Abiogenesis

"Origin of life" redirects here. For non-scientific views on the origins of life, seeCreation myth.

Stages in the origin of life range from the well-understood, such as thehabitable Earthand the abiotic synthesis of simple molecules, to the largely unknown, like the derivation of thelast universal common ancestor(LUCA) with its complex molecular functionalities.

Abiogenesisis the natural process by whichlifearises fromnon-living matter,such as simpleorganic compounds.The prevailing scientifichypothesisis that the transition from non-living toliving entitieson Earth was not a single event, but a process of increasing complexity involving the formation of ahabitable planet,the prebiotic synthesis of organic molecules, molecularself-replication,self-assembly,autocatalysis,and the emergence ofcell 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-lifechemical reactionsgave rise to life under conditions strikingly different from those on Earth today. It primarily uses tools frombiologyandchemistry,with more recent approaches attempting a synthesis of many sciences. Life functions through the specialized chemistry ofcarbonand water, and builds largely upon four key families of chemicals:lipidsfor cell membranes,carbohydratessuch as sugars,amino acidsfor protein metabolism, andnucleic acidDNAandRNAfor 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 anRNA world,although other self-replicating and self-catalyzing molecules may have preceded RNA. Other approaches ("metabolism-first" hypotheses) focus on understanding howcatalysisin chemical systems on the early Earth might have provided theprecursor moleculesnecessary for self-replication. The classic 1952Miller–Urey experimentdemonstrated that most amino acids, the chemical constituents ofproteins,can be synthesized frominorganic compoundsunder conditions intended to replicate those of theearly Earth.External sources of energy may have triggered these reactions, includinglightning,radiation,atmospheric entries of micro-meteorites and implosion of bubbles in sea and ocean waves.

While thelast universal common ancestorof all modern organisms (LUCA) is thought to have been quite different from the origin of life, investigations into LUCA can guide research into early universal characteristics. Agenomicsapproach has sought to characterise LUCA by identifying the genes shared byArchaeaandBacteria,members of the two major branches of life (withEukaryotesincluded in the archaean branch in thetwo-domain system). It appears there are 355 genes common to all life; their functions imply that the LUCA wasanaerobicwith theWood–Ljungdahl pathway,deriving energy bychemiosmosis,and maintaining its hereditary material with DNA, thegenetic code,andribosomes.Although the LUCA lived over 4 billion years ago (4Gya), researchers believe it was far from the first form of life. Earlier cells might have had a leaky membrane and been powered by a naturally occurringproton gradientnear a deep-sea white smokerhydrothermal vent.

Earth remains the only place in theuniverseknown to harbor life.Geochemicalandfossil evidencefrom the Earth informs most studies of abiogenesis. TheEarthwas formed at 4.54 Gya, and the earliest evidence of life on Earth dates from at least 3.8 Gya fromWestern Australia.Fossil micro-organismsappear to have lived within hydrothermal vent precipitates dated 3.77 to 4.28 Gyafrom Quebec,soon afterocean formation4.4 Gya during theHadean.

Overview

Further information:Astrobiology

NASA's 2015 strategy forastrobiologyaimed to solve the puzzle of the origin of life – how a fully functioning living system could emerge from non-living components – through research on the prebiotic origin oflife's chemicals,both inspaceand onplanets,as well as the functioning of early biomolecules tocatalysereactions and supportinheritance.

Lifeconsists of reproduction with (heritable) variations.NASAdefines life as "a self-sustaining chemical system capable ofDarwinian [i.e., biological] evolution."Such a system is complex; thelast universal common ancestor(LUCA), presumably a single-celled organism which lived some 4 billion years ago, already had hundreds ofgenesencoded in theDNAgenetic codethat is universal today. That in turn implies a suite of cellular machinery includingmessenger RNA,transfer RNA,andribosomesto translate the code intoproteins.Those proteins includedenzymesto operate itsanaerobic respirationvia theWood–Ljungdahl metabolic pathway,and aDNA polymeraseto replicate its genetic material.

The challenge for abiogenesis (origin of life)researchers is to explain how such a complex and tightly interlinked system could develop by evolutionary steps, as at first sightall its parts are necessaryto 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.The evolutionary process could have involved molecularself-replication,self-assemblysuch as ofcell membranes,andautocatalysis.Nonetheless, the transition of non-life to life has never been observed experimentally.

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.

Although Earth remains the only place where life is known,the science ofastrobiologyseeks evidence of life on other planets. The 2015 NASA strategy on the origin of life aimed to solve the puzzle by identifying interactions, intermediary structures and functions, energy sources, and environmental factors that contributed to the diversity, selection, and replication of evolvable macromolecular systems,and mapping the chemical landscape of potential primordial informationalpolymers.The advent of polymers that could replicate, store genetic information, and exhibit properties subject to selection was, it suggested, most likely a critical step in theemergenceof prebiotic chemical evolution.Those polymers derived, in turn, from simpleorganic compoundssuch asnucleobases,amino acids,andsugarsthat could have been formed by reactions in the environment.A successful theory of the origin of life must explain how all these chemicals came into being.

Pre-1960s conceptual history

Main article:History of research into the origin of life

TheMiller–Urey experimentwas a synthesis of small organic molecules in a mixture of simple gases in a thermal gradient created by heating (right) and cooling (left) the mixture at the same time, with electrical discharges.

Spontaneous generation

Main article:Spontaneous generation

One ancient view of the origin of life, fromAristotleuntil the 19th century, is ofspontaneous generation.This theory held that "lower" animals such as insects were generated by decaying organic substances, and that life arose by chance.This was questioned from the 17th century, in works likeThomas Browne'sPseudodoxia Epidemica.In 1665,Robert Hookepublished the first drawings of amicroorganism.In 1676,Antonie van Leeuwenhoekdrew and described microorganisms, probablyprotozoaandbacteria.Van Leeuwenhoek disagreed with spontaneous generation, and by the 1680s convinced himself, using experiments ranging from sealed and open meat incubation and the close study of insect reproduction, that the theory was incorrect.In 1668Francesco Redishowed that nomaggotsappeared in meat when flies were prevented from laying eggs.By the middle of the 19th century, spontaneous generation was considered disproven.

Panspermia

Main article:Panspermia

Another ancient idea dating back toAnaxagorasin the 5th century BC ispanspermia,the idea thatlifeexists throughout theuniverse,distributed bymeteoroids,asteroids,cometsandplanetoids.It does not attempt to explain how life originated in itself, but shifts the origin of life on Earth to another heavenly body. The advantage is that life is not required to have formed on each planet it occurs on, but rather in a more limited set of locations, or even a single location, and then spread about thegalaxyto other star systems via cometary or meteorite impact.Panspermia did not get much scientific support because it was largely used to deflect the need of an answer instead of explaining observable phenomena. Although the interest in panspermia grew when the study of meteorites found traces of organic materials in them, it is currently accepted that life started locally on Earth.

"A warm little pond": primordial soup

Main article:Primordial soup

The idea that life originated from non-living matter in slow stages appeared inHerbert Spencer's 1864–1867 bookPrinciples of Biology,and inWilliam Turner Thiselton-Dyer's 1879 paper "On spontaneous generation and evolution". On 1 February 1871Charles Darwinwrote about these publications toJoseph 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 ofammoniaand phosphoricsalts,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. "

Alexander Oparinin 1924 andJ. B. S. Haldanein 1929 proposed that the first molecules constituting the earliest cells slowly self-organized from aprimordial soup,and this theory is called theOparin–Haldane hypothesis.Haldane suggested that the Earth's prebiotic oceans consisted of a "hot dilute soup" in which organic compounds could have formed.J. D. Bernalshowed that such mechanisms could form most of the necessary molecules for life from inorganic precursors.In 1967, he suggested three "stages": the origin of biologicalmonomers;the origin of biologicalpolymers;and the evolution from molecules to cells.

Miller–Urey experiment

Main article:Miller–Urey experiment

In 1952,Stanley MillerandHarold Ureycarried out a chemical experiment to demonstrate how organic molecules could have formed spontaneously from inorganic precursors underprebiotic conditionslike those posited by the Oparin–Haldane hypothesis. It used a highlyreducing(lacking oxygen) mixture of gases—methane,ammonia,andhydrogen,as well aswater vapor—to form simple organic monomers such asamino acids.Bernal said of the Miller–Urey experiment that "it is not enough to explain the formation of such molecules, what is necessary, is a physical-chemical explanation of the origins of these molecules that suggests the presence of suitable sources and sinks for free energy."However, current scientific consensus describes the primitive atmosphere as weakly reducing or neutral,diminishing the amount and variety of amino acids that could be produced. The addition ofironandcarbonateminerals, present in early oceans, however, produces a diverse array of amino acids.Later work has focused on two other potential reducing environments:outer spaceand deep-sea hydrothermal vents.

Producing a habitable Earth

Evolutionary history

Early universe with first stars

See also:Chronology of the universe

Soon after theBig Bang,which occurred roughly 14 Gya, the only chemical elements present in the universe werehydrogen,helium,andlithium,the three lightest atoms in the periodic table. These elements gradually accreted and began orbiting in disks of gas and dust. Gravitational accretion of material at the hot and dense centers of theseprotoplanetary disksformed stars by the fusion of hydrogen.Early stars were massive and short-lived, producing all the heavier elements throughstellar nucleosynthesis.Element formation through stellar nucleosynthesis proceeds to its most stable element Iron-56.Heavier elements were formed during supernovae at the end of a stars lifecycle.Carbon,currently thefourth most abundant chemical elementin the universe (after hydrogen, helium, andoxygen), was formed mainly inwhite dwarf stars,particularly those bigger than twice the mass of the sun.As these stars reached the end of theirlifecycles,they ejected these heavier elements, among them carbon and oxygen, throughout the universe. These heavier elements allowed for the formation of new objects, including rocky planets and other bodies.According to thenebular hypothesis,the formation and evolution of theSolar Systembegan 4.6 Gya with thegravitational collapseof a small part of a giantmolecular cloud.Most of the collapsing mass collected in the center, forming theSun,while the rest flattened into aprotoplanetary diskout of which theplanets,moons,asteroids,and other small Solar System bodies formed.

Emergence of Earth

See also:Geological history of Earth,Circumstellar habitable zone,andPrebiotic atmosphere

The age of theEarthis 4.54 Gya as found by radiometric dating ofcalcium-aluminium-rich inclusionsincarbonaceous chrondritemeteorites, the oldest material in the Solar System.TheHadeanEarth (from its formation until 4 Gya) was at first inhospitable to any living organisms. During its formation, the Earth lost a significant part of its initial mass, and consequentially lacked thegravityto hold molecular hydrogen and the bulk of the original inert gases.Soon after initial accretion of Earth at 4.48 Ga, its collision withTheia,a hypothesised impactor, is thought to have created the ejected debris that would eventually form the Moon.This impact would have removed the Earth's primary atmosphere, leaving behind clouds of viscous silicates and carbon dioxide. This unstable atmosphere was short-lived and condensed shortly after to form the bulk silicate Earth, leaving behind an atmosphere largely consisting of water vapor,nitrogen,andcarbon dioxide,with smaller amounts ofcarbon monoxide,hydrogen, andsulfurcompounds.The solution of carbon dioxide in water is thought to have made the seas slightlyacidic,with apHof about 5.5.

Condensation to form liquidoceansis theorised to have occurred as early as the Moon-forming impact.This scenario has found support from the dating of 4.404 Gyazirconcrystals with highδ¹⁸Ovalues from metamorphosedquartziteofMount Narryerin Western Australia.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.The post-Moon-forming impact Earth likely existed with little if any continental crust, a turbulent atmosphere, and ahydrospheresubject to intenseultravioletlight from aT Tauri stage Sun,fromcosmic radiation,and from continued asteroid andcometimpacts.Despite all this, niche environments likely existed conducive to life on Earth in the Late-Hadean to Early-Archaean.

TheLate Heavy Bombardmenthypothesis posits that a period of intense impact occurred at ~3.9 Gya during the Hadean.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.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.The 3.9 Ga date arises from dating ofApollo mission sample returnscollected mostly near theImbrium Basin,biasing the age of recorded impacts.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.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.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 deephydrothermal ventsthan theMiller–Urey experimentsindicated.The available energy is maximized at 100–150 °C, the temperatures at whichhyperthermophilicbacteria andthermoacidophilicarchaealive.

Earliest evidence of life

Main article:Earliest known life forms

The exact timing at which life emerged on Earth is unknown. Minimum age estimates are based on evidence from thegeologic rock record.The earliest physical evidence of life so far found consists ofmicrobialitesin theNuvvuagittuq Greenstone Beltof Northern Quebec, inbanded iron formationrocks 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 Gyaformation of oceansduring the Hadean. The microbes resembled modern hydrothermal vent bacteria, supporting the view that abiogenesis began in such an environment.

Biogenicgraphitehas been found in 3.7 Gya metasedimentary rocks from southwesternGreenlandand inmicrobial matfossils from 3.49 Gya cherts in thePilbararegion ofWestern Australia.Evidence of early life in rocks fromAkiliaIsland, near theIsua supracrustal beltin southwestern Greenland, dating to 3.7 Gya, have shown biogeniccarbon isotopes.In other parts of the Isua supracrustal belt, graphite inclusions trapped withingarnetcrystals are connected to the other elements of life: oxygen, nitrogen, and possibly phosphorus in the form ofphosphate,providing further evidence for life 3.7 Gya.In thePilbararegion of Western Australia, compelling evidence of early life was found inpyrite-bearing sandstone in a fossilized beach, with rounded tubular cells that oxidized sulfur by photosynthesis in the absence of oxygen.Carbon isotope ratios on graphite inclusions from the Jack Hills zircons suggest that life could have existed on Earth from 4.1 Gya.

The Pilbara region of Western Australia contains theDresser Formationwith rocks 3.48 Gya, including layered structures calledstromatolites.Their modern counterparts are created by photosynthetic micro-organisms includingcyanobacteria.These lie within undeformed hydrothermal-sedimentary strata; their texture indicates a biogenic origin. Parts of the Dresser formation preservehot springson land, but other regions seem to have been shallow seas.A molecular clock analysis suggests the LUCA emerged prior to the Late Heavy Bombardment (3.9 Gya).

• 
  Stromatolitesin the Siyeh Formation,Glacier National Park,dated 3.5 Gya, placing them among the earliest life-forms
• 
  Modern stromatolites inShark Bay,created by photosyntheticcyanobacteria

Producing molecules: prebiotic synthesis

Further information:Nucleosynthesis

Allchemical elementsexcept for hydrogen and helium derive from stellar nucleosynthesis. The basic chemical ingredients of life – thecarbon-hydrogen molecule(CH), the carbon-hydrogen positive ion (CH+) and the carbon ion (C+) – were produced byultraviolet lightfrom stars.Complex molecules, including organic molecules, form naturally both in space and on planets.Organic molecules on the early Earth could have had either terrestrial origins, with organic molecule synthesis driven by impact shocks or by other energy sources, such as ultraviolet light,redoxcoupling, or electrical discharges; or extraterrestrial origins (pseudo-panspermia), with organic molecules formed ininterstellar dust cloudsraining down on to the planet.

Observed extraterrestrial organic molecules

See also:List of interstellar and circumstellar moleculesandPseudo-panspermia

An organic compound is a chemical whose molecules contain carbon. Carbon is abundant in the Sun, stars, comets, and in theatmospheresof most planets.Organic compounds are relatively common in space, formed by "factories of complex molecular synthesis" which occur in molecular clouds andcircumstellar envelopes,and chemically evolve after reactions are initiated mostly byionizing radiation.Purineandpyrimidinenucleobases includingguanine,adenine,cytosine,uracil,andthyminehave been found inmeteorites.These could have provided the materials for DNA andRNAto form on theearly Earth.The amino acidglycinewas found in material ejected from cometWild 2;it had earlier been detected in meteorites.Comets are encrusted with dark material, thought to be atar-like organic substance formed from simple carbon compounds under ionizing radiation. A rain of material from comets could have brought such complex organic molecules to Earth.It is estimated that during the Late Heavy Bombardment, meteorites may have delivered up to five milliontonsof organic prebiotic elements to Earth per year.

PAH world hypothesis

Main article:PAH world hypothesis

TheCat's Paw Nebulais inside theMilky Way Galaxy,in theconstellationScorpius.
Green areas show regions where radiation from hot stars collided with large molecules and small dust grains called "polycyclic aromatic hydrocarbons"(PAHs), causing them tofluoresce.Spitzer Space Telescope,2018.

Polycyclic aromatic hydrocarbons(PAH) are the most common and abundant polyatomic molecules in theobservable universe,and are a major store of carbon.They seem to have formed shortly after the Big Bang,and are associated withnew starsandexoplanets.They are a likely constituent of Earth's primordial sea.PAHs have been detected innebulae,and in theinterstellar medium,in comets, and in meteorites.

The PAH world hypothesis posits PAHs as precursors to the RNA world.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 throughhydrogenation,oxygenation,andhydroxylationto more complex organic compounds used in living cells.

Nucleobases and Nucleotides

Further information:NucleobaseandNucleotide

The majority of organic compounds introduced on Earth by interstellar dust particles have helped to form complex molecules, thanks to their peculiarsurface-catalyticactivities.Studies of the¹²C/¹³Cisotopic ratiosof organic compounds in the Murchison meteorite suggest that the RNA component uracil and related molecules, includingxanthine,were formed extraterrestrially.NASA studies of meteorites suggest that all four DNA nucleobases (adenine, guanine and related organic molecules) have been formed in outer space.Thecosmic dustpermeating the universe contains complex organics ( "amorphous organic solids with a mixedaromatic–aliphaticstructure ") that could be created rapidly by stars.Glycolaldehyde, a sugar molecule and RNA precursor, has been detected in regions of space including aroundprotostarsand on meteorites.

Laboratory synthesis

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 andJoan Oróexperiments.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.Formamide is attractive as a medium that potentially provided a source of amino acid derivatives from simple aldehyde and nitrile feedstocks.

Sugars

The Breslow catalytic cycle for formaldehyde dimerization and C2-C6 sugar formation

Alexander Butlerovshowed in 1861 that theformose reactioncreated sugars including tetroses, pentoses, and hexoses whenformaldehydeis heated under basic conditions with divalent metal ions like calcium. R. Breslow proposed that the reaction was autocatalytic in 1959.

Nucleobases

Nucleobases, such as guanine and adenine, can be synthesized from simple carbon and nitrogen sources, such ashydrogen cyanide(HCN) and ammonia.Formamideproduces 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.HCN is poisonous only toaerobic organisms(eukaryotesand aerobic bacteria), which did not yet exist. It can play roles in other chemical processes such as the synthesis of the amino acid glycine.

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 inred giantstars or in interstellar dust and gas clouds.All four RNA-bases may be synthesized from formamide in high-energy density events like extraterrestrial impacts.

Other pathways for synthesizing bases from inorganic materials have been reported.Freezing temperatures are advantageous for the synthesis of purines, due to the concentrating effect for key precursors such as hydrogen cyanide.However, while adenine and guanine require freezing conditions for synthesis, cytosine and uracil may require boiling temperatures.Seven amino acids and eleven types of nucleobases formed in ice when ammonia andcyanidewere left in a freezer for 25 years.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.The explanation given for the unusual speed of these reactions at such a low temperature iseutectic freezing,which crowds impurities in microscopic pockets of liquid within the ice, causing the molecules to collide more often.

Peptides

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.

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.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.

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).

An experiment reported in 2024 used a saffire substrate with a web of thin cracks under a heat flow, similar to the environment ofdeep-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.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.

While all of these scenarios involve the condensation of amino acids, the prebiotic synthesis of peptides from simpler molecules such as CO, NH₃and C, skipping the step of amino acid formation, is very efficient.

Producing suitable vesicles

Further information:Gard model,Self-organization § Biology,andCellularization

The three main structures composed ofphospholipidsform spontaneously byself-assemblyin solution: theliposome(a closed bilayer), themicelleand the bilayer.

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. Thelipid worldtheory postulates that the first self-replicating object waslipid-like.Phospholipids formlipid bilayersin water while under agitation—the same structure as in cell membranes. These molecules were not present on early Earth, but otheramphiphiliclong-chain molecules also form membranes. These bodies may expand by insertion of additional lipids, and may spontaneously split into twooffspringof 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.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,a positivefeedbackcomposed 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 distinctlineagesof vesicles, which would have allowednatural selection.

Aprotocellis a self-organized, self-ordered, spherical collection of lipids proposed as a stepping-stone to the origin of life.A functional protocell has (as of 2014) not yet been achieved in a laboratory setting.Self-assembledvesiclesare essential components of primitive cells.The theory of classical irreversible thermodynamics treats self-assembly under a generalized chemical potential within the framework ofdissipative systems.Thesecond law of thermodynamicsrequires that overallentropyincreases, yet life is distinguished by its great degree of organization. Therefore, a boundary is needed to separate orderedlife processesfrom chaotic non-living matter.

Irene Chen andJack W. Szostaksuggest that elementary protocells can give rise to cellular behaviors including primitive forms of differential reproduction, competition, and energy storage.Competition for membrane molecules would favor stabilized membranes, suggesting a selective advantage for the evolution of cross-linked fatty acids and even thephospholipidsof today.Suchmicro-encapsulationwould 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 ownelectrochemical gradientto store energy by pumping ions across the membrane.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.

Laboratory experiments have shown that vesicles can undergo an evolutionary process under pressure cycling conditions.Simulating the systemic environment in tectonicfault zoneswithin theEarth's crust,pressure cycling leads to the periodic formation of vesicles.Under the same conditions, randompeptidechains 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,associated with an increase in the survival rate of the vesicles.

Producing biology

Energy and entropy

Further information:Entropy

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 ofcomplexity.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).

Multiple sources of energy were available for chemical reactions on the early Earth. Heat fromgeothermalprocesses is a standard energy source for chemistry. Other examples include sunlight, lightning,atmospheric entries of micro-meteorites,and implosion of bubbles in sea and ocean waves.This has been confirmed by experimentsand simulations. Unfavorable reactions can be driven by highly favorable ones, as in the case of iron-sulfur chemistry. For example, this was probably important forcarbon fixation.Carbon fixation by reaction of CO₂with H₂S via iron-sulfur chemistry is favorable, and occurs at neutral pH and 100 °C. Iron-sulfur surfaces, which are abundant near hydrothermal vents, can drive the production of small amounts of amino acids and other biomolecules.

Chemiosmosis

Further information:Chemiosmosis

ATP synthaseuses the chemiosmotic proton gradient to power ATP synthesis throughoxidative phosphorylation.

In 1961,Peter Mitchellproposedchemiosmosisas a cell's primary system of energy conversion. The mechanism, now ubiquitous in living cells, powers energy conversion in micro-organisms and in themitochondriaof eukaryotes, making it a likely candidate for early life.Mitochondria produceadenosine triphosphate(ATP), the energy currency of the cell used to drive cellular processes such as chemical syntheses. The mechanism of ATP synthesis involves a closed membrane in which theATP synthaseenzyme is embedded. The energy required to release strongly bound ATP has its origin inprotonsthat move across the membrane.In modern cells, those proton movements are caused by the pumping of ions across the membrane, maintaining an electrochemical gradient. In the first organisms, the gradient could have been provided by the difference in chemical composition between the flow from a hydrothermal vent and the surrounding seawater,or perhaps meteoric quinones that were conducive to the development of chemiosmotic energy across lipid membranes if at a terrestrial origin.

Chemiosmoticcoupling in the membranes of amitochondrion

The RNA world

Main article:RNA world

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

RNA is central to the translation process. Small RNAs can catalyze all the chemical groups and information transfers required for life.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 theribosomehas been called the "smoking gun", with a central core of RNA and no amino acid side chains within 18Åof theactive sitethat catalyzes peptide bond formation.

The concept of the RNA world was proposed in 1962 byAlexander Rich,and the term was coined byWalter Gilbertin 1986.There were initial difficulties in the explanation of the abiotic synthesis of the nucleotides cytosine and uracil.Subsequent research has shown possible routes of synthesis; for example, formamide produces all fourribonucleotidesand other biological molecules when warmed in the presence of various terrestrial minerals.

The RNA world hypothesis proposes that undirected polymerisation led to the emergence ofribozymes,and in turn to anRNA replicase.

RNA replicasecan function as both code and catalyst for further RNA replication, i.e. it can be autocatalytic.Jack Szostakhas shown that certain catalytic RNAs can join smaller RNA sequences together, creating the potential for self-replication. The RNA replication systems, which include two ribozymes that catalyze each other's synthesis, showed a doubling time of the product of about one hour, and were subject to natural selection under the experimental conditions.If such conditions were present on early Earth, then natural selection would favor the proliferation of suchautocatalytic sets,to which further functionalities could be added.Self-assembly of RNA may occur spontaneously in hydrothermal vents.A preliminary form of tRNA could have assembled into such a replicator molecule.

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.

Eugene Kooninhas 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."

From RNA to directed protein synthesis

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.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.Aminoacylation ribozymes that can charge tRNAs with their cognate amino acids have also been selected in in vitro experimentation.The authors also extensively mapped fitness landscapes within their selection to find that chance emergence of active sequences was more important that sequence optimization.

Early functional peptides

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.This could have had some effect on early protein sequence diversity. In other work by Anthony Keefe and Jack Szostak,mRNA displayselection on a library of 6*10¹²80-mers was used to search for sequences with ATP binding activity. They concluded that approximately 1 in 10¹¹random sequences had ATP binding function.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.

Phylogeny and LUCA

Further information:Last universal common ancestor

Starting with the work ofCarl Woesefrom 1977,genomicsstudies have placed the last universal common ancestor (LUCA) of all modern life-forms between Bacteria and a clade formed by Archaea andEukaryotain the phylogenetic tree of life. It lived over 4 Gya.A minority of studies have placed the LUCA in Bacteria, proposing that Archaea and Eukaryota are evolutionarily derived from within Eubacteria;Thomas Cavalier-Smithsuggested in 2006 that the phenotypically diverse bacterial phylumChloroflexotacontained the LUCA.

• 
  Phylogenetic treeshowing thelast universal common ancestor(LUCA) at the root. The major clades are theBacteriaon one hand, and theArchaeaandEukaryotaon 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 wasanaerobicwith a Wood–Ljungdahl (reductive Acetyl-CoA) pathway, nitrogen- and carbon-fi xing, thermophilic. Itscofactorssuggest dependence upon an environment rich in hydrogen, carbon dioxide, iron, andtransition 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.The physiology of LUCA has been in dispute.

• 
  LUCA systems and environment included theWood–Ljungdahl pathway.

Leslie Orgel argued that early translation machinery for the genetic code would be susceptible toerror catastrophe.Geoffrey Hoffmann however showed that such machinery can be stable in function against "Orgel's paradox".Metabolic reactions that have also been inferred in LUCA are the incompletereverse Krebs cycle,gluconeogenesis,thepentose phosphate pathway,glycolysis,reductive amination,andtransamination.

Suitable geological environments

Further information:Alternative abiogenesis scenarios

A variety ofgeologic and environmental settings have been proposedfor 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 fromLUCA.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.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.

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.

Deep sea hydrothermal vents

Hot fluids

Further information:Hydrothermal vent

Theearliest known life formsare putative fossilizedmicroorganisms,found in white smokerhydrothermal ventprecipitates. They may have lived as early as 4.28 Gya (billion years ago), relatively soon after theformation of the oceans4.41 Gya, not long after theformation of the Earth4.54 Gya.

Early micro-fossils may have come from a hot world of gases such as methane, ammonia, carbon dioxide, andhydrogen sulfide,toxic to much current life.Analysis of thetree of lifeplaces thermophilic and hyperthermophilic bacteria and archaea closest to the root, suggesting that life may have evolved in a hot environment.The deep sea or alkaline hydrothermal vent theory posits that life began at submarine hydrothermal vents.William MartinandMichael Russellhave 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".

These form where hydrogen-rich fluids emerge from below the sea floor, as a result ofserpentinizationof ultra-maficolivinewith seawater and a pH interface with carbon dioxide-rich ocean water. The vents form a sustained chemical energy source derived from redox reactions, in which electron donors (molecular hydrogen) react with electron acceptors (carbon dioxide); seeiron–sulfur world theory.These areexothermic reactions.

Chemiosmotic gradient

Further information:Hydrothermal ventandChemiosmosis § Emergence of chemiosmosis

Early cell powered by external proton gradient near a deep-sea hydrothermal vent. As long as the membrane (or passive ion channels within it) is permeable to protons, the mechanism can function without ion pumps.

Russell demonstrated that alkaline vents created an abiogenicproton motive forcechemiosmotic gradient,ideal for abiogenesis. Their microscopic compartments "provide a natural means of concentrating organic molecules," composed of iron-sulfur minerals such asmackinawite,endowed these mineral cells with the catalytic properties envisaged byGünter Wächtershäuser.This movement of ions across the membrane depends on a combination of two factors:

1. Diffusionforce caused by concentration gradient—all particles including ions tend to diffuse from higher concentration to lower.
2. Electrostatic force caused by electrical potential gradient—cationslikeprotonsH⁺tend to diffuse down the electrical potential,anionsin 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).

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 asmethanol(CH₃OH) andformic,acetic,andpyruvic acidsout of the dissolved CO₂in the water, if driven by an applied voltage or by reaction with H₂or H₂S.

The research reported by Martin in 2016 supports the thesis that life arose at hydrothermal vents,that spontaneous chemistry in the Earth's crust driven by rock–water interactions at disequilibrium thermodynamically underpinned life's originand that the founding lineages of the archaea and bacteria were H₂-dependent autotrophs that used CO₂as their terminal acceptor in energy metabolism.Martin suggests, based upon this evidence, that the LUCA "may have depended heavily on the geothermal energy of the vent to survive".Pores at deep sea hydrothermal vents are suggested to have been occupied by membrane-bound compartments which promoted biochemical reactions.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.

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 whichhyperthermophilicbacteria andthermoacidophilicarchaealive.Arguments against a hydrothermal origin of life state that hyperthermophily was a result ofconvergent evolutionin bacteria and archaea, and that amesophilicenvironment would have been more likely.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.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.

Arguments against a vent setting

Production of prebiotic organic compounds at hydrothermal vents is estimated to be 1x10⁸kg yr⁻¹.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 aMiller-Urey Experimentsurface atmosphere.

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.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.

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,vesicle formation from a broader, more realistic mix of fatty-acid and 1-alkanol species is more resilient.

Surface bodies of water

Surface bodies of water provide environments able to dry out and be rewetted. Continued wet-dry cycles allow the concentration of prebiotic compounds andcondensation reactionsto polymerise macromolecules. Moreover, lake and ponds on land allow for detrital input from the weathering of continental rocks which containapatite,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.Another line of evidence for a surface start to life is the requirement forUVfor organism function. UV is necessary for the formation of the U+C nucleotidebase pairby partialhydrolysisand nucleobase loss.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 noozone 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.

Hot springs

Most branching phylogenies are thermophilic or hyperthermophilic, making it possible that theLast 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.

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.Mineral deposits in these environments under an anoxic atmosphere would have suitable pH (while current pools in an oxygenated atmosphere would not), contain precipitates of photocatalytic sulfide minerals that absorb harmful ultraviolet radiation, have wet-dry cycles that concentrate substrate solutions to concentrations amenable to spontaneous formation of biopolymerscreated both by chemical reactions in the hydrothermal environment, and by exposure toUV lightduring transport from vents to adjacent pools that would promote the formation of biomolecules.The hypothesized pre-biotic environments are similar to hydrothermal vents, with additional components that help explain peculiarities of the LUCA.

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.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.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.

Temperate surface bodies of water

Amesophilicstart in surface bodies of waters hypothesis has evolved from Darwin's concept of a 'warm little pond' and theOparin-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.

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

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.Thereverse gyrasetopoisomeraseis found exclusively in thermophiles and hyperthermophiles as it allows for coiling of DNA.The reverse gyrase enzyme requiresATPto 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.

Icy surface bodies of water

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.

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.Near-seawater levels of sodium chloride are found to destabilize fatty acid membrane self-assembly, making freshwater settings appealing for early membranous life.

Icy environments would trade the faster reaction rates that occur in warm environments for increased stability and accumulation of larger polymers.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.In anRNA 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 inpsychrophilicorganisms is slower, the process is more successful as hydrolysis is also slower. Shorter nucleotides would not suffer from higher temperatures.

Inside the continental crust

An alternative geological environment has been proposed by the geologist Ulrich Schreiber and the physical chemist Christian Mayer: thecontinental crust.Tectonic faultzones 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 andsupercritical.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 forphase 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).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 thephase transitionpoint between thesupercriticaland the gaseous state. This leads to a natural accumulation zone forlipophilic organic moleculesthat dissolve well insupercritical CO₂,but not in its gaseous state, leading to their local precipitation.Periodic pressure variations such as caused bygeyser activityortidal influencesresult in periodic phase transitions, keeping the local reaction environment in a constantnon-equilibrium state.In presence ofamphiphilic compounds(such as the long chain amines and fatty acids mentioned above), subsequent generations of vesicles are being formedthat are constantly and efficiently being selected for their stability.The resulting structures could provide hydrothermal vents as well as hot springs with raw material for further development.

Homochirality

Main article:Homochirality

Many biomolecules, such asL-glutamic acid,areasymmetric,and occur in living systems in only one of the two possible forms, in the case ofamino acidsthe left-handed form. Prebiotic chemistry would produce both forms, creating a puzzle for abiogenesis researchers.

Homochirality is the geometric uniformity of materials composed ofchiral(non-mirror-symmetric) units. Living organisms use molecules that have the same chirality (handedness): with almost no exceptions,amino acids are left-handed while nucleotides andsugarsare 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 bothforms.Known mechanisms for the production of non-racemic mixtures from racemic starting materials include: asymmetric physical laws, such as theelectroweak interaction;asymmetric environments, such as those caused bycircularly polarizedlight,quartz crystals,or the Earth's rotation,statistical fluctuationsduring racemic synthesis,andspontaneous symmetry breaking.

Once established, chirality would be selected for.A small bias (enantiomeric excess) in the population can be amplified into a large one byasymmetric autocatalysis,such as in theSoai reaction.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.

Homochirality may have started in outer space, as on theMurchison meteoritethe amino acidL-alanine(left-handed) is more than twice as frequent as its D (right-handed) form, andL-glutamic acidis more than three times as abundant as its D counterpart.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.

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".

See also

• 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

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External links

• Making headway with the mysteries of life's origins– Adam Mann (PNAS;14 April 2021)
• Exploring Life's OriginsArchived8 April 2023 at theWayback Machinea virtual exhibit at theMuseum of Science (Boston)
• How life began on Earth– Marcia Malory (Earth Facts; 2015)
• The Origins of Life–Richard Dawkinset al. (BBC Radio;2004)
• Life in the Universe– Essay byStephen Hawking(1996)