example, contain up to 5% water by mass ( Figure 6 ) [ 162 ]. In contrast, the Earth contains 0.05% water by mass.

Radius vs. mass plot for Earth, Venus, and the TRAPPIST-1 exoplanets. Both variables are measured relative to Earth. The black curve represents the composition of a planet composed purely of silicates, such as Venus, while the blue curves represent idealized compositions of planets with various water mass fractions. The potentially habitable TRAPPIST-1 exoplanets (d, e, and f) have significantly higher water mass fractions than the Earth. Figure adapted from Grimm et al. [ 162 ]. Reproduced with permission from the author.

Exoplanets with Earth-like water mass fractions could be conducive to an origin of life in either submarine hydrothermal vents or terrestrial hydrothermal fields. Environmental considerations for these planets are similar to those described for Mars and Venus in Section 3.1 and Section 3.2 . However, there are no analogs for ocean planets in our Solar System, so a discussion of these worlds is warranted here. As they do not have solid surfaces, ocean planets cannot support an origin of life in hot springs. Therefore, microbial life on these worlds must either start in submarine vents, or in tide pools on circumpolar ice caps [ 163 ]. Tide pools seem to lack the energy required to catalyze polymerization reactions [ 14 ], and it is unclear whether or not the oceans of these planets are in contact with their mantles–a prerequisite for the presence of marine hydrothermal systems. On an ocean planet such as TRAPPIST-1e with 5% water by mass, the immense pressure at depth will turn the water above the mantle into ice [ 164 ]. This ice layer may separate seawater from thermal sources, preventing the formation of submarine hydrothermal vents. Like on Europa and Enceladus, phosphorus availability on ocean planets is also a concern [ 165 ]. Other than the erosion of continental crust, which is not present on ocean planets, there is no known method to produce the large amounts of phosphorus needed for biochemistry. An anticipated lack of hydrothermal systems and phosphorus on ocean planets suggests that exoplanets with moderate surface water fractions are most favorable for an origin of life.

4. Discussion of Parameters that Influence Astrobiology

Several planets, moons, and exoplanets possess the environmental conditions required for an origin of life in either submarine hydrothermal vents or terrestrial hydrothermal fields. Given these conditions, there are several supplementary questions which impact the detectability of biosignatures on Mars and other habitable worlds. This speculative section will discuss potential biosignatures which could be discovered in a habitable planet’s fossil record.

4.1. Hydrothermal Systems as First and Last Outposts for Life on Mars

Mars’ surface most likely transitioned out of habitability at some point in the Noachian or the Hesperian; however, the absolute age of this transition out of surface habitability remains uncertain. The MSL Curiosity rover is exploring Mount Sharp in Gale Crater, a 5.5-km-tall central peak with rock layers recording much of Mars’ history. Lacustrine mudstones on the crater floor were dated to between 3.86 and 4.56 billion years in age [ 166 ]. Between 3.3 and 3.7 billion years, the geologic record transitions to phyllosilicate-bearing rocks altered episodically by brine [ 167 ]. This implies that, in at least one location, the surface of Mars supported liquid water after signs of life on land and in the oceans appeared in Earth’s fossil record [ 45 , 70 ].

Therefore, a hypothetical origin of life on Mars could have occurred in roughly the same half-billion-year period as it did on Earth, in the “first outposts” of hydrothermal system such as Home Plate, Nili Tholus, and the Eridania Basin. Unlike lakes and river systems, hydrothermal sites require only small amounts of water and a subsurface heat source to remain active. Models predict that conditions supporting the presence of surface hot springs persisted through the conclusion of the Hesperian [ 168 ]. However, with the loss of the primordial atmosphere and the cessation of the majority of volcanic activity, these hot spring systems would eventually have become inactive. To exist on Mars today, microbes would need to migrate from a surface hydrothermal system into subsurface refugia. On Earth, hot springs and submarine vents are underlain by a system of plumbing which connects them to groundwater and magma sources (e.g., [ 7 , 169 ]). Ancient Martian microbes could have theoretically settled in the subterranean chambers of such a hydrothermal plumbing system. As ancient geothermal fields ceased activity, their waters sublimating into the thinning atmosphere, microbial life could have continued to thrive in the upper levels of the hydrothermal plumbing, protected from the increasingly sterilizing conditions at the surface. These microbes would have been halophilic, as water would have become increasingly salty as they migrated deeper within Mars’ crust to reach warm, liquid regimes as the core cooled [ 116 , 170 ]. Similar to rock-dwelling life on Earth today, these colonies would have been be chemolithotrophs rather than phototrophs, and would have been able to persist in wet, rocky environments with scarce energy resources [ 171 ].

Radar observations by Mars Express have detected at least one subsurface lake 1.5 km below the planet’s surface [ 172 ]. Although these bodies of water are highly saline, some halophilic chemolithotrophs are able to tolerate similar conditions on Earth [ 173 ]. If life on Mars began in the “first outposts” of hydrothermal systems, it may still persist today in these subsurface lakes. Subsurface drilling missions searching for remnants of the Noachian biosphere might consider targeting bodies of water located beneath or near ancient hot springs and submarine vents, as these locations could have been inoculated by surficial colonies.

Hydrothermal systems on Mars would have been most massive and hospitable during the Noachian and the Hesperian due to the large quantities of water on the surface and the large amounts of internal heat radiating out from the core. However, they could have easily experienced episodic upwelling into the Amazonian, serving as “last outposts” for life able to survive on the otherwise uninhabitable surface of Mars. Terrestrial hydrothermal systems are often centered on a magma source for only a few thousand years due to continental drift. This does not apply to a planet such as Mars with low levels of tectonic activity [ 174 ]; a hot spring or hydrothermal vent would have remained stationary over its heat source for the duration of the life of the magma chamber. Even terrestrial hot springs far removed from active magma plumes occasionally re-erupt and create temporary pools of water as trace amounts of magma and water seep to the surface. This is analogous to conditions on Amazonian Mars, where the planetary core was solidifying beneath hotspot plumes with no accompanying tectonic activity. It is a reasonable conjecture that periodic eruptions of hydrothermal systems could have carried organic material and entire microbial communities from subsurface lakes to localities on the surface. Hydrothermal vents and fields on Earth have excellent biosignature preservation potential. It therefore follows that the most recent and most accessible chemical biosignatures, morphological textures indicative of stromatolites, and microfossils may be found at ancient hydrothermal sites such as Home Plate. More recent silica deposits such as those in Nili Patera and Melas Chasma might also be ideal targets for in situ investigation.

If microbes colonized the Martian subsurface via a system of hydrothermal plumbing, they could have also been occasionally carried to the surface by deluges. While some Martian channels formed gradually through steady fluvial erosion, others were carved by short, catastrophic releases of water triggered by volcanism [ 96 , 97 ]. These flooding events began in the Late Hesperian and continued through the early Amazonian; they were the final known appearances of large quantities of water on the surface of Mars [ 175 ]. Temporary paleolakes were created in at least two dozen locations by flooding [ 97 ]. These lakes could have been temporary refugia for halophilic chemotrophs. Jezero Crater, the landing site for NASA’s Mars 2020 rover, is one of the short-lived Martian paleolakes formed by deluges [ 97 ]. In such an environment, the rover could potentially discover microfossils preserved in deltaic sediments. However, due to the temporary nature of the lake, organics and other biosignatures will most likely be present in trace quantities; finding them might require an intensive search.

4.2. Photosynthesis and Other Energy Sources for Microbial Life

Phototrophs comprise a majority of Earth’s biomass and energy production [ 176 ]. The ability to store light energy in carbohydrates enabled photosynthetic bacteria to colonize the majority of Earth’s surface, and it could have led to a similar distribution of life on Mars and/or Venus. Genetic sequencing suggests that complex microbial communities capable of photosynthesis evolved as early as 3.4 billion years ago [ 177 ]. If the development of photosynthesis proceeded at similar paces on Mars and Earth, primitive phototrophs could have conceivably developed before Mars left surface habitability. These microorganisms could have survived through the conclusion of the Hesperian; however, as atmospheric pressure decreased, water levels receded, and radiation flux increased, they would have been unable to retreat underground and most likely would have gone extinct [ 178 ]. However, hypothetical Martian phototrophs could have created stromatolites before going extinct; the prospects for the detection of such biosignatures will be discussed in the next section. On the other hand, photosynthesis would most likely have developed on Venus if its hydrosphere persisted for the 2–3 billion years predicted by some climate models. As the solar energy flux on this planet is 1.9 times that of Earth, photoautrophy would be an advantageous adaptation for hypothetical Venusian microbes.

The distribution of Martian biosignatures may be dependent on the development of photoautrophy. On Earth, chemosynthetic bacteria and archaea are not as widespread as their photosynthetic counterparts [ 176 ]. Photosynthetic life during the Hesperian may have been able to colonize much of the planet’s surface [ 178 ]. Without photosynthesis, however, life is dependent on localized environments such as hydrothermal systems for energy. As hydrothermal systems ceased activity, so too would their microbial colonies. Although a purely chemosynthetic Martian biosphere would still produce microfossils and biogenic organics, they would most likely be concentrated around hydrothermal systems and rare compared to those produced by phototrophs.

Inadequate sources of energy may also pose a challenge to life in the oceans of Europa and Enceladus [ 179 ]. Submarine hydrothermal vents could produce a constant supply of thermal energy for thermophilic microbes. Although the net energy output of a submarine vent is 8–9 orders of magnitude lower than that of sunlight, it is sufficient to support small communities of chemotrophs [ 180 ]. The open ocean, however, has an extremely low energy density. Microbes on Europa cannot be photosynthetic, as light does not penetrate the moon’s icy crust. Therefore, life on ocean worlds may only persist near the seafloor, rather than in the uppermost and most accessible levels of their oceans.

4.3. Stromatolites and Other Advanced Biosignatures

A last piece of conjecture centers around the question of what forms of life we might expect to find on a dying planet. One common assumption is that any Martian microbial community would have produced stromatolites (e.g., [ 111 ]). Stromatolitic structures are created by the growth of communities of phototrophs, which form layered mats of microbial refuse [ 181 ]. They first appeared 3.5 billion years ago during the Archaean [ 182 ], and likely peaked in abundance and diversity about 1.25 billion years ago [ 183 ]. Terrestrial hot springs, such as those in the Pilbara, typically preserve fine-scale stromatolitic textures. One proposed interpretation of an MSL Curiosity image of a mudstone in Gale Crater is that the surface of the rock is covered by a microbial mat [ 184 ]. However, the presence of stromatolites on Mars is dependent on a specific set of cellular functions which may never have evolved while the planet was habitable [ 185 ].

Photosynthesis and the ability to leave stromatolites in the rock record imply advanced cellular machinery [ 186 ]. This would necessarily include a genetic code supporting a protein translation system, featuring ribosome-like organelles. The development of these systems may require evolutionary selection and recombination over tens to hundreds of millions of years across an extensive landscape of watery environments. On the Earth, early life would have been able to expand its range rapidly, actively colonizing new environments in the open oceans, land, and the marine shore [ 49 ]. The subsurface would also expand the extant microbial biosphere. Therefore, life on Earth would have the benefit of a vast and increasing “combinatorial volume” which would support the exploration of many evolutionary pathways [ 187 ].

Mars had rapidly-declining habitability at its surface due to the cessation of its hydrologic cycle and the evaporation of all standing bodies of water. Life might not have had enough combinatorial “runway” to develop the complex and robust machinery required to produce stromatolitic textures. Life discovered in the subsurface of Mars might resemble an early form of chemolithotrophs, the majority being autotrophic and some existing as heterotrophs living off the organic material produced by autotrophs [ 116 , 170 ]. Would such microbial communities leave behind direct fossil evidence of stromatolites, or be robust enough to leave traces of individuals preserved as microfossils? Given the complexity of stromatolites, Martian microbes may not have been capable of such producing such biosignatures. This implies that surface rocks may contain the chemical signatures of life, but limited fossil evidence of microbial communities.

4.4. Testing Origins of Life Hypotheses Using Planetary Exploration

Just as origins of life hypotheses could be used to guide the missions of the next decade, planetary exploration could shed light on whether life began in hot springs, submarine hydrothermal vents, or a different environment altogether. Making such a determination is nearly impossible using the terrestrial point of reference alone, since Earth’s Hadean rock record has been almost entirely destroyed by tectonic and aqueous activity. A search for biosignatures on Europa or Enceladus may be particularly valuable for downselecting between origins of life hypotheses. These moons most likely have marine hydrothermal vents, but lack hot springs and other proposed environments for biogenesis. If their oceans are found to be habitable but lifeless after billions of years of hydrothermal activity, it would strongly suggest that an origin of life in submarine vents is unlikely [ 137 ], or dependent on elements such as phosphorus not present on ocean worlds [ 138 ]. If this knowledge was coupled with convincing evidence of past life in an environment such as Home Plate on Mars, it could indicate that life required subaerial hot springs or similar environments to get started. If, on the other hand, convincing biosignatures were found on Enceladus or Europa, they would probably provide convincing support for an origin of life near submarine vents.

Although few, if any, Hadean rocks survive today, a record of Earth’s early history could be preserved on the Moon [ 188 , 189 ]. During the Late Heavy Bombardment (4.1–3.8 Ga), hundreds of kilometer-scale asteroids impacted the Earth’s surface; each impact would have ejected numerous rocks into Earth orbit. As the Moon was located at one-third of its present distance from the Earth, it would have gravitationally collected 8 million tons of ejecta [ 188 ]. Without aqueous weathering or plate tectonics, fragments of Earth meteorites might be preserved for billions of years. Earth meteorites can be identified on the Moon through the analysis of zircon crystals; one such sample was found in an impact breccia collected by Apollo 14 [ 190 ]. This 4.1 Ga piece of ejecta dates from the Hadean, and is older than any known terrestrial rock formation. Ejecta sourced from submarine vents and hot springs could conceivably preserve a record of the history of life’s origin older than the Nuvvuagittuq Belt or the Dresser Formation [ 191 ]. In fact, the Moon may also preserve a record of Venusian, Martian, and extrasolar biosignatures, albeit in low concentrations [ 191 , 192 ]. NASA’s Artemis Program and related efforts present an excellent opportunity to conduct an extensive search for Hadean impact ejecta within the coming decade (e.g., [ 193 ]).

5. Factoring the Hypotheses into Future Research and Life-Detection Missions

A common assumption in the field of astrobiology is that habitable worlds possessing sources of energy, liquid water, and CHNOPS elements are abodes for life by default. The reality is more nuanced. In order for microbial life to thrive in a habitable environment, it must first originate there or be transported from another location. These processes require prerequisites which are not met by traditional definitions of habitability alone. Therefore, origins of life hypotheses should be factored into the search for biosignatures in the Solar System and on exoplanets. Two alternative hypotheses describing the origins of life propose that microbial life began at submarine hydrothermal vents or in fluctuating terrestrial hydrothermal pools. Alkaline submarine vents provide microbial life with energy in the form of thermal and chemical gradients, vent chimneys where the synthesis of organic compounds can take place, and an environment sheltered from the cataclysms of the Late Heavy Bombardment 3.9 billion years ago. Hot springs are comprised of freshwater pools with wet-dry cycling, which can readily assemble lipid membranes encapsulating polymers of catalytic length and select for increasingly robust protocells. Although both hypotheses have unresolved questions and potential limitations, they are currently our most-investigated models for how microbial life could have begun on Earth and other habitable worlds. As Section 3 and Section 4 proposed, the two alternative hypotheses of “vents and fields” can help guide the search for life beyond our own planet. Factoring in these hypotheses, Figure 7 summarizes the prospects for an origin of life on each of the prospective destinations for future astrobiology missions.

A table of habitable worlds and their relevance to the submarine hydrothermal vent and hot spring origins of life hypotheses. This table summarizes the discussion presented in Section 3 and Section 4 . The complete list of candidate ocean worlds in the Solar System was assembled by Lunine [ 85 ]. All images are credit of NASA.

Future research on the origins of life on other worlds can follow three primary paths: theory, experimentation, and exploration. This review article suggests a modified astrobiology strategy for the next decade, which builds on abiogenesis hypotheses as well as today’s missions and experiments: “Follow the water to where life can start and spread.” It represents an early attempt to define parameters, questions, and assumptions related to the origins of life that can help guide experimentation and planetary exploration in the coming years. Key questions include the following:

• Was early Mars “warm and wet” or “cold and icy?” How would the Noachian climate affect the transport of microbes from hydrothermal systems to other locations?
• How did Martian deluges in the Hesperian and Amazonian periods impact conditions for life? What biosignatures might be left behind from temporary upwellings?
• Could Martian biota develop photoautrophy during the Noachian and Hesperian? How would this development, or lack thereof, impact the search for Martian biosignatures?
• Did Martian hydrothermal systems persist long enough for early microbes to evolve the ability to deposit stromatolitic biosignatures? If not, what simpler biosignatures could be discovered by future missions?
• Have Europa and Enceladus held oceans for the entire history of the Solar System? Can hydrothermal vents form under the conditions of extreme pressure present at Europa’s ocean floor?
• Without continental weathering, are there alternative sources for phosphorus on Europa, Enceladus, and exoplanets with global oceans?
• Can microbial life can be transported between planetary environments (“limited Panspermia”)? If so, could life start in a Martian hot spring and travel to Enceladus or Europa, penetrating the ice shell and accessing the energy-rich environments of a hydrothermal vent? Alternatively, could life start in a hydrothermal vent on an icy moon and be transported to Mars or Earth?

These questions and their answers are likely to change as new research is conducted. Future research could define new parameters, and incorporate additional perspectives and theories which describe requirements for microbial life. Additional research on the submarine vent and hot spring origins of life hypotheses might also influence future developments in astrobiology. If one hypothesis becomes clearly favored over the other, space exploration efforts could be directed to search for life on worlds possessing this specific environment.

Future experiments should aim to simulate extraterrestrial hydrothermal systems in the lab. One such facility is the McMaster Planet Simulator, which has already simulated photochemistry in the atmosphere of Titan [ 194 ]. The University of New South Wales (UNSW) and the University of Cincinnati are constructing hot spring simulators to test the hypothesis that life began in hydrothermal fields [ 195 ]. Multiple submarine hydrothermal vent simulation chambers are active (e.g., [ 196 ]). These experiments provide controlled conditions, and are more predictable than natural hydrothermal systems. One valuable experiment to perform in a facility such as the UNSW hot spring simulator would be to replicate the expected temperatures, pressures, and atmospheric compositions of early Mars. This test could determine whether these variables affect the self-assembly of membranes and polymers in hot springs during wet-dry cycling.

Future experiments in submarine vent simulators could determine whether prebiotic chemistry can occur in a phosphate-poor environment such as the ocean of Europa. Prior studies have determined that amino acids and oligonucleotides can be synthesized in marine hydrothermal vents on Earth, but it is unclear whether this biochemistry is possible in the absence of phosphorus. It would also be advantageous to use a natural experiment to corroborate laboratory results stating that vesicle formation and polymerization are possible in submarine vents. This could be accomplished by dispatching a robotic submersible to an alkaline vent system such as the Lost City Complex. The submarine could drill a hole in a vent chimney, inject Carbon 14-labeled CO 2 , loiter for several hours, and return samples for analysis. This experiment could verify that organic polymers such as formic acid and formaldehyde can be synthesized in submarine vents, and it could enable direct comparisons with similar experiments conducted in hot spring environments.

The search for extraterrestrial life is currently one of the highest priorities of the scientific community [ 197 ]. The missions of the early 21st century have discovered extensive evidence of water on ancient Mars, subsurface oceans on the moons of the outer Solar System, and dozens of potentially habitable exoplanets. Given this abundance of riches, what strategy should we follow to search for evidence of microbial life beyond Earth? One broad proposal would be for every astrobiology mission to incorporate a specific origins of life hypothesis into its scientific rationale. This argument could incorporate questions such as these:

• Is the mission dependent on an origin of life in submarine vents or hot springs?
• If it is searching for life in a different location, such as a lake or an ice sheet, how were microbes transported to this site from a hydrothermal system?
• What biosignatures should the mission search for at its destination?

Beyond this general suggestion, the marine hydrothermal vent and terrestrial hydrothermal field origins of life scenarios suggest several specific mission architectures which could be implemented in the coming decade. One quandary in the search for life on Mars is the destruction of biosignatures on the planet’s surface [ 198 ]. Over billions of years, organic material is decomposed by ultraviolet radiation and microfossil-bearing outcrops are eroded by aeolian activity. Deep drilling could be used to detect organics and other biosignatures in sequences of marine or terrestrial hydrothermal sediments. This strategy enabled the detection of organics in the Dresser Formation [ 71 ], and it will be utilized on Mars by the European Space Agency’s (ESA) ExoMars rover [ 199 ]. Despite the advantages of deep drilling, the experiences of the InSight geophysical lander suggest that hardware reliability and unknown surface properties remain problematic; astronauts on-site may be required to reach depths of more than a few meters below the surface. Until drilling technology improves, sample return from multiple sites could be an alternative approach to biosignature detection. Returning Martian rock and soil samples to laboratories on Earth has the potential to answer numerous high-level questions in planetary science [ 197 ], and NASA’s Mars 2020 rover represents the first step towards this goal [ 200 ]. However, if Noachian Mars had a “cold and icy” climate and/or Martian life never developed photoautrophy, it would be unreasonable to expect that biosignatures would be found during the first sample return mission; each traditional sample return campaign costs 6–8 billion dollars [ 197 ]. If subsequent missions returned small, targeted samples (~100 grams rather than ~500 grams), the price of a sample return mission could potentially drop to about 2 billion dollars. This would enable the exploration of multiple diverse locations where life could have originated and thrived, such as the Eridania Basin, the Columbia Hills, and Nili Patera.

Climate models suggest that Venus could have been habitable for up to 3 billion years, but there is no geomorphologic or spectroscopic evidence for hydrothermal systems in current datasets. Therefore, one next step in the exploration of the planet could be an orbiter similar to NASA’s VERITAS or ESA’s EnVision, which could pinpoint habitable environments on Venus and map its mineralogy. Multiple mission concepts for Europa and Enceladus have proposed flying through the moons’ plumes, using mass spectrometers to search for biosignatures (e.g., [ 201 ]). Such missions would indirectly sample the uppermost levels of the oceans, accomplishing a search for biosignatures for a moderate cost. One challenge for these concepts is that it is unclear how microbes in the open ocean of an icy moon would gather energy; microbial communities might be concentrated around hydrothermal vents on the seafloor to collect thermal energy. If so, a “cryobot” capable of melting through kilometers of ice may be required to conduct a comprehensive search for biosignatures on ocean worlds [ 202 ].

NASA’s Dragonfly mission is a rotorcraft scheduled to land on Titan in 2034 and traverse ~100 km [ 203 ]. During its traverse, the spacecraft will sample a variety of terrains; its data could be used to explore how far prebiotic chemistry can progress in the absence of life, and which organic molecules could naturally be available for an origin of life in a hydrothermal system. Dragonfly’s destination is Selk Crater, where the mission could explore an impact-generated hydrothermal system and search for biosignatures in a cryogenic hydrothermal field. Beyond the Solar System, exoplanets with moderate surface water fractions are more favorable to an origin of life in either submarine vents or hot springs than ocean planets. Therefore, upcoming observations by the Hubble Space Telescope should focus on measuring the percentage of water by mass of transiting habitable-zone exoplanets. This procedure has already been employed for the TRAPPIST-1 system [ 162 ]. Planets hypothesized to have both continents and oceans can then be studied in detail by the upcoming James Webb Space Telescope.

6. Conclusions

The coming decade will see significant advances in the search for life beyond Earth. However, the number of worthy planetary mission concepts will always exceed the resources available to design and build spacecraft [ 197 ]. A planetary environment conducive to an origin of life is likely to either harbor extant life or preserve biosignatures. Therefore, origins of life hypotheses should be factored into astrobiology frameworks and life-detection missions. Conclusive success in the search for extraterrestrial life may be realized only after decades of additional missions, laboratory experiments, and field studies by hundreds of multi-disciplinary researchers integrating and testing many perspectives, including various scenarios for how life might begin.

Acknowledgments

The authors would like to thank D. W. Deamer, S. W. Ruff, A. Omran, and many others for providing constructive reviews of this article. Assistant Editor A. Jiao did an exceptional job shepherding this review through the publication process. D. W. Deamer suggested that a submersible could be used to test the predictions of the submarine vent origins of life hypothesis. This article also benefited greatly from a synthesis of inputs from our colleagues J. W. Rice, K. Campbell, M. Van Kranendonk, and T. Djokic, as well as the entire planetary science community. Finally, we are grateful to NASA and to the thousands of people who wholeheartedly design and build the spacecraft that explore the planets, from Mercury through Pluto and into the vast unknown regions of space. Their effort and dedication makes the search for life on other worlds possible.

Author Contributions

Conceptualization, A.L.; methodology, A.L. and B.D.; validation, B.D.; formal analysis, A.L.; investigation, A.L.; resources, A.L.; writing—original draft preparation, A.L.; writing—review and editing, B.D. and A.L.; visualization, A.L.; supervision, B.D.; project administration, B.D. and A.L. Both authors have read and agreed to the published version of the manuscript.

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.