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Abiogenesis

Precambrian stromatolites in the SiyehFormation, Glacier National Park. In 2002, apaper in the scientific journal Nature suggestedthat these 3.5 Ga (billion years old) geologicalformations contain fossilized cyanobacteriamicrobes. This suggests they are evidence ofone of the earliest known life forms on Earth.

From Wikipedia, the free encyclopedia

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

Abiogenesis (/ˌeɪbaɪ.ɵˈdʒɛnɨsɪs/ AY-by-oh-JEN-ə-siss[1]) or biopoiesis[2] is the natural process bywhich life arose from non-living matter such assimple organic compounds.[3][4][5][6] The Earth wasformed about 4.54 billion years ago. The earliestlife on Earth existed at least 3.5 billion yearsago,[7][8][9] during the Eoarchean Era whensufficient crust had solidified following the moltenHadean Eon. The earliest physical evidence for lifeon Earth is biogenic graphite in 3.7 billion-year-oldmetasedimentary rocks discovered in WesternGreenland[10] and microbial mat fossils found in3.48 billion-year-old sandstone discovered inWestern Australia.[11][12] Nevertheless, severalstudies suggest that life on Earth may have startedeven earlier,[13] as early as 4.25 billion years ago

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according to one study,[14] and even earlier yet, 4.4 billion years ago, according to anotherstudy.[15]

Scientific hypotheses about the origins of life can be divided into three main stages: thegeophysical, the chemical and the biological.[16] Many approaches investigate how self-replicatingmolecules or their components came into existence. On the assumption that life originatedspontaneously on Earth, the Miller–Urey experiment and similar experiments demonstrated thatmost amino acids, often called "the building blocks of life", can be racemically synthesized inconditions which were intended to be similar to those of the early Earth. Several mechanisms havebeen investigated, including lightning and radiation. Other approaches ("metabolism first"hypotheses) focus on understanding how catalysis in chemical systems in the early Earth mighthave provided the precursor molecules necessary for self-replication.[17][18]

Contents [hide]

1 Early geophysical conditions

1.1 The earliest biological evidence for life on Earth

2 Current models

3 Chemical origin of organic molecules

3.1 Chemical synthesis

3.2 Autocatalysis

3.3 Homochirality

4 Reproduction, Duplication and the RNA world

4.1 RNA synthesis and replication

4.2 Pre-RNA world

4.3 Protocells

5 Origin of biological metabolism

5.1 Iron-sulfur world

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5.2 Zn-World hypothesis

5.3 Deep sea vent hypothesis

5.4 Thermosynthesis

6 Other models of abiogenesis

6.1 Clay hypothesis

6.2 Gold's "deep-hot biosphere" model

6.3 Primitive extraterrestrial life

6.4 Extraterrestrial organic molecules

6.5 Lipid world

6.6 Polyphosphates

6.7 PAH world hypothesis

6.8 Radioactive beach hypothesis

6.9 Ultraviolet and temperature-assisted replication model

6.10 Multiple genesis

7 Conceptual history

7.1 Spontaneous generation

7.2 Alternatives to chance: biogenesis

7.3 Pasteur and Darwin

7.4 "Primordial soup" hypothesis

7.5 Proteinoid microspheres

8 More recent theories

9 See also

10 References

11 Notes

12 Further reading

13 External linksEdit links

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Early geophysical conditions [edit]

Main article: Timeline of evolution

Based on recent computer model studies, the complex organic molecules necessary for life mayhave formed in the protoplanetary disk of dust grains surrounding the Sun before the formation ofthe Earth.[19] According to the computer studies, this same process may also occur around otherstars that acquire planets.[19] (Also see Extraterrestrial organic molecules).

The Hadean Earth is thought to have had a secondary atmosphere, formed through degassing ofthe rocks that accumulated from planetesimal impactors. At first, it was thought that the Earth'satmosphere consisted of hydrides—methane, ammonia and water vapour—and that life beganunder such reducing conditions, which are conducive to the formation of organic molecules. Duringits formation, the Earth lost a significant part of its initial mass, with a nucleus of the heavier rockyelements of the protoplanetary disk remaining,.[20] However, based on today's volcanic evidence, itis now thought that the early atmosphere would have probably contained 60% hydrogen, 20%oxygen (mostly in the form of water vapour), 10% carbon dioxide, 5 to 7% hydrogen sulfide, andsmaller amounts of nitrogen, carbon monoxide, free hydrogen, methane and inert gases. As Earthlacked the gravity to hold any molecular hydrogen, this component of the atmosphere would havebeen rapidly lost during the Hadean period, along with the bulk of the original inert gases. Solutionof carbon dioxide in water is thought to have made the seas slightly acidic, with a pH of about5.5.[21] The atmosphere at the time has been characterized as a "gigantic, productive outdoorchemical laboratory."[22] It is similar to the mixture of gases released by volcanoes, which stillsupport some abiotic chemistry today.[22]

Oceans may have appeared first in the Hadean eon, as soon as two hundred million years (200Ma) after the Earth was formed, in a hot 100 °C (212 °F) reducing environment, and the pH of

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about 5.8 rose rapidly towards neutral.[23] This has been supported by the dating of 4.404 Ga-oldzircon crystals from metamorphosed quartzite of Mount Narryer in Western Australia, which areevidence that oceans and continental crust existed within 150 Ma of Earth's formation.[24] Despitethe likely increased vulcanism and existence of many smaller tectonic "platelets", it has beensuggested that between 4.4 and 4.3 Ga, the Earth was a water world, with little if any continentalcrust, an extremely turbulent atmosphere and a hydrosphere subject to high UV, from a T Taurisun, cosmic radiation and continued bolide impact.[25]

The Hadean environment would have been highly hazardous to modern life. Frequent collisionswith large objects, up to 500 kilometres (310 mi) in diameter, would have been sufficient to sterilisethe planet and vaporise the ocean within a few months of impact, with hot steam mixed with rockvapour becoming high altitude clouds that would completely cover the planet. After a few months,the height of these clouds would have begun to decrease but the cloud base would still have beenelevated for about the next thousand years. After that, it would have begun to rain at low altitude.For another two thousand years, rains would slowly have drawn down the height of the clouds,returning the oceans to their original depth only 3,000 years after the impact event.[26]

The earliest biological evidence for life on Earth [edit]

The earliest life on Earth existed at least 3.5 billion years ago,[7][8][9] during the Eoarchean Erawhen sufficient crust had solidified following the molten Hadean Eon. The earliest physicalevidence for life on Earth is biogenic graphite in 3.7 billion-year-old metasedimentary rocksdiscovered in Western Greenland[10] and microbial mat fossils found in 3.48 billion-year-oldsandstone discovered in Western Australia.[11][12] Gustaf Arrhenius of the Scripps Institute ofOceanography using a mass spectrometer has identified what appears to be, on the basis ofbiogenic carbon isotopes, evidence of early life, found in rocks from Akilia Island, near Isua,Greenland, dating to 3.85 billion years old.[27]

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Between 3.8 and 4.1 Ga, changes in the orbits of the gaseous giant planets may have caused alate heavy bombardment[28] that pockmarked the Moon and the other inner planets (Mercury,Mars, and presumably Earth and Venus). This would likely have repeatedly sterilized the planet,had life appeared before that time.[22] Geologically, the Hadean Earth would have been far moreactive than at any other time in its history. Studies of meteorites suggests that radioactive isotopessuch as aluminium-26 with a half-life of 7.17×105 years, and potassium-40 with a half-life of1.250×109 years, isotopes mainly produced in supernovae, were much more common.[29] Coupledwith internal heating as a result of gravitational sorting between the core and the mantle therewould have been a great deal of mantle convection, with the probable result of many more smallerand very active tectonic plates than in modern times.

By examining the time interval between such devastating environmental events, the time intervalwhen life might first have come into existence can be found for different early environments. Astudy by Maher and Stevenson shows that if the deep marine hydrothermal setting provides asuitable site for the origin of life, abiogenesis could have happened as early as 4.0 to 4.2 Ga,whereas if it occurred at the surface of the Earth, abiogenesis could only have occurred between3.7 and 4.0 Ga.[30]

Further evidence of the early appearance of life comes from the Isua supercrustal belt in WesternGreenland and from similar formations in the nearby Akilia Island. Isotopic fingerprints typical oflife, preserved in the sediments, have been used to suggest that life existed on the planet alreadyby 3.85 billion years ago.[31] Christian de Duve argues that the determination of chemistry meansthat "life has to emerge quickly ... Chemical reactions happen quickly or not at all; if any reactiontakes a millennium to complete then the chances are all the reagents will simply dissipate or breakdown in the meantime, unless they are replenished by other faster reactions".[32][33]

Current models [edit]

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There is still no "standard model" of the origin of life. Most currently accepted models draw at leastsome elements from the framework laid out by Alexander Oparin (in 1924) and John Haldane (in1925), who postulated the molecular or chemical evolution theory of life.[34] According to them, thefirst molecules constituting the earliest cells "were synthesized under natural conditions by a slowprocess of molecular evolution, and these molecules then organized into the first molecular systemwith properties with biological order."[34] Oparin and Haldane suggested that the atmosphere ofthe early Earth may have been chemically reducing in nature, composed primarily of methane(CH4), ammonia (NH3), water (H2O), hydrogen sulfide (H2S), carbon dioxide (CO2) or carbonmonoxide (CO), and phosphate (PO4

3-), with molecular oxygen (O2) and ozone (O3) either rare orabsent. In such a reducing atmosphere, electrical activity can catalyze the creation of certain basicsmall molecules (monomers) of life, such as amino acids. This was demonstrated in the Miller–Ureyexperiment by Stanley L. Miller and Harold C. Urey reported in 1953.

John Desmond Bernal coined the term biopoiesis in 1949 to refer to the origin of life,[35] andsuggested that it occurred in three "stages": 1) the origin of biological monomers; 2) the origin ofbiological polymers; and 3) the evolution from molecules to cells. He suggested that evolutioncommenced between stage 1 and 2.[36]

The chemical processes that took place on the early Earth are called chemical evolution. BothManfred Eigen and Sol Spiegelman demonstrated that evolution, including replication, variation,and natural selection, can occur in populations of molecules as well as in organisms.[22]

Spiegelman took advantage of natural selection to synthesize Spiegelman's Monster, which had agenome with just 218 bases. Eigen built on Spiegelman's work and produced a similar system withjust 48 or 54 nucleotides.[37]

Chemical evolution was followed by the initiation of biological evolution, which led to the firstcells.[22] No one has yet synthesized a "protocell" using basic components which would have the

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necessary properties of life (the so-called "bottom-up-approach"). Without such a proof-of-principle, explanations have tended to be focused on chemosynthesis of polymers. However, someresearchers are working in this field, notably Steen Rasmussen and Jack Szostak. Others haveargued that a "top-down approach" is more feasible. One such approach, successfully attemptedby Craig Venter and others at The Institute for Genomic Research, involves engineering existingprokaryotic cells with progressively fewer genes, attempting to discern at which point the mostminimal requirements for life were reached.[38][39]

Chemical origin of organic molecules [edit]

The elements, except for hydrogen, ultimately derive from stellar nucleosynthesis. Complexmolecules, including organic molecules, form naturally both in space and on planets.[40] There aretwo possible sources of organic molecules on the early Earth:

1. Terrestrial origins – organic synthesis driven by impact shocks or by other energy sources(such as ultraviolet light, redox coupling, or electrical discharges) (e.g. Miller's experiments)

2. Extraterrestrial origins – formation of organic molecules in interstellar dust clouds andrained down on planets.[41][42][43] (See pseudo-panspermia)

Estimates of these sources suggest that the heavy bombardment before 3.5 Ga within the earlyatmosphere made available quantities of organics comparable to those produced by other energysources.[44][45]

It has been estimatedthat the Late HeavyBombardment mayalso have effectivelysterilised the Earth's

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A cladogram demonstrating extreme thermophilic bacteria and archae at thebase of the tree of life

surface to a depth oftens of metres. If lifeevolved deeper thanthis, it would have alsobeen shielded fromthe early high levels ofultraviolet radiationfrom the T Tauri stageof the sun's evolution.Simulations ofgeothermically heatedoceanic crust yield farmore organics thanthose found in the Miller-Urey experiments (see below). In the deep hydrothermal vents, EverettShock has found "there is an enormous thermodynamic drive to form organic compounds, asseawater and hydrothermal fluids, which are far from equilibrium, mix and move towards a morestable state".[46] Shock has found that the available energy is maximised at around 100 – 150degrees Celsius, precisely the temperatures at which the hyperthermophilic bacteria and archaea,at the base of the tree of life closest to the Last Universal Common Ancestor.[47]

Chemical synthesis [edit]

While features of self-organization and self-replication are often considered the hallmark of livingsystems, there are many instances of abiotic molecules exhibiting such characteristics underproper conditions. Palasek showed that self-assembly of RNA molecules can occur spontaneouslydue to physical factors in hydrothermal vents.[48] Virus self-assembly within host cells hasimplications for the study of the origin of life,[49] as it lends further credence to the hypothesis that

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life could have started as self-assembling organic molecules.[50][51]

Multiple sources of energy were available for chemical reactions on the early Earth. For example,heat (such as from geothermal processes) is a standard energy source for chemistry. Otherexamples include sunlight and electrical discharges (lightning), among others.[22] Unfavorablereactions can also be driven by highly favorable ones, as in the case of iron-sulfur chemistry. Forexample, this was probably important for carbon fixation (the conversion of carbon from itsinorganic form to an organic one).[note 1] Carbon fixation via iron-sulfur chemistry is highlyfavorable, and occurs at neutral pH and 100 °C (212 °F). Iron-sulfur surfaces, which are abundantnear hydrothermal vents, are also capable of producing small amounts of amino acids and otherbiological metabolites.[22]

Formamide produces all four ribonucleotides and other biological molecules when warmed in thepresence of various terrestrial minerals. Formamide is ubiquitous in the universe, produced by thereaction of water and HCN (hydrogen cyanide). It has several advantages as a prebiotic precursor,including the ability to easily become concentrated through the evaporation of water.[52][53]

Although HCN is poisonous, it only affects aerobic organisms (eukaryotes and aerobic bacteria). Itcan play roles in other chemical processes as well, such as the synthesis of the amino acidglycine.[22]

In 1961, it was shown that the nucleic acid purine base adenine can be formed by heatingaqueous ammonium cyanide solutions.[54] Other pathways for synthesizing bases from inorganicmaterials were also reported.[55] Leslie Orgel and colleagues have shown that freezingtemperatures are advantageous for the synthesis of purines, due to the concentrating effect forkey precursors such as hydrogen cyanide.[56] Research by Stanley Miller and colleaguessuggested that while adenine and guanine require freezing conditions for synthesis, cytosine anduracil may require boiling temperatures.[57] Research by the Miller group notes the formation of

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seven different amino acids and 11 types of nucleobases in ice when ammonia and cyanide wereleft in a freezer from 1972 to 1997.[58][59] Other work demonstrated the formation of s-triazines(alternative nucleobases), pyrimidines (including cytosine and uracil), and adenine from ureasolutions subjected to freeze-thaw cycles under a reductive atmosphere (with spark discharges asan energy source).[60] The explanation given for the unusual speed of these reactions at such alow temperature is eutectic freezing. As an ice crystal forms, it stays pure: only molecules of waterjoin the growing crystal, while impurities like salt or cyanide are excluded. These impurities becomecrowded in microscopic pockets of liquid within the ice, and this crowding causes the molecules tocollide more often.

At the time of the Miller–Urey experiment, scientific consensus was that the early Earth had a

reducing atmosphere with compounds relatively rich in hydrogen and poor in oxygen (e.g., CH4

and NH3 as opposed to CO2 and NO2). However, current scientific consensus describes theprimitive atmosphere as either weakly reducing or neutral[61][62] (see also Oxygen catastrophe).Such an atmosphere would diminish both the amount and variety of amino acids that could beproduced, although studies that include iron and carbonate minerals (thought to be present inearly oceans) in the experimental conditions have again produced a diverse array of aminoacids.[61] Other scientific research has focused on two other potential reducing environments:outer space and deep-sea thermal vents.[63][64][65]

The spontaneous formation of complex polymers from abiotically generated monomers under theconditions posited by the "soup" theory is not at all a straightforward process. Besides thenecessary basic organic monomers, compounds that would have prohibited the formation ofpolymers were formed in high concentration during the Miller–Urey and Oró experiments.[66] TheMiller–Urey experiment, for example, produces many substances that would react with the aminoacids or terminate their coupling into peptide chains.[67]

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Autocatalysis [edit]

Autocatalysts are substances that catalyze the production of themselves, and therefore are simplemolecular replicators. The simplest self-replicating chemical systems are autocatalytic, andtypically contain three components: two precursors that join together to form a product molecule,and the product molecule itself. The product molecule catalyzes the reaction by providing acomplementary template which binds to the precursors, thus bringing them together. Such systemshave been demonstrated both in biological macromolecules and in small organic molecules.[68][69]

Systems that do not proceed by template mechanisms, such as the self-reproduction of micellesand vesicles, have also been observed.[69]

In 1993, Stuart Kauffman proposed that life initially arose as autocatalytic chemical networks.[70]

British ethologist Richard Dawkins wrote about autocatalysis as a potential explanation for theorigin of life in his 2004 book The Ancestor's Tale.[71] In his book, Dawkins cites experimentsperformed by Julius Rebek and his colleagues at the Scripps Research Institute in California inwhich they combined amino adenosine and pentafluorophenyl esters with the autocatalyst aminoadenosine triacid ester (AATE). One system from the experiment contained variants of AATE whichcatalysed the synthesis of themselves. This experiment demonstrated the possibility thatautocatalysts could exhibit competition within a population of entities with heredity, which could beinterpreted as a rudimentary form of natural selection.[citation needed]

In the early 1970s, Manfred Eigen and Peter Schuster examined the transient stages between themolecular chaos and a self-replicating hypercycle in a prebiotic soup.[72] In a hypercycle, theinformation storing system (possibly RNA) produces an enzyme, which catalyzes the formation ofanother information system, in sequence until the product of the last aids in the formation of thefirst information system. Mathematically treated, hypercycles could create quasispecies, whichthrough natural selection entered into a form of Darwinian evolution. A boost to hypercycle theorywas the discovery that RNA, in certain circumstances, forms itself into ribozymes, capable of

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catalyzing their own chemical reactions.[73] The hypercycle theory requires the existence ofcomplex biochemicals such as nucleotides which are not formed under the conditions proposed bythe Miller–Urey experiment.

Geoffrey W. Hoffmann, a student of Eigen, contributed to the concept of life involving bothreplication and metabolism emerging from catalytic noise. His contributions included showing thatan early sloppy translation machinery can be stable against an error catastrophe of the type thathad been envisaged as problematical by Leslie Orgel ("Orgel's paradox")[74][75] and calculationsregarding the occurrence of a set of required catalytic activities together with the exclusion ofcatalytic activities that would be disruptive.[76]

Homochirality [edit]

Main article: Homochirality

Homochirality refers to the geometric property of some materials that are composed of chiral units.Chiral refers to nonsuperimposable 3D forms that are mirror images of one another, as are leftand right hands. Living organisms use molecules that have the same chirality ("handedness"): withsome exceptions, 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, theyare formed in a 50/50 mixture of both enantiomers. This is called a racemic mixture. Knownmechanisms for the production of non-racemic mixtures from racemic starting materials include:asymmetric physical laws, such as the electroweak interaction; asymmetric environments, such asthose caused by circularly polarized light, quartz crystals, or the Earth's rotation; and statisticalfluctuations during racemic synthesis.[77]

Once established, chirality would be selected for.[78] A small enantiomeric excess can be amplifiedinto a large one by asymmetric autocatalysis, such as in the Soai reaction.[79] In asymmetricautocatalysis, the catalyst is a chiral molecule, which means that a chiral molecule is catalysing its

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own production. An initial enantiomeric excess, such as can be produced by polarized light, thenallows the more abundant enantiomer to outcompete the other.[80]

Clark has suggested that homochirality may have started in outer space, as the studies of theamino acids on the Murchison meteorite showed L-alanine to be more than twice as frequent as itsD form, and L-glutamic acid was more than three times prevalent than its D counterpart. Variouschiral crystal surfaces can also act as sites for possible concentration and assembly of chiralmonomer units into macromolecules.[81] Compounds found on meteorites suggest that the chiralityof life derives from abiogenic synthesis, since amino acids from meteorites show a left-handedbias, whereas sugars show a predominantly right-handed bias, the same as found in livingorganisms.[82]

Reproduction, Duplication and the RNA world [edit]

Main article: RNA world hypothesis

The RNA world hypothesis describes anearly Earth with self-replicating andcatalytic RNA but no DNA or proteins. It isgenerally accepted that current life onEarth descends from an RNA world,[84]

although RNA-based life may not havebeen the first life to exist.[85][86] Thisconclusion is drawn from manyindependent lines of evidence, such as theobservations that RNA is central to thetranslation process and that small RNAs

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Atomic structure of the ribosome 30S Subunit fromThermus thermophilus.[83] Proteins are shown in blueand the single RNA chain in orange.

can catalyze all of the chemical groups andinformation transfers required forlife.[86][87] The structure of the ribosomehas been called the "smoking gun," as itshowed that the ribosome is a ribozyme,with a central core of RNA and no aminoacid side chains within 18 angstroms of theactive site where peptide bond formation iscatalyzed.[85] The concept of the RNA world was first proposed in the 1960s by Francis Crick,Leslie Orgel, and Carl Woese, and the term was coined by Walter Gilbert in 1986.[86][88]

Possible precursors for the evolution of protein synthesis include a mechanism to synthesize shortpeptide cofactors or from a mechanism for the duplication of RNA. It is likely that the ancestralribosome was composed entirely of RNA, although some roles have since been taken over byproteins. Major remaining questions on this topic include identifying the selective force for theevolution of the ribosome and determining how the genetic code arose.[89]

Eugene Koonin said, "Despite considerable experimental and theoretical effort, no compellingscenarios currently exist for the origin of replication and translation, the key processes thattogether comprise the core of biological systems and the apparent pre-requisite of biologicalevolution. The RNA World concept might offer the best chance for the resolution of this conundrumbut so far cannot adequately account for the emergence of an efficient RNA replicase or thetranslation system. The MWO (Ed.: "many worlds in one"[90]) version of the cosmological model ofeternal inflation could suggest a way out of this conundrum because, in an infinite multiverse with afinite number of distinct macroscopic histories (each repeated an infinite number of times),emergence of even highly complex systems by chance is not just possible but inevitable."[90]

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RNA synthesis and replication [edit]

The RNA world has spurred scientists to try to determine if RNA molecules could havespontaneously formed that were capable of catalyzing their own replication.[91][92][93] Evidencessuggest chemical conditions (including the presence of boron, molybdenum and oxygen) forinitially producing RNA molecules may have been better on the planet Mars than those on theplanet Earth.[91][92] If so, life-suitable molecules, originating on Mars, may have later migrated toEarth via meteor ejections.[91][92]

A number of hypotheses of modes of formation have been put forward. As of 1994, there weredifficulties in the abiotic synthesis of the nucleotides cytosine and uracil.[94] Subsequent researchhas shown possible routes of synthesis; for example, formamide produces all four ribonucleotidesand other biological molecules when warmed in the presence of various terrestrial minerals.[52][53]

Early cell membranes could have formed spontaneously from proteinoids, which are protein-likemolecules produced when amino acid solutions are heated while in the correct concentration inaqueous solution. These are seen to form micro-spheres which are observed to behave similarlyto membrane-enclosed compartments. Other possibilities include systems of chemical reactionsthat take place within clay substrates or on the surface of pyrite rocks.

Factors supportive of an important role for RNA in early life include its ability to act both to storeinformation and to catalyze chemical reactions (as a ribozyme); its many important roles as anintermediate in the expression and maintenance of the genetic information (in the form of DNA) inmodern organisms; and the ease of chemical synthesis of at least the components of the moleculeunder the conditions that approximated the early Earth. Relatively short RNA molecules have beenartificially produced in labs, which are capable of replication.[95] Such replicase RNA, whichfunctions as both code and catalyst provides its own template upon which copying can occur. JackSzostak has shown that certain catalytic RNAs can, indeed, join smaller RNA sequences together,creating the potential, in the right conditions for self-replication. If these conditions were present,

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Darwinian selection would favour the proliferation of such self-catalysing structures, to whichfurther functionalities could be added.[96] Lincoln and Joyce have identified RNA systems capableof self-sustained replication.[97] The systems, which include two ribozymes that catalyze eachother's synthesis, replicated with doubling time of about one hour, and were subject to naturalselection.[98] In evolutionary competition experiments, this led to the emergence of new systemswhich replicated more efficiently.[85] This was the first demonstration of evolutionary adaptationoccurring in a molecular genetic system.[98]

Life can be considered to have emerged when RNA chains began to express the basic conditionsnecessary for natural selection to operate as conceived by Darwin: heritability, variation of type,and competition for limited resources. Fitness of an RNA replicator (its per capita rate of increase)would likely be a function of adaptive capacities that were intrinsic (in the sense that they weredetermined by the nucleotide sequence) and the availability of resources.[99][100] The threeprimary adaptive capacities may have been (1) the capacity to replicate with moderate fidelity(giving rise to both heritability and variation of type), (2) the capacity to avoid decay, and (3) thecapacity to acquire and process resources.[99][100] These capacities would have been determinedinitially by the folded configurations of the RNA replicators that, in turn, would be encoded in theirindividual nucleotide sequences. Competitive success among different replicators would havedepended on the relative values of these adaptive capacities.

Pre-RNA world [edit]

It is possible that a different type of nucleic acid, such as PNA, TNA or GNA, was the first one toemerge as a self-reproducing molecule, to be replaced by RNA only later.[101][102] Larralde et al.,say that "the generally accepted prebiotic synthesis of ribose, the formose reaction, yieldsnumerous sugars without any selectivity."[103] and they conclude that their "results suggest thatthe backbone of the first genetic material could not have contained ribose or other sugars

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because of their instability." The ester linkage of ribose and phosphoric acid in RNA is known to beprone to hydrolysis.[104]

Pyrimidine ribonucleosides and their respective nucleotides have been prebiotically synthesised bya sequence of reactions which by-pass the free sugars, and are assembled in a stepwise fashionby using nitrogenous or oxygenous chemistries. John Sutherland has demonstrated high yieldingroutes to cytidine and uridine ribonucleotides built from small 2 and 3 carbon fragments such asglycolaldehyde, glyceraldehyde or glyceraldehyde-3-phosphate, cyanamide and cyanoacetylene.One of the steps in this sequence allows the isolation of enantiopure ribose aminooxazoline if theenantiomeric excess of glyceraldehyde is 60% or greater.[105] This can be viewed as a prebioticpurification step, where the said compound spontaneously crystallised out from a mixture of theother pentose aminooxazolines. Ribose aminooxazoline can then react with cyanoacetylene in amild and highly efficient manner to give the alpha cytidine ribonucleotide. Photoanomerization withUV light allows for inversion about the 1' anomeric centre to give the correct betastereochemistry.[106] In 2009 they showed that the same simple building blocks allow access, viaphosphate controlled nucleobase elaboration, to 2',3'-cyclic pyrimidine nucleotides directly, whichare known to be able to polymerise into RNA. This paper also highlights the possibility for thephoto-sanitization of the pyrimidine-2',3'-cyclic phosphates.[107] James Ferris's studies have shownthat clay minerals of montmorillonite will catalyze the formation of RNA in aqueous solution, byjoining activated mono RNA nucleotides to join together to form longer chains.[108] Although thesechains have random sequences, the possibility that one sequence began to non-randomlyincrease its frequency by increasing the speed of its catalysis is possible to "kick start" biochemicalevolution.

Protocells [edit]

Main article: Protocell

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The three main structuresphospholipids form spontaneouslyin solution: the liposome (a closedbilayer), the micelle and the bilayer

A protocell is self-organized, endogenously ordered, sphericalcollection of lipids proposed as a stepping-stone to the originof life.[109] A central question in evolution is how simpleprotocells first arose and began the competitive process thatdrove the evolution of life. Although a functional protocell hasnot yet been achieved in a laboratory setting, the goalappears well within reach.[110][111][112]

Self-assembled vesicles are essential components of primitivecells.[109] The second law of thermodynamics requires thatthe universe move in a direction in which disorder (or entropy)increases, yet life is distinguished by its great degree oforganization. Therefore, a boundary is needed to separate lifeprocesses from non-living matter.[113] Researchers Irene A.Chen and Jack W. Szostak (Nobel Prize in Physiology orMedicine 2009) amongst others, demonstrated that simplephysicochemical properties of elementary protocells can giverise to essential cellular behaviors, including primitive forms of Darwinian competition and energystorage. Such cooperative interactions between the membrane and encapsulated contents couldgreatly simplify the transition from replicating molecules to true cells.[111] Furthermore, competitionfor membrane molecules would favor stabilized membranes, suggesting a selective advantage forthe evolution of cross-linked fatty acids and even the phospholipids of today.[111] This micro-encapsulation allowed for metabolism within the membrane, exchange of small molecules andprevention of passage of large substances across it.[114] The main advantages of encapsulationinclude increased solubility of the cargo and storing energy in the form of a chemical gradient.

Scientists have come to conclude that life began in hydrothermal vents in the deep sea, but a

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2012 study led by Armen Mulkidjanian of Germany's University of Osnabrück, suggests that inlandpools of condensed and cooled geothermal vapour have the ideal characteristics for the origin oflife.[115] Scientists discovered in 2002 that by adding a montmorillonite clay to a solution of fattyacid micelles (lipid spheres), the clay sped up the rate of vesicles formation 100-fold.[112] So thisone mineral can get precursors (nucleotides) to spontaneously assemble into RNA and membraneprecursors to assemble into membrane.

Another protocell model is the Jeewanu. First synthesized in 1963 from simple minerals and basicorganics while exposed to sunlight, it is still reported to have some metabolic capabilities, thepresence of semipermeable membrane, amino acids, phospholipids, carbohydrates and RNA-likemolecules.[116][117] However, the nature and properties of the Jeewanu remains to be clarified.

Origin of biological metabolism [edit]

Laboratory research suggests that metabolism-like reactions could have occurred naturally inearly oceans, before the first organisms evolved.[17][18] The findings suggests that metabolismpredates the origin of life and evolved through the chemical conditions that prevailed in the worldsearliest oceans. Reconstructions in laboratories show that some of these reactions can produceRNA, and some others resemble two essential reaction cascades of metabolism: glycolysis and thepentose phosphate pathway, that provide essential precursors for nucleic acids, amino acids andlipids.[17] Following are some observed discoveries and related hypotheses.

Iron-sulfur world [edit]

Main article: Iron–sulfur world theory

Another possible answer to the polymerization conundrum was provided in the 1980s by GünterWächtershäuser, encouraged and supported by Karl R. Popper,[118][119][120] in his iron–sulfurworld theory. In this theory, he postulated the evolution of (bio)chemical pathways as fundamentals

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of the evolution of life. Moreover, he presented a consistent system of tracing today's biochemistryback to ancestral reactions that provide alternative pathways to the synthesis of organic buildingblocks from simple gaseous compounds.

In contrast to the classical Miller experiments, which depend on external sources of energy (suchas simulated lightning or ultraviolet irradiation), "Wächtershäuser systems" come with a built-insource of energy, sulfides of iron and other minerals (e.g. pyrite). The energy released from redoxreactions of these metal sulfides is not only available for the synthesis of organic molecules, butalso for the formation of oligomers and polymers. It is therefore hypothesized that such systemsmay be able to evolve into autocatalytic sets of self-replicating, metabolically active entities thatwould predate the life forms known today.[17][18] The experiment produced a relatively small yieldof dipeptides (0.4% to 12.4%) and a smaller yield of tripeptides (0.10%) although under the sameconditions, dipeptides were quickly broken down.[121]

Several models reject the idea of the self-replication of a "naked-gene" and postulate theemergence of a primitive metabolism which could provide an environment for the later emergenceof RNA replication. The centrality of the Krebs cycle to energy production in aerobic organisms,and in drawing in carbon dioxide and hydrogen ions in biosynthesis of complex organic chemicals,including amino acids and nucleotides, suggests that it was one of the first parts of the metabolismto evolve.[122] Somewhat in agreement with these notions, Mike Russell has proposed that "thepurpose of life is to hydrogenate carbon dioxide" (as part of a "metabolism-first", rather than a"genetics-first", scenario).[123][124] Physicist Jeremy England of MIT has proposed thatthermodynamically, life was bound to eventually arrive, as based on established physics, hemathematically indicates "that when a group of atoms is driven by an external source of energy(like the sun or chemical fuel) and surrounded by a heat bath (like the ocean or atmosphere), it willoften gradually restructure itself in order to dissipate increasingly more energy. This could meanthat under certain conditions, matter inexorably acquires the key physical attribute associated with

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life.".[125][126]

One of the earliest incarnations of this idea was put forward in 1924 with Alexander Oparin's notionof primitive self-replicating vesicles which predated the discovery of the structure of DNA. Variantsin the 1980s and 1990s include Günter Wächtershäuser's iron-sulfur world theory and modelsintroduced by Christian de Duve based on the chemistry of thioesters. More abstract andtheoretical arguments for the plausibility of the emergence of metabolism without the presence ofgenes include a mathematical model introduced by Freeman Dyson in the early 1980s and StuartKauffman's notion of collectively autocatalytic sets, discussed later in that decade.

Leslie Orgel summarized his analysis of the proposal by stating, "There is at present no reason toexpect that multistep cycles such as the reductive citric acid cycle will self-organize on the surfaceof FeS/FeS2 or some other mineral."[127] It is possible that another type of metabolic pathway wasused at the beginning of life. For example, instead of the reductive citric acid cycle, the "open"acetyl-CoA pathway (another one of the five recognised ways of carbon dioxide fixation in naturetoday) would be compatible with the idea of self-organisation on a metal sulfide surface. The keyenzyme of this pathway, carbon monoxide dehydrogenase/acetyl-CoA synthase harbours mixednickel-iron-sulfur clusters in its reaction centers and catalyses the formation of acetyl-CoA (whichmay be regarded as a modern form of acetyl-thiol) in a single step.

Zn-World hypothesis [edit]

The Zn-World (zinc world) theory of Armen Mulkidjanian[128] is an extension of Wächtershäuser'spyrite hypothesis. Wächtershäuser based his theory of the initial chemical processes leading toinformational molecules (i.e. RNA, peptides) on a regular mesh of electric charges at the surface ofpyrite that may have made the primeval polymerization thermodynamically more favourable byattracting reactants and arranging them appropriately relative to each other.[129] The Zn-Worldtheory specifies and differentiates further.[128][130] Hydrothermal fluids rich in H2S interacting with

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cold primordial ocean (or "Darwin pond") water leads to the precipitation of metal sulfide particles.Oceanic vent systems and other hydrothermal systems have a zonal structure reflected in ancientvolcanogenic massive sulfide deposits (VMS) of hydrothermal origin. They reach many kilometersin diameter and date back to the Archean eon. Most abundant are pyrite (FeS2), chalcopyrite(CuFeS2), and sphalerite (ZnS), with additions of galena (PbS) and alabandite (MnS). ZnS andMnS have a unique ability to store radiation energy, e.g. provided by UV light. Since during therelevant time window of the origins of replicating molecules the primordial atmospheric pressurewas high enough (>100 bar) to precipitate near the Earth's surface and UV irradiation was 10 to100 times more intense than now, the unique photosynthetic properties mediated by ZnS providedjust the right energy conditions to energize the synthesis of informational and metabolic moleculesand the selection of photostable nucleobases.

The Zn-World theory has been further filled out with experimental and theoretical evidence for theionic constitution of the interior of the first proto-cells before Archea, Eubacteria and Proto-Eukarya evolved. Archibald Maccallum noted the resemblance of organismal fluids such as blood,lymph to seawater;[131] however, the inorganic composition of all cells differ from that of modernsea water, which led Mulkidjanian and colleagues to reconstruct the "hatcheries" of the first cellscombining geochemical analysis with phylogenomic scrutiny of the inorganic ion requirements ofuniversal components of modern cells. The authors conclude that ubiquitous, and by inferenceprimordial, proteins and functional systems show affinity to and functional requirement for K+,Zn2+, Mn2+, and phosphate. Geochemical reconstruction shows that the ionic compositionconducive to the origin of cells could not have existed in what we today call marine settings but iscompatible with emissions of vapor-dominated zones of what we today call inland geothermalsystems. Under the anoxic, CO2-dominated primordial atmosphere, the chemistry of watercondensates and exhalations near geothermal fields would resemble the internal milieu of moderncells. Therefore, the precellular stages of evolution may have taken place in shallow "Darwin-

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Deep-sea hydrothermal vent or'black smoker'

ponds" lined with porous silicate minerals mixed with metal sulfides and enriched in K+, Zn2+, andphosphorus compounds.[132][133]

Deep sea vent hypothesis [edit]

The deep sea vent, or alkaline hydrothermal vent, theoryfor the origin of life on Earth posits that life may havebegun at submarine hydrothermal vents, where hydrogen-rich fluids emerge from below the sea floor, as a result ofserpentinization of ultra-mafic olivine with sea water and apH interface with carbon dioxide-rich ocean water.Sustained chemical energy in such systems is derived fromredox reactions, in which electron donors, such asmolecular hydrogen, react with electron acceptors, such ascarbon dioxide (see iron-sulfur world theory). These arehighly exothermic reactions.[note 2]

Michael Russell demonstrated that alkaline vents createdan abiogenic proton-motive force chemiosmoticgradient,[134] in which conditions are ideal for an abiogenichatchery for life. Their microscopic compartments "providea natural means of concentrating organic molecules",composed of iron-sulfur minerals such as mackinawite,endowed these mineral cells with the catalytic propertiesenvisaged by Günter Wächtershäuser.[122] This movement of ions across the membrane dependson a combination of two factors:

1. Diffusion force caused by concentration gradient – all particles including ions tend to diffuse

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from higher concentration to lower.

2. Electrostatic force caused by electrical potential gradient – cations like protons H+ tend todiffuse down the electrical potential, anions in the opposite direction.

These two gradients taken together can be expressed as an electrochemical gradient, providingenergy for abiogenic synthesis. The proton-motive force (PMF) can be described as the measureof the potential energy stored as a combination of proton and voltage gradients across amembrane (differences in proton concentration and electrical potential).

Nobel laureate Szostak suggested that geothermal activity provides greater opportunities for theorigination of life in open lakes where there is a buildup of minerals. In 2010, based on spectralanalysis of sea and hot mineral water as well as cactus juice, Ignat Ignatov and Oleg Mosindemonstrated that life may have predominantly originated in hot mineral water. The hot mineralwater that contains bicarbonate and calcium ions has the most optimal range.[135] This is similarcase as the origin of life in hydrothermal vents, but with bicarbonate and calcium ions in hot water.This water has a pH of 9–11 and is possible to have the reactions in sea water. According to Nobelwinner Melvin Calvin, certain reactions of condensation-dehydration of amino acids andnucleotides in individual blocks of peptides and nucleic acids can take place in the primaryhydrosphere with pH 9-11 at a later evolutionary stage.[136] Some of these compounds likehydrocyanic acid (HCN) have been proven in the experiments of Miller. This is the environment inwhich the stromatolites have been created. David Ward described the formation of stromatolites inhot mineral water at the Yellowstone National Park. Stromatolites have lived in hot mineral waterand in proximity to areas with volcanic activity.[137] Processes have evolved in the sea neargeysers of hot mineral water. In 2011 Tadashi Sugawara created a protocell in hot water.[138]

Thermosynthesis [edit]

Today's bioenergetic process of fermentation is carried out by either the aforementioned citric acid

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cycle or the Acetyl-CoA pathway, both of which have been connected to the primordial iron-sulfurworld. In a different approach, the thermosynthesis hypothesis considers the bioenergetic processof chemiosmosis, which plays an essential role in cellular respiration and photosynthesis, morebasal than fermentation: the ATP synthase enzyme, which sustains chemiosmosis, is proposed asthe currently extant enzyme most closely related to the first metabolic process.[139][140]

First, life needed an energy source to bring about the condensation reaction that yielded thepeptide bonds of proteins and the phosphodiester bonds of RNA. In a generalization and thermalvariation of the binding change mechanism of today's ATP synthase, the "first protein" would havebound substrates (peptides, phosphate, nucleosides, RNA 'monomers') and condensed them to areaction product that remained bound until after a temperature change it was released by thermalunfolding.

The energy source under the thermosynthesis hypothesis was thermal cycling, the result ofsuspension of protocells in a convection current, as is plausible in a volcanic hot spring; theconvection accounts for the self-organization and dissipative structure required in any origin of lifemodel. The still ubiquitous role of thermal cycling in germination and cell division is considered arelic of primordial thermosynthesis.

By phosphorylating cell membrane lipids, this "first protein" gave a selective advantage to the lipidprotocell that contained the protein. This protein also synthesized a library of many proteins, ofwhich only a minute fraction had thermosynthesis capabilities. As proposed by Dyson,[141] itpropagated functionally: it made daughters with similar capabilities, but it did not copy itself.Functioning daughters consisted of different amino acid sequences.

Whereas the iron-sulfur world identifies a circular pathway as the most simple—and thereforeassumes the existence of enzymes—the thermosynthesis hypothesis does not even invoke apathway, and does not assume the existence of regular enzymes: ATP synthase's binding change

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mechanism resembles a physical adsorption process that yields free energy,[142] rather than aregular enzyme's mechanism, which decreases the free energy. The RNA world also implies theexistence of several enzymes. It has been claimed that the emergence of cyclic systems of proteincatalysts is implausible.[143]

Other models of abiogenesis [edit]

Clay hypothesis [edit]

A model for the origin of life based on clay was forwarded by A. Graham Cairns-Smith in 1985 andexplored as a plausible illustration by several scientists.[144] The Clay hypothesis postulates thatcomplex organic molecules arose gradually on a pre-existing, non-organic replication platform ofsilicate crystals in solution.

Cairns-Smith is a trenchant critic of other models of chemical evolution.[145] However, he admitsthat like many models of the origin of life, his own also has its shortcomings.

In 2007, Kahr and colleagues reported their experiments that tested the idea that crystals can actas a source of transferable information, using crystals of potassium hydrogen phthalate. "Mother"crystals with imperfections were cleaved and used as seeds to grow "daughter" crystals fromsolution. They then examined the distribution of imperfections in the new crystals and found thatthe imperfections in the mother crystals were reproduced in the daughters, but the daughtercrystals also had many additional imperfections. For gene-like behavior to be observed, thequantity of inheritance of these imperfections should have exceeded that of the mutations in thesuccessive generations, but it did not. Thus Kahr concluded that the crystals, "were not faithfulenough to store and transfer information from one generation to the next".[146][147]

Gold's "deep-hot biosphere" model [edit]

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In the 1970s, Thomas Gold proposed the theory that life first developed not on the surface of theEarth, but several kilometers below the surface. The discovery in the late 1990s of nanobes(filamental structures that are smaller than bacteria, but that may contain DNA) in deep rocks[148]

might be seen as lending support to Gold's theory.

It is now reasonably well established that microbial life is plentiful at shallow depths in the Earth, upto 5 kilometres (3.1 mi) below the surface,[148] in the form of extremophile archaea, rather than thebetter-known eubacteria (which live in more accessible conditions). It is claimed that discovery ofmicrobial life below the surface of another body in our solar system would lend significant credenceto this theory. Thomas Gold also asserted that a trickle of food from a deep, unreachable, sourceis needed for survival because life arising in a puddle of organic material is likely to consume all ofits food and become extinct. Gold's theory is that the flow of such food is due to out-gassing ofprimordial methane from the Earth's mantle; more conventional explanations of the food supply ofdeep microbes (away from sedimentary carbon compounds) is that the organisms subsist onhydrogen released by an interaction between water and (reduced) iron compounds in rocks.

Primitive extraterrestrial life [edit]

Main article: Panspermia

Exogenesis is related to, but not the same as, the notion of panspermia. Neither hypothesisactually answers the question of how life first originated, but merely shifts it to another planet or acomet. However, the advantage of an extraterrestrial origin of primitive life is that life is notrequired to have evolved on each planet it occurs on, but rather in a single location, and thenspread about the galaxy to other star systems via cometary and/or meteorite impact. Evidence tosupport the hypothesis is scant, but it finds support in studies of Martian meteorites found inAntarctica and in studies of extremophile microbes' survival in outerspace.[149][150][151][152][153][154][155]

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Regression of complexity increase.

An alternative to Earthly abiogenesis is the hypothesis thatprimitive life may have originally formed extraterrestrially(extraterrestrial life),[156][157] either in space or elsewhere.Studies which apply the equivalent of Moore's Law tobiological evolution and extrapolate backwards haveproposed that life began 9.7 ± 2.5 billion years ago, billionsof years before the Earth was formed. They noted that lifemay have started "from systems with single heritable elements that are functionally equivalent to anucleotide".[156][157] In the case of evolution, empirical evidence suggested a doubling ofcomplexity every 376 million years. As the age of trees can be measured by the number of rings,the hypothesis that the age of life could be measured by biological complexity (i.e., the length offunctional non-redundant DNA in the genome) was studied.[156][157] If log-transformed complexityis plotted against the time of origin of large evolutionary lineages, then the points fit to a straightline (see figure). The exponential increase in complexity can be explained by a positive self-activating feedback loop.[157] The regression line hits zero (i.e., one nucleotide) at 9.7 ± 2.5 billionyears ago.[156] If this model is correct, and since our Solar System is 4.6 billion years old,[158] thenlife somehow arrived to Earth from older stellar systems. This hypothesis was criticized by EugeneKoonin, who suggested that the rates of early biological evolution might have been much fasterdue to the absence of competition on early Earth.[159] Chris Adami argued that "it is inconceivablethat life began with just a few nucleotides."[157] To answer this criticism, Sharov proposed ahypothetical abiogenesis scenario that starts from coenzyme-like molecules that are functionallyequivalent to single nucleotides.[160][161]

On 24 January 2014, NASA reported that current studies on the planet Mars by the Curiosity andOpportunity rovers will now be searching for evidence of ancient life, including a biosphere basedon autotrophic, chemotrophic and/or chemolithoautotrophic microorganisms, as well as ancient

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Methane is one of thesimplest organic compounds

water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that mayhave been habitable.[162][163][164][165] The search for evidence of habitability, taphonomy (relatedto fossils), and organic carbon on the planet Mars is now a primary NASA objective.[162]

Extraterrestrial organic molecules [edit]

See also: List of molecules in interstellar space and Panspermia § Pseudo-panspermia

An organic compound is any member of a large class of gaseous,liquid, or solid chemicals whose molecules contain carbon. Carbonis the fourth most abundant element in the universe by mass afterhydrogen, helium, and oxygen.[166] Carbon is abundant in the Sun,stars, comets, and in the atmospheres of most planets.[167]

Organic compounds are relatively common in space, formed by"factories of complex molecular synthesis" which occur in molecularclouds and circumstellar envelopes, and chemically evolve afterreactions are initiated mostly by ionizing radiation.[40][168] Basedon computer model studies, the complex organic moleculesnecessary for life may have formed on dust grains in theprotoplanetary disk surrounding the Sun before the formation ofthe Earth.[19] According to the computer studies, this same process may also occur around otherstars that acquire planets.[19]

Observations suggest that the majority of organic compounds introduced on Earth by interstellardust particles are considered principal agents in the formation of complex molecules, thanks totheir peculiar surface-catalytic activities.[169][170] Studies reported in 2008, based on 12C/13Cisotopic ratios of organic compounds found in the Murchison meteorite, suggested that the RNAcomponent uracil and related molecules, including xanthine, were formed extraterrestrially.[171][172]

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Formation of Glycolaldehyde in star dust

On 8 August 2011, a report based on NASA studies of meteorites found on Earth was publishedsuggesting DNA components (adenine, guanine and related organic molecules) were made inouter space.[169][173][174][175] Scientists also found that the cosmic dust permeating the universecontains complex organics ("amorphous organic solids with a mixed aromatic-aliphatic structure")that could be created naturally, and rapidly, by stars.[176][177][178] A scientist who suggested thatthese compounds may have been related to the development of life on Earth said that "If this is thecase, life on Earth may have had an easier time getting started as these organics can serve asbasic ingredients for life."[176]

Glycolaldehyde, the first example of an interstellarsugar molecule, was detected in the star-formingregion near the center of our galaxy. It was discoveredin 2000 by Jes Jørgensen and Jan M. Hollis.[179]

Then, on 29 August 2012, the same team reportedthe detection of glycolaldehyde in a distant starsystem. The molecule was found around theprotostellar binary IRAS 16293-2422 400 light yearsfrom Earth.[180][181][182] Glycolaldehyde is needed toform ribonucleic acid (RNA), which is similar in functionto DNA. These findings suggest that complex organicmolecules may form in stellar systems prior to theformation of planets, eventually arriving on youngplanets early in their formation.[183] Because sugarsare associated with both metabolism and the geneticcode, two of the most basic aspects of life, it isthought the discovery of extraterrestrial sugar

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An illustration of typical polycyclicaromatic hydrocarbons. Clockwise from

increases the likelihood that life may exist elsewherein our galaxy.[179]

NASA announced in 2009 that scientists had identified another fundamental chemical buildingblock of life in a comet for the first time, glycine, an amino acid, which was detected in materialejected from Comet Wild-2 in 2004 and grabbed by NASA's Stardust probe. Glycine has beendetected in meteorites before. Carl Pilcher, who leads NASA's Astrobiology Institute commentedthat "The discovery of glycine in a comet supports the idea that the fundamental building blocks oflife are prevalent in space, and strengthens the argument that life in the universe may be commonrather than rare."[184] Comets are encrusted with outer layers of dark material, thought to be a tar-like substance composed of complex organic material formed from simple carbon compounds afterreactions initiated mostly by ionizing radiation. It is possible that a rain of material from cometscould have brought significant quantities of such complex organic molecules to Earth.[185] Aminoacids which were formed extraterrestrially may also have arrived on Earth via comets.[22] It isestimated that during the Late Heavy Bombardment, meteorites may have delivered up to fivemillion tons of biogenic elements to Earth per year.[22]

Polycyclic aromatic hydrocarbons (PAH) are the mostcommon and abundant of the known polyatomicmolecules in the visible universe, and are considered alikely constituent of the primordial sea.[186][187][188]

PAHs, along with fullerenes (or "buckyballs"), havebeen recently detected in nebulae.[189][190]

On 3 April 2013, NASA reported that complex organicchemicals could arise on Titan, a moon of Saturn,based on studies simulating the atmosphere ofTitan.[191]

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top left: benz(e)acephenanthrylene, pyreneand dibenz(ah)anthracene.Lipid world [edit]

Main article: Gard model

The lipid world theory postulates that the first self-replicating object was lipid-like.[192][193] It isknown that phospholipids form lipid bilayers in water while under agitation – the same structure asin cell membranes. These molecules were not present on early Earth, but other amphiphilic longchain molecules also form membranes. Furthermore, these bodies may expand (by insertion ofadditional lipids), and under excessive expansion may undergo spontaneous splitting whichpreserves the same size and composition of lipids in the two progenies. The main idea in thistheory is that the molecular composition of the lipid bodies is the preliminary way for informationstorage, and evolution led to the appearance of polymer entities such as RNA or DNA that maystore information favorably. Studies on vesicles from potentially prebiotic amphiphiles have so farbeen limited to systems containing one or two types of amphiphiles. This in contrast to the outputof simulated prebiotic chemical reactions, which typically produce very heterogeneous mixtures ofcompounds.[194] Within the hypothesis of a lipid bilayer membrane composed of a mixture ofvarious distinct amphiphilic compounds there is the opportunity of a huge number of theoreticallypossible combinations in the arrangements of these amphiphiles in the membrane. Among allthese potential combinations, a specific local arrangement of the membrane would have favoredthe constitution of an hypercycle,[195][196] according to the terminology by Manfred Eigen, actuallya positive feedback composed of two mutual catalysts represented by a membrane site and aspecific compound trapped in the vesicle. Such site/compound pairs are transmissible to thedaughter vesicles leading to the emergence of distinct lineages of vesicles which would haveallowed Darwinian natural selection.[197]

Polyphosphates [edit]

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A problem in most scenarios of abiogenesis is that the thermodynamic equilibrium of amino acidversus peptides is in the direction of separate amino acids. What has been missing is some forcethat drives polymerization. The resolution of this problem may well be in the properties ofpolyphosphates.[198][199] Polyphosphates are formed by polymerization of ordinarymonophosphate ions PO4

−3. Several mechanisms for such polymerization have been suggested.Polyphosphates cause polymerization of amino acids into peptides. They are also logicalprecursors in the synthesis of such key biochemical compounds as ATP. A key issue seems to bethat calcium reacts with soluble phosphate to form insoluble calcium phosphate (apatite), so someplausible mechanism must be found to keep calcium ions from causing precipitation of phosphate.There has been much work on this topic over the years, but an interesting new idea is thatmeteorites may have introduced reactive phosphorus species on the early Earth.[200]

PAH world hypothesis [edit]

Main article: PAH world hypothesis

Polycyclic aromatic hydrocarbons (PAHs) are known to be abundant in the universe,[186][187][188]

including in the interstellar medium, in comets, and in meteorites, and are some of the mostcomplex molecules so far found in space.[167]

Other sources of complex molecules have been postulated, including extraterrestrial stellar orinterstellar origin. For example, from spectral analyses, organic molecules are known to be presentin comets and meteorites. In 2004, a team detected traces of PAHs in a nebula.[201] In 2010,another team also detected PAHs, along with fullerenes (or "buckyballs"), in nebulae.[202] The useof PAHs has also been proposed as a precursor to the RNA world in the PAH worldhypothesis.[citation needed] The Spitzer Space Telescope has detected a star, HH 46-IR, which isforming by a process similar to that by which the sun formed. In the disk of material surroundingthe star, there is a very large range of molecules, including cyanide compounds, hydrocarbons,

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and carbon monoxide. In September 2012, NASA scientists reported that PAHs, subjected tointerstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation andhydroxylation, to more complex organics – "a step along the path toward amino acids andnucleotides, the raw materials of proteins and DNA, respectively".[203][204] Further, as a result ofthese transformations, the PAHs lose their spectroscopic signature which could be one of thereasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions ofcold, dense clouds or the upper molecular layers of protoplanetary disks."[203][204]

On 21 February 2014, NASA announced a greatly upgraded database [167] for tracking PAHs inthe universe. According to scientists, more than 20% of the carbon in the universe may beassociated with PAHs, possible starting materials for the formation of life. PAHs seem to have beenformed shortly after the Big Bang, are widespread throughout the universe,[186][187][188] and areassociated with new stars and exoplanets.[167]

Radioactive beach hypothesis [edit]

Zachary Adam claims that tidal processes that occurred during a time when the moon was muchcloser may have concentrated grains of uranium and other radioactive elements at the high-watermark on primordial beaches, where they may have been responsible for generating life's buildingblocks.[205] According to computer models reported in Astrobiology,[206] a deposit of suchradioactive materials could show the same self-sustaining nuclear reaction as that found in theOklo uranium ore seam in Gabon. Such radioactive beach sand might have provided sufficientenergy to generate organic molecules, such as amino acids and sugars from acetonitrile in water.Radioactive monazite also have released soluble phosphate into the regions between sand-grains,making it biologically "accessible". Thus amino acids, sugars, and soluble phosphates might havebeen produced simultaneously, according to Adam. Radioactive actinides, left behind in someconcentration by the reaction, might have formed part of organo-metallic complexes. These

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complexes could have been important early catalysts to living processes.

John Parnell has suggested that such a process could provide part of the "crucible of life" in theearly stages of any early wet rocky planet, so long as the planet is large enough to havegenerated a system of plate tectonics which brings radioactive minerals to the surface. As theearly Earth is believed to have had many smaller "platelets" it might have provided a suitableenvironment for such processes.[207]

Ultraviolet and temperature-assisted replication model [edit]

From a thermodynamic perspective of the origin of life springs the ultraviolet and temperature-assisted replication (UVTAR) model. Karo Michaelian points out that any model for the origin of lifemust take into account the fact that life is an irreversible thermodynamic process which arises andpersists because it produces entropy. Entropy production is not incidental to the process of life,but rather the fundamental reason for its existence. Present day life augments the entropyproduction of Earth by catalysing the water cycle through evapotranspiration.[208][209] Michaelianargues that if the thermodynamic function of life today is to produce entropy through coupling withthe water cycle, then this probably was its function at its very beginnings. It turns out that both RNAand DNA when in water solution are very strong absorbers and extremely rapid dissipaters ofultraviolet light within the 200–300 nm wavelength range, which is that part of the sun's spectrumthat could have penetrated the dense prebiotic atmosphere.[210] have shown that the amount ofultraviolet (UV) light reaching the Earth's surface in the Archean eon could have been up to 31orders of magnitude greater than it is today at 260 nm where RNA and DNA absorb most strongly.Absorption and dissipation of UV light by the organic molecules at the Archean ocean surfacewould have significantly increased the temperature of the surface and led to enhancedevaporation and thus to have augmented the primitive water cycle. Since absorption anddissipation of high energy photons is an entropy producing process, argues that non-equilbrium

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abiogenic synthesis of RNA and DNA utilizing UV light would have been thermodynamicallyfavored.[107]

A simple mechanism that could explain the replication of RNA and DNA without resort to the use ofenzymes could also be provided within the same thermodynamic framework by assuming that lifearose when the temperature of the primitive seas had cooled to somewhat below the denaturingtemperature of RNA or DNA (based on the ratio of 18O/16O found in cherts of the Barbertongreenstone belt of South Africa of about 3.5 to 3.2 Ga., surface temperatures are predicted tohave been around 70±15 °C,[211] close to RNA or DNA denaturing (uncoiling and separation)temperatures. During the night, the surface water temperature would drop below the denaturingtemperature and single strand RNA/DNA could act as a template for the formation of double strandRNA/DNA. During the daylight hours, RNA and DNA would absorb UV light and convert this directlyto heat the ocean surface, thereby raising the local temperature enough to allow for denaturing ofRNA and DNA. The copying process would have been repeated with each diurnal cycle.[212][213]

Such a temperature assisted mechanism of replication bears similarity to polymerase chainreaction (PCR), a routine laboratory procedure employed to multiply DNA segments. Michaeliansuggests that the traditional origin of life research, that expects to describe the emergence of lifefrom near-equilibrium conditions, is erroneous and that non-equilibrium conditions must beconsidered, in particular, the importance of entropy production to the emergence of life.

Since denaturation would be most probable in the late afternoon when the Archean sea surfacetemperature would be highest, and since late afternoon submarine sunlight is somewhat circularlypolarized, the homochirality of the organic molecules of life can also be explained within theproposed thermodynamic framework.[214][215]

Multiple genesis [edit]

Different forms of life with variable origin processes may have appeared quasi-simultaneously in

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the early history of Earth.[216] The other forms may be extinct, leaving distinctive fossils throughtheir different biochemistry (e.g., using arsenic instead of phosphorus), survive as extremophiles,or simply be unnoticed through their being analogous to organisms of the current life tree.Hartman[217] for example combines a number of theories together, by proposing that:

The first organisms were self-replicating iron-rich clays which fixed carbon dioxide intooxalic and other dicarboxylic acids. This system of replicating clays and theirmetabolic phenotype then evolved into the sulfide rich region of the hotspringacquiring the ability to fix nitrogen. Finally phosphate was incorporated into theevolving system which allowed the synthesis of nucleotides and phospholipids. Ifbiosynthesis recapitulates biopoiesis, then the synthesis of amino acids preceded thesynthesis of the purine and pyrimidine bases. Furthermore the polymerization of theamino acid thioesters into polypeptides preceded the directed polymerization of aminoacid esters by polynucleotides.

Lynn Margulis's endosymbiotic theory suggests that multiple forms of archea entered intosymbiotic relationship to form the eukaryotic cell. The horizontal transfer of genetic materialbetween archea promotes such symbiotic relationships, and thus many separate organisms mayhave contributed to building what has been recognised as the Last Universal Common Ancestor(LUCA) of modern organisms.

Conceptual history [edit]

John Desmond Bernal has identified a number of "outstanding difficulties in accounts of the originof life". Earlier theories, he suggests, such as spontaneous generation were based upon anexplanation that life was continuously created as a result of chance events.[218]

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Spontaneous generation [edit]

Main article: Spontaneous generation

Belief in the present ongoing spontaneous generation of certain forms of life from non-living mattergoes back to Aristotle and ancient Greek philosophy and continued to have support in Westernscholarship until the 19th century. This belief was paired with a belief in heterogenesis, i.e., thatone form of life derived from a different form (e.g. bees from flowers).[219] Classical notions ofspontaneous generation, which can be considered under the modern term abiogenesis, held thatcertain complex, living organisms are generated by decaying organic substances. According toAristotle, it was a readily observable truth that aphids arise from the dew which falls on plants, fliesfrom putrid matter, mice from dirty hay, crocodiles from rotting logs at the bottom of bodies ofwater, and so on.[220] In the 17th century, such assumptions started to be questioned. In 1646, SirThomas Browne published his Pseudodoxia Epidemica (subtitled Enquiries into Very manyReceived Tenets, and Commonly Presumed Truths), which was an attack on false beliefs and"vulgar errors." His conclusions were not widely accepted at the time. His contemporary, AlexanderRoss wrote: "To question this (i.e., spontaneous generation) is to question reason, sense andexperience. If he doubts of this let him go to Egypt, and there he will find the fields swarming withmice, begot of the mud of Nylus, to the great calamity of the inhabitants."[221]

In 1665, Robert Hooke published the first drawings of a microorganism. Hooke was followed in1676 by Anton van Leeuwenhoek, who drew and described microorganisms that are now thoughtto have been protozoa and bacteria.[222] Many felt the existence of microorganisms was evidencein support of spontaneous generation, since microorganisms seemed too simplistic for sexualreproduction, and asexual reproduction through cell division had not yet been observed. VanLeeuwenhoek took issue with the ideas common at the time that fleas and lice couldspontaneously result from putrefaction, and that frogs could likewise arise from slime. Using abroad range of experiments ranging from sealed and open meat incubation and the close study of

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insect reproduction, by the 1680s he became convinced that spontaneous generation wasincorrect.[223]

The first experimental evidence against spontaneous generation came in 1668 when FrancescoRedi showed that no maggots appeared in meat when flies were prevented from laying eggs. It wasgradually shown that, at least in the case of all the higher and readily visible organisms, theprevious sentiment regarding spontaneous generation was false. The alternative seemed to bebiogenesis: that every living thing came from a pre-existing living thing (omne vivum ex ovo, Latinfor "every living thing from an egg").

In 1768, Lazzaro Spallanzani demonstrated that microbes were present in the air, and could bekilled by boiling. In 1861, Louis Pasteur performed a series of experiments that demonstrated thatorganisms such as bacteria and fungi do not spontaneously appear in sterile, nutrient-rich media,but only invade them from outside.

Alternatives to chance: biogenesis [edit]

The belief that spontaneous self-ordering of spontaneous generation is impossible led to analternative. By the middle of the 19th century, the theory of biogenesis had accumulated so muchevidential support, due to the work of Louis Pasteur and others, that the alternative theory ofspontaneous generation had been effectively disproven.

Pasteur and Darwin [edit]

Pasteur himself remarked, after a definitive finding in 1864, "Never will

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Charles Darwin in1879.

Pasteur himself remarked, after a definitive finding in 1864, "Never willthe doctrine of spontaneous generation recover from the mortal blowstruck by this simple experiment."[224][225] One alternative was thatlife's origins on Earth had come from somewhere else in the Universe.Periodically resurrected (see Panspermia, above) Bernaldemonstrates that this approach "is equivalent in the last resort toasserting the operation of metaphysical, spiritual entities... it turns onthe argument of creation by design by a creator or demiurge".[226]

Such a theory, Bernal demonstrated was unscientific and a number ofscientists defined life as a result of an inner "life force", which in thelate 19th century was championed by Henri Bergson.

The concept of evolution proposed by Charles Darwin put an end tothese metaphysical theologies. In a letter to Joseph Dalton Hooker on 1 February 1871,[227]

Charles Darwin addressed the question, suggesting that the original spark of life may have begunin a "warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc.present, so that a protein compound was chemically formed ready to undergo still more complexchanges". He went on to explain that "at the present day such matter would be instantly devouredor absorbed, which would not have been the case before living creatures were formed."[228] Inother words, the presence of life itself makes the search for the spontaneous origin of lifedependent on the artificial production of organic compounds in the sterile conditions of thelaboratory.

"Primordial soup" hypothesis [edit]

Further information: Miller–Urey experiment

No new notable research or theory on the subject appeared until

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Alexander Oparin (right)at the laboratory.

1924, when Alexander Oparin reasoned that atmospheric oxygenprevents the synthesis of certain organic compounds that arenecessary building blocks for the evolution of life. In his book TheOrigin of Life,[229][230] Oparin proposed that the "spontaneousgeneration of life" that had been attacked by Louis Pasteur did infact occur once, but was now impossible because the conditionsfound on the early Earth had changed, and preexisting organismswould immediately consume any spontaneously generatedorganism. Oparin argued that a "primeval soup" of organicmolecules could be created in an oxygenless atmosphere throughthe action of sunlight. These would combine in ever more complexways until they formed coacervate droplets. These droplets would"grow" by fusion with other droplets, and "reproduce" through fissioninto daughter droplets, and so have a primitive metabolism in which those factors which promote"cell integrity" survive, and those that do not become extinct. Many modern theories of the origin oflife still take Oparin's ideas as a starting point.

Robert Shapiro has summarized the "primordial soup" theory of Oparin and Haldane in its "matureform" as follows:[231]

1. The early Earth had a chemically reducing atmosphere.

2. This atmosphere, exposed to energy in various forms, produced simple organic compounds("monomers").

3. These compounds accumulated in a "soup", which may have been concentrated at variouslocations (shorelines, oceanic vents etc.).

4. By further transformation, more complex organic polymers – and ultimately life – developedin the soup.

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Around the same time, J. B. S. Haldane suggested that the Earth's prebiotic oceans—differentfrom their modern counterparts—would have formed a "hot dilute soup" in which organiccompounds could have formed. J.D. Bernal, a pioneer in x-ray crystallography, called this ideabiopoiesis or biopoesis, the process of living matter evolving from self-replicating but nonlivingmolecules,[232][233] and proposed that biopoiesis passes through a number of intermediatestages.

One of the most important pieces of experimental support for the "soup" theory came in 1952. Agraduate student, Stanley Miller, and his professor, Harold Urey, performed an experiment thatdemonstrated how organic molecules could have spontaneously formed from inorganic precursors,under conditions like those posited by the Oparin-Haldane Hypothesis. The now-famous "Miller–Urey experiment" used a highly reduced mixture of gases—methane, ammonia and hydrogen—toform basic organic monomers, such as amino acids.[234] This provided direct experimental supportfor the second point of the "soup" theory, and it is around the remaining two points of the theorythat much of the debate now centers. In the Miller–Urey experiment, a mixture of water, hydrogen,methane, and ammonia was cycled through an apparatus that delivered electrical sparks to themixture. After one week, it was found that about 10% to 15% of the carbon in the system was nowin the form of a racemic mixture of organic compounds, including amino acids, which are thebuilding blocks of proteins.

Bernal shows that based upon this and subsequent work there is no difficulty in principle in formingmost of the molecules which we recognise as the basic molecules of life from their inorganicprecursors. The underlying hypothesis held by Oparin, Haldane, Bernal, Miller and Urey, forinstance, was that multiple conditions on the primeval Earth favored chemical reactions thatsynthesized the same set of complex organic compounds from such simple precursors. A 2011reanalysis of the saved vials containing the original extracts that resulted from the Miller and Ureyexperiments, using current and more advanced analytical equipment and technology, has

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uncovered more biochemicals than originally discovered in the 1950s. One of the more importantfindings was 23 amino acids, far more than the five originally found.[235] However Bernal rightlyshows that "it is not enough to explain the formation of such molecules, what is necessary he says"..is a physical-chemical explanation of the origins of these molecules that suggests the presenceof suitable sources and sinks for free energy".[236]

Proteinoid microspheres [edit]

Main article: Proteinoid

In trying to uncover the intermediate stages of abiogenesis mentioned by Bernal, Sidney W. Fox inthe 1950s and 1960s, studied the spontaneous formation of peptide structures under conditionsthat might plausibly have existed early in Earth's history. He demonstrated that amino acids couldspontaneously form small chains called peptides. In one of his experiments, he allowed aminoacids to dry out as if puddled in a warm, dry spot in prebiotic conditions. He found that, as theydried, the amino acids formed long, often cross-linked, thread-like, submicroscopic polypeptidemolecules now named "proteinoid microspheres".[237]

In another experiment using a similar method to set suitable conditions for life to form, Foxcollected volcanic material from a cinder cone in Hawaii. He discovered that the temperature wasover 100 °C (212 °F) just 4 inches (100 mm) beneath the surface of the cinder cone, andsuggested that this might have been the environment in which life was created—molecules couldhave formed and then been washed through the loose volcanic ash and into the sea. He placedlumps of lava over amino acids derived from methane, ammonia and water, sterilized all materials,and baked the lava over the amino acids for a few hours in a glass oven. A brown, stickysubstance formed over the surface and when the lava was drenched in sterilized water a thick,brown liquid leached out. It turned out that the amino acids had combined to form proteinoids, andthe proteinoids had combined to form small globules that Fox called "microspheres". His

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proteinoids were not cells, although they formed clumps and chains reminiscent of cyanobacteria,but they contained no functional nucleic acids or any encoded information. Based upon suchexperiments, Colin S. Pittendrigh stated in December 1967 that "laboratories will be creating aliving cell within ten years," a remark that reflected the typical contemporary levels of innocence ofthe complexity of cell structures.[238]

More recent theories [edit]

Bernal in 1967 identified three different sorts of difficulties in the abiogenetic origins of life[239]

* Stage 1: he saw as the origins of organic molecules, and this is now fairly well understood. Thenecessity of a source and sink of energy, and the necessity of a fluid medium has been muchstudied (see above).

* Stage 2: he saw as the necessity to explain how organic monomers became ordered intobiologically active polymers. Once again there is the necessity of sources and sinks for thisprocess. The discovery of alkaline vents and the similarity with the "proton pump" found as thebasis of biological life has begun to provide evidence for this. The second problem foreseen byBernal was the origin of replication. The work with the RNA world is specifically intended to findanswers to this problem.

* Stage 3: he saw was the most difficult. This was the discovery of methods by which biologicalreactions were incorporated behind cell walls. Modern work on the self organising capacities bywhich cell membranes self-assemble, and the work on micropores in various substrates as a half-way house towards the development of independent free-living cells is ongoing research designedto answer this problem.[240][241]

See also [edit]

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Artificial cell

Astrochemistry

Biological immortality

Common descent

Emergence

Entropy and life

Mediocrity principle

Mimivirus

Planetary habitability

Rare Earth hypothesis

Shadow biosphere

Stromatolite

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Notes [edit]

1. ^ The reactions are:

FeS + H2S → FeS2 + 2H+ + 2e-

FeS + H2S → FeS2 + HCOOH

2. ^ The reactions are:Reaction 1: Fayalite + water → magnetite + aqueous silica + hydrogen

3Fe2SiO4 + 2H2O → 2Fe3O4 + 3SiO2 + 2H2

Reaction 2: Forsterite + aqueous silica → serpentine

3Mg2SiO4 + SiO2 + 4H2O → 2Mg3Si2O5(OH)4

Reaction 3: Forsterite + water → serpentine + brucite

2Mg2SiO4 + 3H2O → Mg3Si2O5(OH)4 + Mg(OH)2

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Reaction 3 describes the hydration of olivine with water only to yield serpentine and Mg(OH)2(brucite). Serpentine is stable at high pH in the presence of brucite like calcium silicate hydrate, (C-S-H) phases formed along with portlandite (Ca(OH)2) in hardened Portland cement paste after thehydration of belite (Ca2SiO4), the artificial calcium equivalent of forsterite.Analogy of reaction 3 with belite hydration in ordinary Portland cement: Belite + water → C-S-Hphase + portlandite

2 Ca2SiO4 + 4 H2O → 3 CaO · 2 SiO2 · 3 H2O + Ca(OH)2

Further reading [edit]

Arrhenius, Gustaf et al. (1997). "Entropy and Charge in Molecular Evolution—the Case of Phosphate".Journal of Theoretical Biology 187 (4): 503–22. doi:10.1006/jtbi.1996.0385 . PMID 9299295 .

Buehler, Lukas K. (2000–2005) The physico-chemical basis of life,http://www.whatislife.com/about.html accessed 27 October 2005.

Davies, Paul (1998). The Fifth Miracle. Penguin Science, London. ISBN 0-14-028226-2.

De Duve, Christian (January 1996). Vital Dust: The Origin and Evolution of Life on Earth. Basic Books.ISBN 0-465-09045-1.

Egel, R.; Lankenau, D.-H.; Mulkidjanian, A. Y. (2011). Origins of Life: The Primal Self-Organization .Berlin Heidelberg: Springer-Verlag. pp. 1–366,. doi:10.1007/978-3-642-21625-1 . ISBN 978-3-642-21624-4.

Fernando, CT; Rowe, J (2007). "Natural selection in chemical evolution". Journal of Theoretical Biology247 (1): 152–67. doi:10.1016/j.jtbi.2007.01.028 . PMID 17399743 .

Hartman, Hyman (1998). "Photosynthesis and the Origin of Life". Origins of Life and Evolution ofBiospheres 28 (4–6): 515–521. doi:10.1023/A:1006548904157 .

Harris, Henry (2002). Things come to life. Spontaneous generation revisited. Oxford: Oxford UniversityPress. ISBN 0-19-851538-3.

Hazen, Robert M. (December 2005). Genesis: The Scientific Quest for Life's Origins . Joseph HenryPress. ISBN 0-309-09432-1.

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Gribbon, John (1998). The Case of the Missing Neutrinos and other Curious Phenomena of theUniverse. Penguin Science, London. ISBN 0-14-028734-5.

Horgan, J (1991). "In the beginning". Scientific American 264 (6): 100–109.Bibcode:1991SciAm.264..100P . doi:10.1038/scientificamerican0691-100 . (Cited on p. 108).

Huber, C; Wächtershäuser, G. (1998). "Peptides by activation of amino acids with CO on (Ni,Fe)Ssurfaces: implications for the origin of life". Science 281 (5377): 670–2. Bibcode:1998Sci...281..670H .doi:10.1126/science.281.5377.670 . PMID 9685253 . (Cited on p. 108).

Ignatov, I., Mosin, O. V. (2013) Modeling of Possible Processes for Origin of Life and Living Matter in HotMineral and Seawater with Deuterium, Journal of Environment and Earth Science, Vol. 3, No. 14,pp. 103–118.http://www.iiste.org/Journals/index.php/JEES/article/view/9903

Klotz, I., Did Life Start in a Pond, Not Oceans? – opinion of Jack Szostak in Discovery Newshttp://news.discovery.com/earth/oceans/life-pond-ocean-122402.htm

Knoll, Andrew H. (2003). Life on a Young Planet: The First Three Billion Years of Evolution on Earth.Princeton University Press. ISBN 0-691-00978-3.

Kurihara, K., Tamura, M., Shohda, K., Toyota, T., Suzuki, K., Sugawara, T. (2011) Self-Reproduction ofSupramolecular Giant Vesicles Combined with the Amplification of Encapsulated DNA, NatureChemistry, Vol. 4, No.10, pp. 775–781.http://www.nature.com/nchem/journal/v3/n10/full/nchem.1127.html

Luisi, Pier Luigi (2006). The Emergence of Life: From Chemical Origins to Synthetic Biology. CambridgeUniversity Press. ISBN 0-521-82117-7.

Martin, W.; Russell, M.J. (2002). "On the origins of cells: a hypothesis for the evolutionary transitionsfrom abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells" .Philosophical Transactions of the Royal Society B 358 (1429): 59–83; discussion 83–5.doi:10.1098/rstb.2002.1183 . PMC 1693102 . PMID 12594918 .

Maynard Smith, John; Szathmary, Eors (16 March 2000). The Origins of Life: From the Birth of Life tothe Origin of Language. Oxford Paperbacks. ISBN 0-19-286209-X.

Morowitz, Harold J. (1992) "Beginnings of Cellular Life: Metabolism Recapitulates Biogenesis". YaleUniversity Press. ISBN 0-300-05483-1

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Listen to this article (4 parts) · (info)Part 1 • Part 2 • Part 3 • Part 4

This audio f ile was created f rom a rev ision ofthe "Abiogenesis" article dated 2012-06-13,

NASA Astrobiology Institute: Earth's Early Environment and Life

NASA Specialized Center of Research and Training in Exobiology: Gustaf O. Arrhenius

Pitsch, Stefan; Krishnamurthy, Ramanarayanan; Arrhenius, Gustaf (2000). "Concentration of SimpleAldehydes by Sulfite-Containing Double-Layer Hydroxide Minerals: Implications for Biopoesis" .Helvetica Chimica Acta 83 (9): 2398 2411. doi:10.1002/1522-2675(20000906)83:9<2398::AID-HLCA2398>3.0.CO;2-5 .

Pons, M.L., Quitte, G., Fujii, T., et al. (2011) Early Archean Serpentine Mud Volcanoes at Isua,Greenland, as a Niche for Early Life, PNAS, Vo;. 108, No.43, pp. 17639–17643.http://www.pnas.org/content/early/2011/10/10/1108061108.abstract

Pross, Addy (2012). What is Life?: How chemistry becomes biology. Oxford University Press. ISBN 0-19-964101-3.

Russell, MJ; Hall, AJ; Cairns-Smith, AG; Braterman, PS (1988). "Submarine hot springs and the originof life". Nature 336 (6195): 117. Bibcode:1988Natur.336..117R . doi:10.1038/336117a0 .

Shock, E. (1997) High Temperature Life without Photosyntesis as a Model for Mars, Journal ofGeophysical Research, Vol. 102, No. E10, pp. 23687–23694.http://www.igpp.ucla.edu/public/mkivelso/refs/PUBLICATIONS/shcok%20hiT%20life%20Mars7JE01087.pdf

Ward, D. (2010) First Fossil-makers in Hot Water. Astrobiology Magazine.http://www.astrobio.net/exclusive/3418/first-fossil-makers-in-hot-water

Dedicated issue of Philosophical Transactions B on Major Steps in Cell Evolution freely available.

Dedicated issue of Philosophical Transactions B on the Emergence of Life on the Early Earth freelyavailable.

External links [edit]

A.E.Zlobin, Tunguska similar impacts and origin oflife (mathematical theory of origin of life, incoming ofpattern recognition algorithm due to comets)

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the "Abiogenesis" article dated 2012-06-13,and does not ref lect subsequent edits to thearticle. (Audio help)

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