Abiogenesis

Abiogenesis is the natural process by which life arises from non-living matter, such as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-living to living entities on Earth was not a single event, but a process of increasing complexity involving the formation of a habitable planet, the prebiotic synthesis of organic molecules, molecular self-replication, self-assembly, autocatalysis, and the emergence of cell membranes. The transition from non-life to life has never been observed experimentally, but many proposals have been made for different stages of the process.

The study of abiogenesis aims to determine how pre-life chemical reactions gave rise to life under conditions strikingly different from those on Earth today. It primarily uses tools from biology and chemistry, with more recent approaches attempting a synthesis of many sciences. Life functions through the specialized chemistry of carbon and water, and builds largely upon four key families of chemicals: lipids for cell membranes, carbohydrates such as sugars, amino acids for protein metabolism, and nucleic acid DNA and RNA for the mechanisms of heredity. Any successful theory of abiogenesis must explain the origins and interactions of these classes of molecules.

Many approaches to abiogenesis investigate how self-replicating molecules, or their components, came into existence. Researchers generally think that current life descends from an RNA world, although other self-replicating and self-catalyzing molecules may have preceded RNA. Other approaches ("metabolism-first" hypotheses) focus on understanding how catalysis in chemical systems on the early Earth might have provided the precursor molecules necessary for self-replication. The classic 1952 Miller–Urey experiment demonstrated that most amino acids, the chemical constituents of proteins, can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth. External sources of energy may have triggered these reactions, including lightning, radiation, atmospheric entries of micro-meteorites and implosion of bubbles in sea and ocean waves.

While the

Earth remains the only place in the universe known to harbor life. Geochemical and fossil evidence from the Earth informs most studies of abiogenesis. The Earth was formed at 4.54 Gya, and the earliest evidence of life on Earth dates from at least 3.8 Gya from Western Australia. Fossil micro-organisms appear to have lived within hydrothermal vent precipitates dated 3.77 to 4.28 Gya from Quebec, soon after ocean formation 4.4 Gya during the Hadean.

Life consists of reproduction with (heritable) variations.^[3] NASA defines life as "a self-sustaining chemical system capable of Darwinian [i.e., biological] evolution."^[4] Such a system is complex; the last universal common ancestor (LUCA), presumably a single-celled organism which lived some 4 billion years ago, already had hundreds of genes encoded in the DNA genetic code that is universal today. That in turn implies a suite of cellular machinery including messenger RNA, transfer RNA, and ribosomes to translate the code into proteins. Those proteins included enzymes to operate its anaerobic respiration via the Wood–Ljungdahl metabolic pathway, and a DNA polymerase to replicate its genetic material.^[5][6]

The challenge for abiogenesis (origin of life)^[7][8][9] researchers is to explain how such a complex and tightly interlinked system could develop by evolutionary steps, as at first sight all its parts are necessary to enable it to function. For example, a cell, whether the LUCA or in a modern organism, copies its DNA with the DNA polymerase enzyme, which is in turn produced by translating the DNA polymerase gene in the DNA. Neither the enzyme nor the DNA can be produced without the other.^[10] The evolutionary process could have involved molecular self-replication, self-assembly such as of cell membranes, and autocatalysis.^[5][6][11] Nonetheless, the transition of non-life to life has never been observed experimentally.^[12]

The precursors to the development of a living cell like the LUCA are clear enough, if disputed in their details: a habitable world is formed with a supply of minerals and liquid water. Prebiotic synthesis creates a range of simple organic compounds, which are assembled into polymers such as proteins and RNA. The process after the LUCA is readily understood: biological evolution caused the development of a wide range of species with varied forms and biochemical capabilities. The derivation of living things such as the LUCA from simple components, however, is far from understood.^[1]

Although Earth remains the only place where life is known,

One ancient view of the origin of life, from

Another ancient idea dating back to

The idea that life originated from non-living matter in slow stages appeared in Herbert Spencer's 1864–1867 book Principles of Biology, and in William Turner Thiselton-Dyer's 1879 paper "On spontaneous generation and evolution". On 1 February 1871 Charles Darwin wrote about these publications to Joseph Hooker, and set out his own speculation, suggesting that the original spark of life may have begun in a "warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, &c., present, that a proteine compound was chemically formed ready to undergo still more complex changes." Darwin went on to explain that "at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed."^[34][35][36]

Alexander Oparin in 1924 and J. B. S. Haldane in 1929 proposed that the first molecules constituting the earliest cells slowly self-organized from a primordial soup, and this theory is called the Oparin–Haldane hypothesis.^[37][38] Haldane suggested that the Earth's prebiotic oceans consisted of a "hot dilute soup" in which organic compounds could have formed.^[21][39] J. D. Bernal showed that such mechanisms could form most of the necessary molecules for life from inorganic precursors.^[40] In 1967, he suggested three "stages": the origin of biological monomers; the origin of biological polymers; and the evolution from molecules to cells.^[41][42]

In 1952,

Soon after the

Condensation to form liquid oceans is theorised to have occurred as early as the Moon-forming impact.^[62][63] This scenario has found support from the dating of 4.404 Gya zircon crystals with high δ¹⁸O values from metamorphosed quartzite of Mount Narryer in Western Australia.^[64][65] The Hadean atmosphere has been characterized as a "gigantic, productive outdoor chemical laboratory," similar to volcanic gases today which still support some abiotic chemistry. Despite the likely increased volcanism from early plate tectonics, the Earth may have been a predominantly water world between 4.4 and 4.3 Gya. It is debated whether or not crust was exposed above this ocean due to uncertainties of what early plate tectonics looked like. For early life to have developed, it is generally thought that a land setting is required, so this question is essential to determining when in Earth's history life evolved.^[66] The post-Moon-forming impact Earth likely existed with little if any continental crust, a turbulent atmosphere, and a hydrosphere subject to intense ultraviolet light from a T Tauri stage Sun, from cosmic radiation, and from continued asteroid and comet impacts.^[67] Despite all this, niche environments likely existed conducive to life on Earth in the Late-Hadean to Early-Archaean.

The Late Heavy Bombardment hypothesis posits that a period of intense impact occurred at ~3.9 Gya during the Hadean.^[68][69] A cataclysmic impact event would have had the potential to sterilise all life on Earth by volatilising liquid oceans and blocking the Sun needed for photosynthesising primary producers, pushing back the earliest possible emergence of life to after Late Heavy Bombardment.^[70] Recent research questions both the intensity of the Late Heavy Bombardment as well as its potential for sterilisation. Uncertainties as to whether Late Heavy Bombardment was one giant impact or a period of greater impact rates greatly changed the implication of its destructive power.^[71][72] The 3.9 Ga date arises from dating of Apollo mission sample returns collected mostly near the Imbrium Basin, biasing the age of recorded impacts.^[73] Impact modelling of the lunar surface reveals that rather than a cataclysmic event at 3.9 Ga, multiple small-scale, short-lived periods of bombardment likely occurred.^[74] Terrestrial data backs this idea by showing multiple periods of ejecta in the rock record both before and after the 3.9 Ga marker, suggesting that the early Earth was subject to continuous impacts that would not have had as great an impact on extinction as previously thought.^[75] If the Late Heavy Bombardment did not take place, this allows for the emergence of life to have taken place far before 3.9 Ga.

If life evolved in the ocean at depths of more than ten meters, it would have been shielded both from late impacts and the then high levels of ultraviolet radiation from the sun. Geothermically heated oceanic crust could have yielded far more organic compounds through deep hydrothermal vents than the Miller–Urey experiments indicated.^[76] The available energy is maximized at 100–150 °C, the temperatures at which hyperthermophilic bacteria and thermoacidophilic archaea live.^[77]

The exact timing at which life emerged on Earth is unknown. Minimum age estimates are based on evidence from the geologic rock record. The earliest physical evidence of life so far found consists of microbialites in the Nuvvuagittuq Greenstone Belt of Northern Quebec, in banded iron formation rocks at least 3.77 and possibly as old as 4.32 Gya. The micro-organisms lived within hydrothermal vent precipitates, soon after the 4.4 Gya formation of oceans during the Hadean. The microbes resembled modern hydrothermal vent bacteria, supporting the view that abiogenesis began in such an environment.^[78]

Biogenic

The Pilbara region of Western Australia contains the Dresser Formation with rocks 3.48 Gya, including layered structures called stromatolites. Their modern counterparts are created by photosynthetic micro-organisms including cyanobacteria.^[86] These lie within undeformed hydrothermal-sedimentary strata; their texture indicates a biogenic origin. Parts of the Dresser formation preserve hot springs on land, but other regions seem to have been shallow seas.^[87] A molecular clock analysis suggests the LUCA emerged prior to the Late Heavy Bombardment (3.9 Gya).^[88]

• 
  Stromatolites in the Siyeh Formation, Glacier National Park, dated 3.5 Gya, placing them among the earliest life-forms
• 
  Modern stromatolites in Shark Bay, created by photosynthetic cyanobacteria

All

An organic compound is a chemical whose molecules contain carbon. Carbon is abundant in the Sun, stars, comets, and in the

Polycyclic aromatic hydrocarbons (PAH) are the most common and abundant polyatomic molecules in the observable universe, and are a major store of carbon.^{[93][100][101][102]} They seem to have formed shortly after the Big Bang,^{[103][101][102]} and are associated with new stars and exoplanets.^[93] They are a likely constituent of Earth's primordial sea.^{[103][101][102]} PAHs have been detected in nebulae,^[104] and in the interstellar medium, in comets, and in meteorites.^[93]

The PAH world hypothesis posits PAHs as precursors to the RNA world.^[105] A star, HH 46-IR, resembling the sun early in its life, is surrounded by a disk of material which contains molecules including cyanide compounds, hydrocarbons, and carbon monoxide. PAHs in the interstellar medium can be transformed through hydrogenation, oxygenation, and hydroxylation to more complex organic compounds used in living cells.^[106]

The majority of organic compounds introduced on Earth by interstellar dust particles have helped to form complex molecules, thanks to their peculiar surface-catalytic activities.^[107][108] Studies of the ¹²C/¹³C isotopic ratios of organic compounds in the Murchison meteorite suggest that the RNA component uracil and related molecules, including xanthine, were formed extraterrestrially.^[109] NASA studies of meteorites suggest that all four DNA nucleobases (adenine, guanine and related organic molecules) have been formed in outer space.^{[107][110][111]} The cosmic dust permeating the universe contains complex organics ("amorphous organic solids with a mixed aromatic–aliphatic structure") that could be created rapidly by stars.^[112] Glycolaldehyde, a sugar molecule and RNA precursor, has been detected in regions of space including around protostars and on meteorites.^[113][114]

As early as the 1860s, experiments demonstrated that biologically relevant molecules can be produced from interaction of simple carbon sources with abundant inorganic catalysts. The spontaneous formation of complex polymers from abiotically generated monomers under the conditions posited by the "soup" theory is not straightforward. Besides the necessary basic organic monomers, compounds that would have prohibited the formation of polymers were also formed in high concentration during the Miller–Urey and Joan Oró experiments.^[115] Biology uses essentially 20 amino acids for its coded protein enzymes, representing a very small subset of the structurally possible products. Since life tends to use whatever is available, an explanation is needed for why the set used is so small.^[116] Formamide is attractive as a medium that potentially provided a source of amino acid derivatives from simple aldehyde and nitrile feedstocks.^[117]

Alexander Butlerov showed in 1861 that the formose reaction created sugars including tetroses, pentoses, and hexoses when formaldehyde is heated under basic conditions with divalent metal ions like calcium. R. Breslow proposed that the reaction was autocatalytic in 1959.^[118]

Nucleobases, such as guanine and adenine, can be synthesized from simple carbon and nitrogen sources, such as hydrogen cyanide (HCN) and ammonia.^[119] Formamide produces all four ribonucleotides when warmed with terrestrial minerals. Formamide is ubiquitous in the Universe, produced by the reaction of water and HCN. It can be concentrated by the evaporation of water.^[120][121] HCN is poisonous only to aerobic organisms (eukaryotes and aerobic bacteria), which did not yet exist. It can play roles in other chemical processes such as the synthesis of the amino acid glycine.^[60]

DNA and RNA components including uracil, cytosine and thymine can be synthesized under outer space conditions, using starting chemicals such as pyrimidine found in meteorites. Pyrimidine may have been formed in red giant stars or in interstellar dust and gas clouds.^[122] All four RNA-bases may be synthesized from formamide in high-energy density events like extraterrestrial impacts.^[123]

Other pathways for synthesizing bases from inorganic materials have been reported.^[124] Freezing temperatures are advantageous for the synthesis of purines, due to the concentrating effect for key precursors such as hydrogen cyanide.^[125] However, while adenine and guanine require freezing conditions for synthesis, cytosine and uracil may require boiling temperatures.^[126] Seven amino acids and eleven types of nucleobases formed in ice when ammonia and cyanide were left in a freezer for 25 years.^[127][128] S-triazines (alternative nucleobases), pyrimidines including cytosine and uracil, and adenine can be synthesized by subjecting a urea solution to freeze-thaw cycles under a reductive atmosphere, with spark discharges as an energy source.^[129] The explanation given for the unusual speed of these reactions at such a low temperature is eutectic freezing, which crowds impurities in microscopic pockets of liquid within the ice, causing the molecules to collide more often.^[130]

Prebiotic peptide synthesis is proposed to have occurred through a number of possible routes. Some center on high temperature/concentration conditions in which condensation becomes energetically favorable, while others focus on the availability of plausible prebiotic condensing agents.^{[131][further explanation needed]}

Experimental evidence for the formation of peptides in uniquely concentrated environments is bolstered by work suggesting that wet-dry cycles and the presence of specific salts can greatly increase spontaneous condensation of glycine into poly-glycine chains.^[132] Other work suggests that while mineral surfaces, such as those of pyrite, calcite, and rutile catalyze peptide condensation, they also catalyze their hydrolysis. The authors suggest that additional chemical activation or coupling would be necessary to produce peptides at sufficient concentrations. Thus, mineral surface catalysis, while important, is not sufficient alone for peptide synthesis.^[133]

Many prebiotically plausible condensing/activating agents have been identified, including the following: cyanamide, dicyanamide, dicyandiamide, diaminomaleonitrile, urea, trimetaphosphate, NaCl, CuCl_2, (Ni,Fe)S, CO, carbonyl sulfide (COS), carbon disulfide (CS₂)_, SO_2, and diammonium phosphate (DAP).^[131]

An experiment reported in 2024 used a saffire substrate with a web of thin cracks under a heat flow, similar to the environment of deep-ocean vents, as a mechanism to separate and concentrate prebiotically relevant building blocks from a dilute mixture, purifying their concentration by up to three orders of magnitude. The authors propose this as a plausible model for the origin of complex biopolymers.^[134] This presents another physical process that allows for concentrated peptide precursors to combine in the right conditions. A similar role of increasing amino acid concentration has been suggested for clays as well.^[135]

While all of these scenarios involve the condensation of amino acids, the prebiotic synthesis of peptides from simpler molecules such as CO, NH₃ and C, skipping the step of amino acid formation, is very efficient.^[136][137]  

The largest unanswered question in evolution is how simple protocells first arose and differed in reproductive contribution to the following generation, thus initiating the evolution of life. The lipid world theory postulates that the first self-replicating object was lipid-like.^[138][139] Phospholipids form lipid bilayers in water while under agitation—the same structure as in cell membranes. These molecules were not present on early Earth, but other amphiphilic long-chain molecules also form membranes. These bodies may expand by insertion of additional lipids, and may spontaneously split into two offspring of similar size and composition. Lipid bodies may have provided sheltering envelopes for information storage, allowing the evolution and preservation of polymers like RNA that store information. Only one or two types of amphiphiles have been studied which might have led to the development of vesicles.^[140] There is an enormous number of possible arrangements of lipid bilayer membranes, and those with the best reproductive characteristics would have converged toward a hypercycle reaction,^[141][142] a positive feedback composed of two mutual catalysts represented by a membrane site and a specific compound trapped in the vesicle. Such site/compound pairs are transmissible to the daughter vesicles leading to the emergence of distinct lineages of vesicles, which would have allowed natural selection.^[143]

A protocell is a self-organized, self-ordered, spherical collection of lipids proposed as a stepping-stone to the origin of life.^[140] A functional protocell has (as of 2014) not yet been achieved in a laboratory setting.^{[144][145][146]} Self-assembled vesicles are essential components of primitive cells.^[140] The theory of classical irreversible thermodynamics treats self-assembly under a generalized chemical potential within the framework of dissipative systems.^{[147][148][149]} The second law of thermodynamics requires that overall entropy increases, yet life is distinguished by its great degree of organization. Therefore, a boundary is needed to separate ordered life processes from chaotic non-living matter.^[150]

Irene Chen and Jack W. Szostak suggest that elementary protocells can give rise to cellular behaviors including primitive forms of differential reproduction, competition, and energy storage.^[145] Competition for membrane molecules would favor stabilized membranes, suggesting a selective advantage for the evolution of cross-linked fatty acids and even the phospholipids of today.^[145] Such micro-encapsulation would allow for metabolism within the membrane and the exchange of small molecules, while retaining large biomolecules inside. Such a membrane is needed for a cell to create its own electrochemical gradient to store energy by pumping ions across the membrane.^[151][152] Fatty acid vesicles in conditions relevant to alkaline hydrothermal vents can be stabilized by isoprenoids which are synthesized by the formose reaction; the advantages and disadvantages of isoprenoids incorporated within the lipid bilayer in different microenvironments might have led to the divergence of the membranes of archaea and bacteria.^[153]

Laboratory experiments have shown that vesicles can undergo an evolutionary process under pressure cycling conditions.^[154] Simulating the systemic environment in tectonic fault zones within the Earth's crust, pressure cycling leads to the periodic formation of vesicles.^[155] Under the same conditions, random peptide chains are being formed, which are being continuously selected for their ability to integrate into the vesicle membrane. A further selection of the vesicles for their stability potentially leads to the development of functional peptide structures,^{[156][157][158]} associated with an increase in the survival rate of the vesicles.

Life requires a loss of entropy, or disorder, as molecules organize themselves into living matter. At the same time, the emergence of life is associated with the formation of structures beyond a certain threshold of complexity.^[159] The emergence of life with increasing order and complexity does not contradict the second law of thermodynamics, which states that overall entropy never decreases, since a living organism creates order in some places (e.g. its living body) at the expense of an increase of entropy elsewhere (e.g. heat and waste production).^{[160][161][162]}

Multiple sources of energy were available for chemical reactions on the early Earth. Heat from geothermal processes is a standard energy source for chemistry. Other examples include sunlight, lightning,^[60] atmospheric entries of micro-meteorites,^[163] and implosion of bubbles in sea and ocean waves.^[164] This has been confirmed by experiments^[165][166] and simulations.^[167] Unfavorable reactions can be driven by highly favorable ones, as in the case of iron-sulfur chemistry. For example, this was probably important for

In 1961,

The RNA world hypothesis describes an early Earth with self-replicating and catalytic RNA but no DNA or proteins.^[172] Many researchers concur that an RNA world must have preceded the DNA-based life that now dominates.^[173] However, RNA-based life may not have been the first to exist.^[174][175] Another model echoes Darwin's "warm little pond" with cycles of wetting and drying.^[176]

RNA is central to the translation process. Small RNAs can catalyze all the chemical groups and information transfers required for life.^[175][177] RNA both expresses and maintains genetic information in modern organisms; and the chemical components of RNA are easily synthesized under the conditions that approximated the early Earth, which were very different from those that prevail today. The structure of the ribosome has been called the "smoking gun", with a central core of RNA and no amino acid side chains within 18 Å of the active site that catalyzes peptide bond formation.^{[178][174][179]}

The concept of the RNA world was proposed in 1962 by Alexander Rich,^[180] and the term was coined by Walter Gilbert in 1986.^[175][181] There were initial difficulties in the explanation of the abiotic synthesis of the nucleotides cytosine and uracil.^[182] Subsequent research has shown possible routes of synthesis; for example, formamide produces all four ribonucleotides and other biological molecules when warmed in the presence of various terrestrial minerals.^[120][121]

Possible precursors to protein synthesis include the synthesis of short peptide cofactors or the self-catalysing duplication of RNA. It is likely that the ancestral ribosome was composed entirely of RNA, although some roles have since been taken over by proteins. Major remaining questions on this topic include identifying the selective force for the evolution of the ribosome and determining how the genetic code arose.[192]

Eugene Koonin has argued that "no compelling scenarios currently exist for the origin of replication and translation, the key processes that together comprise the core of biological systems and the apparent pre-requisite of biological evolution. The RNA World concept might offer the best chance for the resolution of this conundrum but so far cannot adequately account for the emergence of an efficient RNA replicase or the translation system."^[193]

In line with the RNA world hypothesis, much of modern biology's templated protein biosynthesis is done by RNA molecules—namely tRNAs and the ribosome (consisting of both protein and rRNA components). The most central reaction of peptide bond synthesis is understood to be carried out by base catalysis by the 23S rRNA domain V.^[194] Experimental evidence has demonstrated successful di- and tripeptide synthesis with a system consisting of only aminoacyl phosphate adaptors and RNA guides, which could be a possible stepping stone between an RNA world and modern protein synthesis.^[194][195] Aminoacylation ribozymes that can charge tRNAs with their cognate amino acids have also been selected in in vitro experimentation.^[196] The authors also extensively mapped fitness landscapes within their selection to find that chance emergence of active sequences was more important that sequence optimization.^[196]

The first proteins would have had to arise without a fully-fledged system of protein biosynthesis. As discussed above, numerous mechanisms for the prebiotic synthesis of polypeptides exist. However, these random sequence peptides would not have likely had biological function. Thus, significant study has gone into exploring how early functional proteins could have arisen from random sequences. First, some evidence on hydrolysis rates shows that abiotically plausible peptides likely contained significant "nearest-neighbor" biases.^[197] This could have had some effect on early protein sequence diversity. In other work by Anthony Keefe and Jack Szostak, mRNA display selection on a library of 6*10¹² 80-mers was used to search for sequences with ATP binding activity. They concluded that approximately 1 in 10¹¹ random sequences had ATP binding function.^[198] While this is a single example of functional frequency in the random sequence space, the methodology can serve as a powerful simulation tool for understanding early protein evolution.^[199]

Starting with the work of

• 
  Eukaryota

  on the other.

In 2016, a set of 355 genes likely present in the LUCA was identified. A total of 6.1 million prokaryotic genes from Bacteria and Archaea were sequenced, identifying 355 protein clusters from among 286,514 protein clusters that were probably common to the LUCA. The results suggest that the LUCA was anaerobic with a Wood–Ljungdahl (reductive Acetyl-CoA) pathway, nitrogen- and carbon-fixing, thermophilic. Its cofactors suggest dependence upon an environment rich in hydrogen, carbon dioxide, iron, and transition metals. Its genetic material was probably DNA, requiring the 4-nucleotide genetic code, messenger RNA, transfer RNA, and ribosomes to translate the code into proteins such as enzymes. LUCA likely inhabited an anaerobic hydrothermal vent setting in a geochemically active environment. It was evidently already a complex organism, and must have had precursors; it was not the first living thing.^[10][204] The physiology of LUCA has been in dispute.^{[205][206][207]}

• 
  LUCA systems and environment included the Wood–Ljungdahl pathway.^[10]

Leslie Orgel argued that early translation machinery for the genetic code would be susceptible to error catastrophe. Geoffrey Hoffmann however showed that such machinery can be stable in function against "Orgel's paradox".^{[208][209][210]} Metabolic reactions that have also been inferred in LUCA are the incomplete reverse Krebs cycle, gluconeogenesis, the pentose phosphate pathway, glycolysis, reductive amination, and transamination.^[211][212]

A variety of geologic and environmental settings have been proposed for an origin of life. These theories are often in competition with one another as there are many differing views of prebiotic compound availability, geophysical setting, and early life characteristics. The first organism on Earth likely looked different from LUCA. Between the first appearance of life and where all modern phylogenies began branching, an unknown amount of time passed, with unknown gene transfers, extinctions, and evolutionary adaptation to various environmental niches.^[213] One major shift is believed to be from the RNA world to an RNA-DNA-protein world. Modern phylogenies provide more pertinent genetic evidence about LUCA than about its precursors.^[214]

The most popular hypotheses for settings for the origin of life are deep sea hydrothermal vents and surface bodies of water. Surface waters can be classified into hot springs, moderate temperature lakes and ponds, and cold settings.

Early micro-fossils may have come from a hot world of gases such as methane, ammonia, carbon dioxide, and hydrogen sulfide, toxic to much current life.^[215] Analysis of the tree of life places thermophilic and hyperthermophilic bacteria and archaea closest to the root, suggesting that life may have evolved in a hot environment.^[216] The deep sea or alkaline hydrothermal vent theory posits that life began at submarine hydrothermal vents.^[217][218] William Martin and Michael Russell have suggested "that life evolved in structured iron monosulphide precipitates in a seepage site hydrothermal mound at a redox, pH, and temperature gradient between sulphide-rich hydrothermal fluid and iron(II)-containing waters of the Hadean ocean floor. The naturally arising, three-dimensional compartmentation observed within fossilized seepage-site metal sulphide precipitates indicates that these inorganic compartments were the precursors of cell walls and membranes found in free-living prokaryotes. The known capability of FeS and NiS to catalyze the synthesis of the acetyl-methylsulphide from carbon monoxide and methylsulphide, constituents of hydrothermal fluid, indicates that pre-biotic syntheses occurred at the inner surfaces of these metal-sulphide-walled compartments".^[219]

These form where hydrogen-rich fluids emerge from below the sea floor, as a result of

Russell demonstrated that alkaline vents created an abiogenic

1. Diffusion force caused by concentration gradient—all particles including ions tend to diffuse from higher concentration to lower.
2. Electrostatic force caused by electrical potential gradient—
   anions
   in the opposite direction.

These two gradients taken together can be expressed as an electrochemical gradient, providing energy for abiogenic synthesis. The proton motive force can be described as the measure of the potential energy stored as a combination of proton and voltage gradients across a membrane (differences in proton concentration and electrical potential).^[152]

The surfaces of mineral particles inside deep-ocean hydrothermal vents have catalytic properties similar to those of enzymes and can create simple organic molecules, such as methanol (CH₃OH) and formic, acetic, and pyruvic acids out of the dissolved CO₂ in the water, if driven by an applied voltage or by reaction with H₂ or H₂S.^[221][222]

The research reported by Martin in 2016 supports the thesis that life arose at hydrothermal vents,^[223][224] that spontaneous chemistry in the Earth's crust driven by rock–water interactions at disequilibrium thermodynamically underpinned life's origin^[225][226] and that the founding lineages of the archaea and bacteria were H₂-dependent autotrophs that used CO₂ as their terminal acceptor in energy metabolism.^[227] Martin suggests, based upon this evidence, that the LUCA "may have depended heavily on the geothermal energy of the vent to survive".^[10] Pores at deep sea hydrothermal vents are suggested to have been occupied by membrane-bound compartments which promoted biochemical reactions.^[228][229] Metabolic intermediates in the Krebs cycle, gluconeogenesis, amino acid bio-synthetic pathways, glycolysis, the pentose phosphate pathway, and including sugars like ribose, and lipid precursors can occur non-enzymatically at conditions relevant to deep-sea alkaline hydrothermal vents.^[230]

If the deep marine hydrothermal setting was the site for the origin of life, then abiogenesis could have happened as early as 4.0-4.2 Gya. If life evolved in the ocean at depths of more than ten meters, it would have been shielded both from impacts and the then high levels of ultraviolet radiation from the sun. The available energy in hydrothermal vents is maximized at 100–150 °C, the temperatures at which hyperthermophilic bacteria and thermoacidophilic archaea live.^[231][232] Arguments against a hydrothermal origin of life state that hyperthermophily was a result of convergent evolution in bacteria and archaea, and that a mesophilic environment would have been more likely.^[233][234] This hypothesis, suggested in 1999 by Galtier, was proposed one year before the discovery of the Lost City Hydrothermal Field, where white-smoker hydrothermal vents average ~45-90 °C.^[235] Moderate temperatures and alkaline seawater at Lost City are now the favoured hydrothermal vent setting in contrast to acidic, high temperature (~350 °C) black-smokers.

Production of prebiotic organic compounds at hydrothermal vents is estimated to be 1x10⁸ kg yr⁻¹.^[236] While a large amount of key prebiotic compounds, such as methane, are found at vents, they are in far lower concentrations than estimates of a Miller-Urey Experiment environment. In the case of methane, the production rate at vents is around 2-4 orders of magnitude lower than predicted amounts in a Miller-Urey Experiment surface atmosphere.^[236][237]

Other arguments against an oceanic vent setting for the origin of life include the inability to concentrate prebiotic materials due to strong dilution from seawater. This open-system cycles compounds through minerals that make up vents, leaving little residence time to accumulate.^[238] All modern cells rely on phosphates and potassium for nucleotide backbone and protein formation respectively, making it likely that the first life forms also shared these functions. These elements were not available in high quantities in the Archaean oceans as both primarily come from the weathering of continental rocks on land, far from vent settings. Submarine hydrothermal vents are not conducive to condensation reactions needed for polymerisation to form macromolecules.^[239][240]

An older argument was that key polymers were encapsulated in vesicles after condensation, which supposedly would not happen in saltwater because of the high concentrations of ions. However, while it is true that salinity inhibits vesicle formation from low-diversity mixtures of fatty acids,^[241] vesicle formation from a broader, more realistic mix of fatty-acid and 1-alkanol species is more resilient.^[242][241]

Surface bodies of water provide environments able to dry out and be rewetted. Continued wet-dry cycles allow the concentration of prebiotic compounds and condensation reactions to polymerise macromolecules. Moreover, lake and ponds on land allow for detrital input from the weathering of continental rocks which contain apatite, the most common source of phosphates needed for nucleotide backbones. The amount of exposed continental crust in the Hadean is unknown, but models of early ocean depths and rates of ocean island and continental crust growth make it plausible that there was exposed land.^[243] Another line of evidence for a surface start to life is the requirement for UV for organism function. UV is necessary for the formation of the U+C nucleotide base pair by partial hydrolysis and nucleobase loss.^[244] Simultaneously, UV can be harmful and sterilising to life, especially for simple early lifeforms with little ability to repair radiation damage. Radiation levels from a young Sun were likely greater, and, with no ozone layer, harmful shortwave UV rays would reach the surface of Earth. For life to begin, a shielded environment with influx from UV-exposed sources is necessary to both benefit and protect from UV. Shielding under ice, liquid water, mineral surfaces (e.g. clay) or regolith is possible in a range of surface water settings. While deep sea vents may have input from raining down of surface exposed materials, the likelihood of concentration is lessened by the ocean's open system.^[245]

Most branching phylogenies are thermophilic or hyperthermophilic, making it possible that the Last universal common ancestor (LUCA) and preceding lifeforms were similarly thermophilic. Hot springs are formed from the heating of groundwater by geothermal activity. This intersection allows for influxes of material from deep penetrating waters and from surface runoff that transports eroded continental sediments. Interconnected groundwater systems create a mechanism for distribution of life to wider area.^[246]

Mulkidjanian and co-authors argue that marine environments did not provide the ionic balance and composition universally found in cells, or the ions required by essential proteins and ribozymes, especially with respect to high K⁺/Na⁺ ratio, Mn²⁺, Zn²⁺ and phosphate concentrations. They argue that the only environments that mimic the needed conditions on Earth are hot springs similar to ones at Kamchatka.

A phylogenomic and geochemical analysis of proteins plausibly traced to the LUCA shows that the ionic composition of its intracellular fluid is identical to that of hot springs. The LUCA likely was dependent upon synthesized organic matter for its growth.[247] Experiments show that RNA-like polymers can be synthesized in wet-dry cycling and UV light exposure. These polymers were encapsulated in vesicles after condensation.^[241] Potential sources of organics at hot springs might have been transport by interplanetary dust particles, extraterrestrial projectiles, or atmospheric or geochemical synthesis. Hot springs could have been abundant in volcanic landmasses during the Hadean.^[171]

A mesophilic start in surface bodies of waters hypothesis has evolved from Darwin's concept of a 'warm little pond' and the Oparin-Haldane hypothesis. Freshwater bodies under temperate climates can accumulate prebiotic materials while providing suitable environmental conditions conducive to simple life forms. The climate during the Archaean is still a highly debated topic, as there is uncertainty about what continents, oceans, and the atmosphere looked like then. Atmospheric reconstructions of the Archaean from geochemical proxies and models state that sufficient greenhouse gases were present to maintain surface temperatures between 0-40 °C. Under this assumption, there is a greater abundance of moderate temperature niches in which life could begin.^[251]

Strong lines of evidence for mesophily from biomolecular studies include Galtier's G+C nucleotide thermometer. G+C are more abundant in thermophiles due to the added stability of an additional hydrogen bond not present between A+T nucleotides. rRNA sequencing on a diverse range of modern lifeforms show that LUCA's reconstructed G+C content was likely representative of moderate temperatures.^[234]

Although most modern phylogenies are thermophilic or hyperthermophilic, it is possible that their widespread diversity today is a product of convergent evolution and horizontal gene transfer rather than an inherited trait from LUCA.^[252] The reverse gyrase topoisomerase is found exclusively in thermophiles and hyperthermophiles as it allows for coiling of DNA.^[253] The reverse gyrase enzyme requires ATP to function, both of which are complex biomolecules. If an origin of life is hypothesised to involve a simple organism that had not yet evolved a membrane, let alone ATP, this would make the existence of reverse gyrase improbable. Moreover, phylogenetic studies show that reverse gyrase had an archaeal origin, and that it was transferred to bacteria by horizontal gene transfer. This implies that reverse gyrase was not present in the LUCA.^[254]

Cold-start origin of life theories stem from the idea there may have been cold enough regions on the early Earth that large ice cover could be found. Stellar evolution models predict that the Sun's luminosity was ~25% weaker than it is today. Fuelner states that although this significant decrease in solar energy would have formed an icy planet, there is strong evidence for liquid water to be present, possibly driven by a greenhouse effect. This would create an early Earth with both liquid oceans and icy poles.^[255]

Ice melts that form from ice sheets or glaciers melts create freshwater pools, another niche capable of experiencing wet-dry cycles. While these pools that exist on the surface would be exposed to intense UV radiation, bodies of water within and under ice are sufficiently shielded while remaining connected to UV exposed areas through ice cracks. Suggestions of impact melting of ice allow freshwater paired with meteoritic input, a popular vessel for prebiotic components.^[256] Near-seawater levels of sodium chloride are found to destabilize fatty acid membrane self-assembly, making freshwater settings appealing for early membranous life.^[257]

Icy environments would trade the faster reaction rates that occur in warm environments for increased stability and accumulation of larger polymers.^[258] Experiments simulating Europa-like conditions of ~20 °C have synthesised amino acids and adenine, showing that Miller-Urey type syntheses can still occur at cold temperatures.^[259] In an RNA world, the ribozyme would have had even more functions than in a later DNA-RNA-protein-world. For RNA to function, it must be able to fold, a process that is hindered by temperatures above 30 °C. While RNA folding in psychrophilic organisms is slower, the process is more successful as hydrolysis is also slower. Shorter nucleotides would not suffer from higher temperatures.^[260][261]

An alternative geological environment has been proposed by the geologist Ulrich Schreiber and the physical chemist Christian Mayer: the continental crust.^[262] Tectonic fault zones could present a stable and well-protected environment for long-term prebiotic evolution. Inside these systems of cracks and cavities, water and carbon dioxide present the bulk solvents. Their phase state would depend on the local temperature and pressure conditions and could vary between liquid, gaseous and supercritical. When forming two separate phases (e.g., liquid water and supercritical carbon dioxide in depths of little more than 1 km), the system provides optimal conditions for phase transfer reactions. Concurrently, the contents of the tectonic fault zones are being supplied by a multitude of inorganic educts (e.g., carbon monoxide, hydrogen, ammonia, hydrogen cyanide, nitrogen, and even phosphate from dissolved apatite) and simple organic molecules formed by hydrothermal chemistry (e.g. amino acids, long-chain amines, fatty acids, long-chain aldehydes).^[263][264] Finally, the abundant mineral surfaces provide a rich choice of catalytic activity.

An especially interesting section of the tectonic fault zones is located at a depth of approximately 1000 m. For the carbon dioxide part of the bulk solvent, it provides temperature and pressure conditions near the phase transition point between the supercritical and the gaseous state. This leads to a natural accumulation zone for lipophilic organic molecules that dissolve well in supercritical CO₂, but not in its gaseous state, leading to their local precipitation.^[265] Periodic pressure variations such as caused by geyser activity or tidal influences result in periodic phase transitions, keeping the local reaction environment in a constant non-equilibrium state. In presence of amphiphilic compounds (such as the long chain amines and fatty acids mentioned above), subsequent generations of vesicles are being formed^[266] that are constantly and efficiently being selected for their stability.^[267] The resulting structures could provide hydrothermal vents as well as hot springs with raw material for further development.

Homochirality is the geometric uniformity of materials composed of chiral (non-mirror-symmetric) units. Living organisms use molecules that have the same chirality (handedness): with almost no exceptions,^[269] amino acids are left-handed while nucleotides and sugars are right-handed. Chiral molecules can be synthesized, but in the absence of a chiral source or a chiral catalyst, they are formed in a 50/50 (racemic) mixture of both forms. Known mechanisms for the production of non-racemic mixtures from racemic starting materials include: asymmetric physical laws, such as the electroweak interaction; asymmetric environments, such as those caused by circularly polarized light, quartz crystals, or the Earth's rotation, statistical fluctuations during racemic synthesis,^[268] and spontaneous symmetry breaking.^{[270][271][272]}

Once established, chirality would be selected for.^[273] A small bias (enantiomeric excess) in the population can be amplified into a large one by asymmetric autocatalysis, such as in the Soai reaction.^[274] In asymmetric autocatalysis, the catalyst is a chiral molecule, which means that a chiral molecule is catalyzing its own production. An initial enantiomeric excess, such as can be produced by polarized light, then allows the more abundant enantiomer to outcompete the other.^[275]

Homochirality may have started in outer space, as on the Murchison meteorite the amino acid L-alanine (left-handed) is more than twice as frequent as its D (right-handed) form, and L-glutamic acid is more than three times as abundant as its D counterpart.^[276][277] Amino acids from meteorites show a left-handed bias, whereas sugars show a predominantly right-handed bias: this is the same preference found in living organisms, suggesting an abiogenic origin of these compounds.^[278]

In a 2010 experiment by Robert Root-Bernstein, "two D-RNA-oligonucleotides having inverse base sequences (D-CGUA and D-AUGC) and their corresponding L-RNA-oligonucleotides (L-CGUA and L-AUGC) were synthesized and their affinity determined for Gly and eleven pairs of L- and D-amino acids". The results suggest that homochirality, including codon directionality, might have "emerged as a function of the origin of the genetic code".^[279]

• Autopoiesis
• Formamide-based prebiotic chemistry – Scientific efforts aimed at reconstructing the beginnings of life
• Proto-metabolism
• GADV-protein world hypothesis – A hypothetical stage of abiogenesis
• Genetic recombination – Production of offspring with combinations of traits that differ from those found in either parent
• Shadow biosphere – Hypothetical biosphere of Earth
• Manganese metallic nodules