View source for Abiogenesis

{{dablink|This article focuses on modern scientific research on the origin of life. For religious beliefs about the creation of life, see [[creation myth]].}}

[[Image:Stromatolites.jpg|right|thumbnail|250px|[[Pre-Cambrian]] [[stromatolites]] in the Siyeh Formation, [[Glacier National Park (U.S.)|Glacier National Park]]. In 2002, William Schopf of [[UCLA]] published a controversial paper in the [[scientific journal]] ''[[Nature (journal)|Nature]]'' arguing that geological formations such as this possess 3.5 billion year old [[fossilized]] [[algae]] microbes.<ref>{{cite web|url=http://www.abc.net.au/science/news/space/SpaceRepublish_497964.htm|title=Is this life? ABC Science Online|accessdate=2007-07-10}}</ref> If true, they would be the earliest known life on earth.]]

In the [[natural sciences]], [[abiogenesis]], the question of the '''origin of life''', is the study of how [[life]] on [[Earth]] might have emerged from [[non-life]]. Scientific consensus is that abiogenesis occurred sometime between 4.4 [[1000000000 (number)|billion]] years ago, when [[water]] vapor first liquefied,<ref>Simon A. Wilde, John W. Valley, William H. Peck and Colin M. Graham, ''Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago'', Nature 409, 175–178 (2001) {{doi|10.1038/35051550}}</ref> and 2.7 billion years ago, when the ratio of stable [[isotopes]] of [[carbon]] ([[Carbon-12|<sup>12</sup>C]] and [[Carbon-13|<sup>13</sup>C]]), [[iron]] (<sup>56</sup>Fe, <sup>57</sup>Fe, and <sup>58</sup>Fe) and [[sulfur]] (<sup>32</sup>S, <sup>33</sup>S, <sup>34</sup>S, and <sup>36</sup>S) points to a biogenic origin of minerals and sediments<ref>{{cite web|url=http://www.journals.royalsoc.ac.uk/content/01273731t4683245/|title=www.journals.royalsoc.ac.uk/content/01273731t4683245/<!--INSERT TITLE-->|accessdate=2007-07-10}}</ref><ref>{{cite web|url=http://geology.geoscienceworld.org/cgi/content/abstract/34/3/153|title=geology.geoscienceworld.org/cgi/content/abstract/34/3/153<!--INSERT TITLE-->|accessdate=2007-07-10}}</ref> and molecular biomarkers indicate [[photosynthesis]].<ref>{{cite web|url=http://www.journals.royalsoc.ac.uk/content/887701846v502u58/|title=www.journals.royalsoc.ac.uk/content/887701846v502u58/<!--INSERT TITLE-->|accessdate=2007-07-10}}</ref><ref>{{cite web|url=http://www.journals.royalsoc.ac.uk/content/814615517u5757r6/|title=www.journals.royalsoc.ac.uk/content/814615517u5757r6/<!--INSERT TITLE-->|accessdate=2007-07-10}}</ref>
This topic also includes [[panspermia]] and other exogenic theories regarding possible extra-planetary or extraterrestrial origins of life, thought to have possibly occurred sometime over the last 13.7 billion years in the evolution of the [[Universe]] since the [[Big Bang]].<ref>{{cite web|url=http://map.gsfc.nasa.gov/m_mm/mr_age.html|title=map.gsfc.nasa.gov/m_mm/mr_age.html<!--INSERT TITLE-->|accessdate=2007-07-10}}</ref>

Origin of life studies is a limited field of research despite its profound impact on [[biology]] and human understanding of the natural world. Progress in this field is generally slow and sporadic, though it still draws the attention of many due to the eminence of the question being investigated. Several theories have been proposed, most notably the [[iron-sulfur world theory]] (metabolism first) and the [[RNA world hypothesis]] (genetics first).<ref>Chapter 6, last section in Alberts B, Johnson A, Lewis J, Raff M, Roberts K and Walter P, ''Molecular Biology of the Cell'', 4th Edition, Routledge, March, 2002, ISBN 0-8153-3218-1.</ref>

== History of the concept in science ==
[[Image:Aleksandr Oparin and Andrei Kursanov in enzymology laboratory 1938.jpg|thumb|right|Aleksandr Oparin (right) at the laboratory]]

Until the early [[19th century]] people frequently believed in spontaneous generation of life from non-living matter. 

=== Spontaneous generation ===

Classical notions of abiogenesis, now more precisely known as  '''spontaneous generation''', held that complex, living [[organism]]s are generated by decaying organic substances, e.g. that [[mouse|mice]] spontaneously appear in stored grain or maggots spontaneously appear in meat.

According to [[Aristotle]] it was a readily observable truth that [[aphid]]s arise from the dew which falls on plants, [[flea]]s from putrid matter, mice from dirty hay, and crocodiles from rotting logs at the bottom of bodies of water, and so forth. In the 17th century such assumptions started to be questioned; such as that by [[Sir Thomas Browne]] in his [[Pseudodoxia Epidemica]], subtitled ''Enquiries into Very many Received Tenets, and Commonly Presumed Truths'', of 1646, an attack on false beliefs and "vulgar errors." His conclusions were not widely accepted, e.g. his contemporary, [[Alexander Ross (writer)|Alexander Ross]] wrote: "To question this (i.e., spontaneous generation) is to question reason, sense and experience.  If he doubts of this let him go to [[Egypt]], and there he will find the fields swarming with mice, begot of the mud of [[Nile|Nylus]], to the great calamity of the inhabitants."<ref>Balme, D.M. (1962), "Development of Biology in Aristotle and Theophrastus: Theory of Spontaneous Generation" (Phronesis: A journal for Ancient Philosophy, Volume 7, Numbers 1–2, 1962), pp. 91–104(14)</ref>

In 1546 the physician [[Girolamo Fracastoro]] theorized that epidemic diseases were caused by tiny, invisible particles or "spores", which might not be living creatures, but this was not widely accepted. Next, [[Robert Hooke]] published the first drawings of a microorganism in 1665. He is also credited for naming the cell which he discovered while observing cork samples.

Then in 1676 Anthony van Leeuwenhoek discovered microorganisms that, based on his drawings and descriptions are thought to have been protozoa and bacteria. This sparked a renewal in interest in the microscopic world.<ref>Dobell, C. (1960), "Antony Van Leeuwenhoek and his little animals"New York (EUA)</ref> 

The first step was taken by the [[Italy|Italian]] [[Francesco Redi]], who, in 1668, proved that no [[maggot]]s appeared in meat when flies were prevented from laying eggs.  From the 17th century onwards it was gradually shown that, at least in the case of all the higher and readily visible organisms, the previous sentiment regarding spontaneous generation was false. The alternative seemed to be [[omne vivum ex ovo]]: that every living thing came from a pre-existing living thing (literally, from an egg). 

In 1768 [[Lazzaro Spallanzani]] proved that [[microbe]]s came from the air, and could be killed by boiling. Yet it was not until 1861 that [[Louis Pasteur]] performed a series of careful experiments which proved that organisms such as bacteria and fungi do not appear in nutrient rich media of their own accord in non-living material, and which supported [[cell theory]].

=== Darwin and Pasteur ===

By the middle of the 19th century [[Pasteur]] and other scientists demonstrated that living organisms did not arise spontaneously from non-living matter; the question therefore arose of how life might have come about within a [[naturalism (philosophy)|naturalistic]] framework. In a letter to [[Joseph Dalton Hooker]] on [[February 1]], [[1871]],<ref>[http://www.windmillministries.org/frames/CH5A.htm First life on Earth] windmillministries.org, Retrieved on [[2008-01-18]]</ref> [[Charles Darwin]] made the suggestion that the original spark of life may have begun in 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 complex changes". He went on to explain that "at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed."<ref>"It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. But if (and oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, &c., present, that a proteine compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed."  written in 1871, published in [[Francis Darwin|Darwin, Francis]], ed. 1887. ''The life and letters of Charles Darwin, including an autobiographical chapter.'' London: John Murray. Volume 3. p. [http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F1452.3&pageseq=30 18]</ref> In other words, the presence of life itself makes the search for the origin of life dependent on the sterile conditions of the laboratory.

=== Haldane and Oparin ===

No real progress was made until 1924 when [[Aleksandr Ivanovich Oparin]] experimentally showed that atmospheric oxygen prevented the synthesis of the organic molecules that are the necessary building blocks for the evolution of life. In his ''The Origin of Life on Earth'',<ref>Oparin, A. I. (1968), The Origin and Development of Life (NASA TTF-488). Washington: D.C.L GPO,1968 </ref><ref>Oparin, A. I. The Origin of Life. New York: Dover (1952)</ref> Oparin argued that a "primeval soup" of organic molecules could be created in an oxygen-less atmosphere through the action of sunlight. These would combine in ever-more complex fashions until they dissolved into a [[coacervate]] droplet. These droplets would "[[cell growth|grow]]" by fusion with other droplets, and "[[biological reproduction|reproduce]]" through fission into daughter droplets, and so have a primitive [[metabolism]] in which those factors which promote "cell integrity" survive, those that do not become extinct. Many modern theories of the origin of life still take Oparin's ideas as a starting point.
Around the same time [[J. B. S. Haldane]] also suggested that the earth's pre-biotic oceans – very different from their modern counterparts – would have formed a "hot dilute soup" in which organic compounds, the building blocks of life, could have formed. This idea was called biopoiesis or biopoesis, the process of living matter evolving from self-replicating but nonliving molecules.<ref>Bryson, Bill (2003) A Short History of Nearly Everything pp. 300–302; ISBN 0-552-99704-8</ref>

== Early conditions ==
Morse and MacKenzie<ref>Morse, J.W. and MacKenzie, F.T. (1998). "Hadean Ocean Carbonate chemistry." In: ''Aquatic Geochemistry'' '''4''': 301–319</ref> have suggested that oceans may have appeared first in the Hadean era, as soon as 200 million years after the Earth was formed, in a hot (100 °C) [[Redox|reducing]] environment, and that the [[pH]] of about 5.8 rose rapidly towards neutral.  This has been supported by Wilde<ref>Wilde, S.A. et al (2001), "Evidence from detrital zircons for the existence of continental crust and oceans 4.4 Gyr ago", ''"Nature"''409 pp.175–178</ref> who has pushed the date of the [[zircon]] crystals found in the metamorphosed [[quartzite]] of [[Narryer Gneiss Terrane|Mount Narryer]] in Western Australia, previously thought to be 4.1–4.2 billion years old, to 4.404 billion years.  This means that oceans and [[continental crust]] existed within 150 million years of Earth's formation.  Despite this, the [[Hadean]] environment was one highly hazardous to life.  Frequent collisions with large objects, up to 500 kilometres in diameter, would have been sufficient to vaporise the ocean within a few months of impact, with hot steam mixed with rock vapour leading to high altitude clouds completely covering the planet. After a few months the height of these clouds began to decrease but the cloud base would still have been elevated for about the next thousand years. After that, it began to rain at low altitude. For another two thousand years rains slowly draw down the height of the clouds, returning the oceans to their original depth only 3,000 years after the impact event.<ref>Sleep, N.H. et al (1989) "Annihilation of ecosystems by large asteroid impacts on early Earth" ''"Nature"''342, pp139–142</ref>  The possible [[Late Heavy Bombardment]] possibly caused by the movements in position of the Gaseous Giant planets, that pockmarked the moon, and other inner planets (Mercury, Mars, and presumably Earth and Venus), between 3.8 and 4.1 billion years would likely have sterilised the planet if life had already evolved by that time.

Examining the time interval that could have existed between devastating environmental insults by impact exceeded the timescale for establishing self-replicating protoorganisms, the interval in time when life might first have bootstrapped itself into existence can be found for different early environments. The study by Maher and Stephenson<ref>Maher, Kevin A. and Stephenson, David J (19880 "Impact frustration of the origin of life" (Nature. Vol. 331, pp. 612–614. 18 Feb. 1988)</ref> shows that if the deep marine hydrothermal setting provides a suitable site for the origin of life, abiogenesis could have happened as early as 4000 to 4200 Myr ago, whereas if it occurred at the surface of the earth abiogenesis could only have occurred between 3700 and 4000 Myr.

Other research suggests a colder start to life. Research by [[Stanley Miller]] showed the ingredients [[adenine]] and [[guanine]] require freezing conditions to synthesize, but [[cytosine]] and [[uracil]] require boiling temperatures.<ref>Michael P. Robertson and Stanley L. Miller, "An Efficient Prebiotic Synthesis of Cytosine and Uracil," ''Nature 375'' (1995), pp. 772–774</ref> Based on his research he suggested a beginning of life involving freezing conditions and exploding meteorites.<ref>J.L. Bada, C. Bigham, and S.L. Miller, "Impact Melting of Frozen Oceans on the Early Earth: Implications for the Origin of Life," ''Proceedings of the National Academy of Sciences'', USA 91 (February 1994), pp. 1248–1250</ref> A new article in [[Discover (magazine)|Discover Magazine]] points to research by Stanley Miller indicating the formation of seven different amino acids and 11 types of nucleobases in ice when [[ammonia]] and [[cyanide]] were left in the Antarctic ice from 1972–1997,<ref>{{cite journal |last=Levy |first=M |coauthors=Miller SL, Brinton K, Bada JL. |year=2000 |month=June |title=Prebiotic synthesis of adenine and amino acids under Europa-like conditions |journal=Icarus |volume=145 |issue=2 |pages=609–13 |pmid=11543508 |url=http://www.ncbi.nlm.nih.gov/sites/entrez |accessdate= 2008-02-11 |quote= 
 | doi = 10.1006/icar.2000.6365 <!--Retrieved from Yahoo! by DOI bot-->}}</ref> and research by Hauke Trinks showing the formation of RNA molecules 400 bases long under freezing conditions using an RNA template, a single-strand chain of RNA that guides the formation of a new strand of RNA. As that new RNA strand grows, it adheres to the template.<ref>{{cite journal |last=Trinks |first=Hauke |coauthors=Schröder, Wolfgang; Biebricher, Christof
 |year=2005 |month=October |title=Ice And The Origin Of Life  |journal=Origins of Life and Evolution of the Biosphere |volume=35 |issue=5 |pages=429–445 |doi=10.1007 |url=http://www.ingentaconnect.com/content/klu/orig/2005/00000035/00000005/00005009#aff_1 |accessdate= 2008-02-11 |quote= }}</ref> The explanation given for the unusual speed of these reactions at such a low temperature is [[Eutectic point|eutectic freezing]]. As an ice crystal forms, it stays pure: Only molecules of water join the growing crystal, while impurities like salt or cyanide are excluded. These impurities become crowded in microscopic pockets of liquid within the ice, and this crowding causes the molecules to collide more often.<ref>[http://discovermagazine.com/2008/feb/did-life-evolve-in-ice/article_view?b_start:int=0&-C= Discover Magazine article ''Did Life Evolve in Ice?'' published February 2008]</ref>  

Evidence of the early appearance of life comes from the [[Isua]] supercrustal belt in Western Greenland and from similar formations in the nearby [[Akilia Island]]s.  Carbon entering into rock formations has a concentration of elemental δ<sup>13</sup>C of about −5.5, where because of a preferential biotic uptake of <sup>12</sup>C, biomass has a δ<sup>13</sup>C of between −20 and −30.  These isotopic fingerprints are preserved in the sediments, and Mojzis<ref>Mojzis, S.J. et al (1996), "Evidence for life on earth before 3,800 million years ago", ''"Nature"'' 384 pp.55–59</ref> has used this technique to suggest that life existed on the planet already by 3.85 billion years ago.  Lazcano and Miller (1994) suggest that the rapidity of the evolution of life is dictated by the rate of recirculating water through mid-ocean submarine vents. Complete recirculation takes 10 million years, thus any organic compounds produced by then would be altered or destroyed by temperatures exceeding 300 °C.  They estimate that the development of a 100 kilobase genome of a DNA/protein primitive [[heterotroph]] into a 7000 gene filamentous [[cyanobacteria|cyanobacterium]] would have required only 7 million years.<ref>Lazcano A, and S.L. Miller (1994) "How long did it take for life to begin and evolve to cyanobacteria"" ''"Journal of Molecular Evolution" 39 pp.546–554</ref>

== Current models ==
There is no truly "standard model" of the origin of life. But most currently accepted models build in one way or another upon a number of discoveries about the origin of molecular and cellular components for life, which are listed in a rough order of postulated emergence:
# Plausible pre-biotic conditions result in the creation of certain basic small [[molecule]]s ([[monomer]]s) of life, such as [[amino acid]]s. This was demonstrated in the [[Miller experiment|Miller-Urey experiment]] by [[Stanley L. Miller]] and [[Harold C. Urey]] in 1953.
# [[Phospholipid]]s (of an appropriate length) can spontaneously form [[lipid bilayer]]s, a basic component of the [[cell membrane]].
# The [[polymer]]ization of [[nucleotide]]s into random [[RNA]] molecules might have resulted in self-replicating ''[[ribozyme]]s'' (''[[RNA world hypothesis]]'').
# [[Selection]] pressures for catalytic efficiency and diversity result in ribozymes which catalyse [[peptidyl transfer]] (hence formation of small proteins), since oligopeptides complex with RNA to form better catalysts. Thus the first [[ribosome]] is born, and protein synthesis becomes more prevalent.
# [[Proteins]] outcompete ribozymes in catalytic ability, and therefore become the dominant biopolymer. Nucleic acids are restricted to predominantly [[genomic]] use.

The origin of the basic [[biomolecule]]s, while not settled, is less controversial than the significance and order of steps 2 and 3. The basic chemicals from which life was thought to have formed are:
#[[Methane]] (CH<sub>4</sub>),
#[[Ammonia]] (NH<sub>3</sub>),
#[[Water]] (H<sub>2</sub>O),
#[[Hydrogen sulfide]] (H<sub>2</sub>S),
#[[Carbon dioxide]] (CO<sub>2</sub>) or [[carbon monoxide]] (CO), and
#[[Phosphate]] (PO<sub>4</sub><sup>3-</sup>).
Molecular [[oxygen]] (O<sub>2</sub>) and [[ozone]] (O<sub>3</sub>) were either rare or absent.

As of [[2008]], no one has yet synthesized a "protocell" using basic components which would have the necessary properties of life (the so-called ''"bottom-up-approach"''). Without such a proof-of-principle, explanations have tended to be short on specifics. However, some researchers are working in this field, notably [[Steen Rasmussen]] at [[Los Alamos National Laboratory]] and [[Jack Szostak]] at [[Harvard University]]. Others have argued that a ''"top-down approach"'' is more feasible. One such approach, attempted by [[Craig Venter]] and others at [[The Institute for Genomic Research]], involves engineering existing prokaryotic cells with progressively fewer genes, attempting to discern at which point the most minimal requirements for life were reached. The biologist [[John Desmond Bernal]], coined the term [[Abiogenesis|Biopoesis]] for this process, and suggested that there were a number of clearly defined "stages" that could be recognised in explaining the origin of life.
*Stage 1: The origin of biological [[monomers]]
*Stage 2: The origin of biological [[polymers]]
*Stage 3: The evolution from molecules to cell

Bernal suggested that [[evolution|Darwinian evolution]] may have commenced early, some time between Stage 1 and 2.

=== Origin of organic molecules ===
There are three sources of organic molecules on the early Earth: 
#organic synthesis by other energy sources (such as ultraviolet light or electrical discharges) (eg.Miller's experiments). 
#delivery by extraterrestrial objects (eg carbonaceous [[chondrites]]); 
#organic synthesis driven by impact shocks. 

Recently estimates of these sources suggest that the heavy bombardment before 3.5 Gyr ago within the early atmosphere made available quantities of organics comparable to those produced by other energy sources.<ref>Chyba, Christopher and Sagan, Carl (1992),"Endogenous production, exogenous delivery and impact-shock synthesis of organic molecules: an inventory for the origins of life" (Nature 355, 125–132 ([[09 January]] [[1992]]))</ref>
[[Image:UreyMillerExperiment.jpeg|thumb|right|The Miller-Urey experiment attempted to recreate the chemical conditions of the primitive Earth in the laboratory, and synthesized some of the building blocks of life.]]

==== Miller's experiments (The Primordial Soup Theory) ====
{{Main|Miller experiment}}
In 1953 a graduate student, [[Stanley Miller]], and his professor, [[Harold Urey]], performed an experiment that proved organic molecules could have spontaneously formed on [[early Earth]] from inorganic precursors. The now-famous “[[Miller experiment|Miller-Urey experiment]]” used a highly reduced mixture of gases – [[methane]], [[ammonia]] and [[hydrogen]] – to form basic organic [[monomer]]s, such as amino acids. Whether the mixture of gases used in the Miller-Urey experiment truly reflects the atmospheric content of [[early Earth]] is a controversial topic. Other less reducing gases produce a lower yield and variety. It was once thought that appreciable amounts of molecular oxygen were present in the prebiotic atmosphere, which would have essentially prevented the formation of organic molecules; however, the current scientific consensus is that such was not the case. See [[Oxygen Catastrophe]].

Simple organic molecules are, of course, a long way from a fully functional [[self-replication|self-replicating]] life form. But in an environment with no pre-existing life these molecules may have accumulated and provided a rich environment for [[chemical evolution]] ("'''soup theory'''"). On the other hand, the spontaneous formation of complex [[polymer]]s from abiotically generated monomers under these conditions is not at all a straightforward process. Besides the necessary basic organic monomers, compounds that would have prohibited the formation of polymers were formed in high concentration during the experiments.

It can be argued that the most crucial challenge unanswered by this theory is how the relatively simple organic building blocks polymerise and form more complex structures, interacting in consistent ways to form a protocell. For example, in an aqueous environment [[hydrolysis]] of oligomers/polymers into their constituent monomers would be favored over the condensation of individual monomers into polymers. Also, the Miller experiment produces many substances that would undergo cross-reactions with the amino acids or terminate the peptide chain.

==== The Deep Sea Vent Theory ====

The deep sea vent theory for the origin of life on Earth states that life may have began at the interface where chemically rich fluids, heated by some mechanisms like tidal forces of surrounding moons or planets, emerge from below the sea floor. Chemical energy is derived from the reduced gases by the [[redox reaction]]s, such as hydrogen-sulfide and hydrogen coming out from the vent in contact with a suitable oxidant, such as carbon dioxide<ref>This theory may be a way life could have started on the Saturn moon Enceladus. See [http://saturn.jpl.nasa.gov/news/features/feature20080326.cfm ''A Perspective on Life on Enceladus: A World of Possibilities''] for details.</ref>.

==== Fox's experiments ====
In the 1950s and 1960s [[Sidney W. Fox]], studied the spontaneous formation of [[peptide]] structures under conditions that might plausibly have existed early in Earth's history. He demonstrated that amino acids could spontaneously form small peptides. These amino acids and small peptides could be encouraged to form closed spherical membranes, called [[microspheres]].<ref name="foxexp">[http://nitro.biosci.arizona.edu/courses/EEB105/lectures/Origins_of_Life/origins.html Experiments on origin of organic molecules] Nitro.biosci.arizona.edu, Retrieved on [[2008-13-01]]</ref>

==== Eigen's hypothesis ====
In the early 1970s the problem of the origin of life was approached by [[Manfred Eigen]] and [[Peter Schuster]] of the [[Max Planck Institute for Biophysical Chemistry]]. They examined the transient stages between the molecular chaos and a self-replicating [[hypercycle]] in a prebiotic soup.<ref>Manfred Eigen and Peter Schuster: The Hypercycle: A principle of natural self-organization, 1979, Springer ISBN 0-387-09293-5</ref>

In a hypercycle, the [[information|information storing system]] (possibly [[RNA]]) produces an [[enzyme]], which catalyzes the formation of another information system, in sequence until the product of the last aids in the formation of the first information system. Mathematically treated, hypercycles could create [[Quasispecies model|quasispecies]], which through natural selection entered into a form of Darwinian evolution. A boost to hypercycle theory was the discovery that RNA, in certain circumstances forms itself into [[ribozyme]]s, capable of catalyzing their own chemical reactions.<ref name="eigen">[http://www.thebioreview.com/evolution/origin.html origin of life] thebioreview.com Retrieved on [[2008-01-14]]</ref> However, these reactions are limited to self-excisions (in which a longer RNA molecule becomes shorter), and much rarer small additions that are incapable of coding for any useful protein.  The hypercycle theory is further degraded since the hypothetical RNA would require the existence of complex biochemicals such as nucleotides which are not formed under the conditions proposed by the Miller-Urey experiment.

==== Wächtershäuser's hypothesis ====
{{Main|iron-sulfur world theory}}
[[Image:Nur04506.jpg|thumb|Deep-sea [[black smoker]]]]
Another possible answer to this polymerization conundrum was provided in 1980s by [[Günter Wächtershäuser]], in his [[iron-sulfur world theory]]. In this theory, he postulated the evolution of (bio)chemical pathways as fundamentals of the evolution of life. Moreover, he presented a consistent system of tracing today's biochemistry back to ancestral reactions that provide alternative pathways to the synthesis of organic building blocks from simple gaseous compounds.

In contrast to the classical Miller experiments, which depend on external sources of energy (such as simulated lightning or UV irradiation), "Wächtershäuser systems" come with a built-in source of energy, [[sulfide]]s of [[iron]] and other minerals (e.g. pyrite). The energy released from [[redox]] reactions of these metal sulfides is not only available for the synthesis of organic molecules, but also for the formation of [[oligomer]]s and [[polymer]]s. It is therefore hypothesized that such systems may be able to evolve into [[autocatalytic sets]] of self-replicating, metabolically active entities that would predate the life forms known today.

The experiment produced a relatively small yield of [[dipeptides]] (0.4% to 12.4%) and a smaller yield of [[tripeptide]]s (0.10%) but the authors also noted that: "under these same conditions dipeptides hydrolysed rapidly."<ref>Huber, C. and Wächterhäuser, G., (1998). "Peptides by activation of amino acids with CO on (Ni,Fe)S surfaces: implications for the origin of life". Science 281: 670–672.</ref>

====Radioactive beach theory====
Zachary Adam<ref>Dartnell, Lewis "Life's a beach on planet Earth" in ''[[New Scientist]]'' [[12 January]] [[2008]]</ref> at the [[University of Washington]], Seattle, claims that stronger tidal processes from a much closer moon may have concentrated radioactive grains of [[uranium]] and other radioactive elements at the high water mark on primordial beaches where they may have been responsible for generating life's building blocks.  According to computer models reported in Astrobiology, vol 7 p 852, a deposit of such radioactive materials could show the same self-sustaining nuclear reaction as that found in the [[Oklo]] uranium ore seam in [[Gabon]]. Such radioactive beach sand provides sufficient energy to generate organic molecules, such as [[amino acid]]s and [[sugar]]s from [[acetonitrile]] in water. Radioactive [[monazite]] also releases soluble [[phosphate]] into regions between sand-grains, making it biologically "accessible".  Thus amino acids, sugars and soluble phosphates can all be simultaneously produced, according to Adam.  Radioactive [[actinide]]s, then in greater concentrations, could have formed part of organo-metallic complexes. These complexes could have been important early [[catalysts]] to living processes.

John Parnell of the University of Aberdeen suggests that such a process could provide part of the "crucible of life" on any early wet rocky planet, so long as the planet is large enough to have generated a system of [[plate tectonics]] which brings radioactive minerals to the surface.  As the early Earth is believed to have many smaller "platelets" it would provide a suitable environment for such processes.

====Homochirality====
{{Main|Homochirality}}
Some process in chemical evolution must account for the origin of [[homochirality]], i.e. all building blocks in living organisms having the same "handedness" ([[amino acids]] being left-handed, nucleic acid sugars ([[ribose]] and [[deoxyribose]]) being right-handed, and chiral [[Phospholipid#Phosphoglycerides|phosphoglycerides]]). Chiral molecules can be synthesized, but in the absence of a chiral source or a chiral catalyst are formed in a 50/50 mixture of both [[enantiomers]]. This is called a [[racemic]] mixture. Clark has suggested that homochirality may have started in space, as the studies of the amino acids on the [[Murchison meteorite]] showed L-alanine to be more than twice as frequent as its D form, and L-glutamic acid was more than 3 times prevalent than its D counterpart. It is suggested that [[polarised light]] has the power to destroy one [[enantiomer]] within the [[proto-planetary disk]]. Noyes<ref>Noyes, H. Pierre; Bonner, William A.; and Tomlin, J. A. (1977), "On the origin of biological chirality via natural beta-decay" (Origins of Life and Evolution of Biospheres, Volume 8, Number 1 / April, 1977)</ref> showed that [[beta decay]] caused the breakdown of D-[[leucine]], in a [[racemic]] mixture, and that the presence of [[carbon-14|<sup>14</sup>C]], present in larger amounts in organic chemicals in the early Earth environment, could have been the cause.  Robert M. Hazen reports upon experiments conducted in which various chiral crystal surfaces, act as sites for possible concentration and assembly of chiral monomer units into macromolecules<ref>Hazen, Robert M (2005), "genesis: the scientific quest for life's origin" (JosephHenry Books)</ref>.  Once established, chirality would be selected for.<ref>Clark, S. (1999), "Polarised starlight and the handedness of Life" (American Scientist 97, pp336–343)</ref>

==== Self-organization and replication ====
{{main|Self-organization}}
{{sect-stub}}
While features of self-organization and self-replication are often considered the hallmark of living systems, there are many instances of abiotic molecules exhibiting such characteristics under proper conditions.  For example Martin and Russel<ref>Martin, William; and Russel, Michael J (2003), "On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells 
" (Philosophiocal Transactions of the Royal Society, Volume 358, Number 1429/[[January 29]], [[2003]])</ref> show that physical compartmentation by [[cell membranes]] from the environment and self-organization of self-contained [[redox]] reactions are the most conserved attributes of living things, and they argue therefore that inorganic matter with such attributes would be life's most likely last common ancestor.

===From organic molecules to protocells===
The question "How do simple organic molecules form a protocell?" is largely unanswered but there are many hypotheses. Some of these postulate the early appearance of nucleic acids ("[[gene]]s-first") whereas others postulate the evolution of biochemical reactions and pathways first ("[[metabolism]]-first"). Recently, trends are emerging to create hybrid models that combine aspects of both.

===="Genes first" models: the RNA world====
{{main|RNA world hypothesis}}

[[The RNA world hypothesis]] suggests that relatively short [[RNA]] molecules could have spontaneously formed that were capable of catalyzing their own continuing replication. It is difficult to gauge the probability of this formation. A number of theories of modes of formation have been put forward. Early cell membranes could have formed spontaneously from [[proteinoid]]s, protein-like molecules that are produced when amino acid solutions are heated – when present at the correct concentration in aqueous solution, these form microspheres which are observed to behave similarly to membrane-enclosed compartments. Other possibilities include systems of chemical reactions taking 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 store information and catalyse chemical reactions (as a [[ribozyme]]); its many important roles as an intermediate in the expression and maintenance of the genetic information (in the form of [[DNA]]) in modern organisms; and the ease of chemical synthesis of at least the components of the molecule under conditions approximating the early Earth. Relatively short RNA molecules which can duplicate others have been artificially produced in the lab.<ref>W. K. Johnston, P. J. Unrau, M. S. Lawrence, M. E. Glasner and D. P. Bartel[http://www.sciencemag.org/cgi/content/abstract/292/5520/1319 RNA-Catalyzed RNA Polymerization: Accurate and General RNA-Templated Primer Extension], Science 292, 1319 (2001)</ref>

Researchers have pointed out difficulties for the abiogenic synthesis of nucleotides from [[cytosine]] and [[uracil]].<ref>L. Orgel, The origin of life on earth. Scientific American. 271 (4) p. 81, 1994.</ref> Cytosine has a half-life of 19 days at 100 °C and 17,000 years in freezing water.<ref>Matthew Levy and Stanley L. Miller, ''The stability of the RNA bases: Implications for the origin of life'', Proceedings of the National Academy of Science USA 95, 7933–7938 (1998)</ref> Larralde et al, say that "the generally accepted prebiotic synthesis of ribose, the formose reaction, yields numerous sugars without any selectivity."<ref>Larralde R, Robertson M P, Miller S L. Proc Natl Acad Sci USA. 1995;92:8158–8160.</ref> and they conclude that their "results suggest that the backbone of the first genetic material could not have contained ribose or other sugars because of their instability." The ester linkage of ribose and phosphoric acid in RNA is known to be prone to hydrolysis.<ref> Lindahl T. Nature (London). 1993;362:709–715. </ref>

A slightly different version of this hypothesis is that a different type of [[nucleic acid]], such as [[Peptide nucleic acid|PNA]], [[Threose nucleic acid|TNA]] or [[Glycerol nucleic acid|GNA]], was the first one to emerge as a self-reproducing molecule, to be replaced by RNA only later.<ref>Orgel, Leslie (Nov 2000). "A Simpler Nucleic Acid". Science 290 (5495): 1306–1307</ref><ref>Nelson, K.E., Levy, M., and Miller, S.L. [http://www.pnas.org/cgi/content/abstract/97/8/3868 Peptide nucleic acids rather than RNA may have been the first genetic molecule] (2000) Proc. Natl. Acad. Sci. USA 97, 3868–3871.</ref>

===="Metabolism first" models: iron-sulfur world and others====
Several models reject the idea of the self-replication of a "naked-gene" and postulate the emergence of a primitive metabolism which could provide an environment for the later emergence of RNA replication.

One of the earliest incarnations of this idea was put forward in [[1924]] with [[Aleksandr Ivanovich Oparin]]'s notion of primitive self-replicating [[vesicle (biology)|vesicles]] which predated the discovery of the structure of DNA. More recent variants in the 1980s and 1990s include [[Günter Wächtershäuser]]'s [[iron-sulfur world theory]] and models introduced by [[Christian de Duve]] based on the chemistry of [[thioester]]s. More abstract and theoretical arguments for the plausibility of the emergence of metabolism without the presence of genes include a mathematical model introduced by [[Freeman Dyson]] in the early 1980s and [[Stuart Kauffman]]'s notion of collectively [[autocatalytic set]]s, discussed later in that decade.

However, the idea that a closed metabolic cycle, such as the reductive [[citric acid cycle]], could form spontaneously (proposed by Günter Wächtershäuser) remains unsupported. According to [[Leslie Orgel]], a leader in origin-of-life studies for the past several decades, there is reason to believe the assertion will remain so. In an article entitled "Self-Organizing Biochemical Cycles",<ref>''PNAS, vol. 97, no. 23, [[November 7]] [[2000]], p12503–12507''</ref> Orgel summarizes his analysis of the proposal by stating, "There is at present no reason to expect that multistep cycles such as the reductive citric acid cycle will self-organize on the surface of FeS/FeS2 or some other mineral." It is possible that another type of metabolic pathway was used at the beginning of life. For example, instead of the reductive citric acid cycle, the "open" [[acetyl-CoA]] pathway (another one of the four recognised ways of carbon dioxide fixation in nature today) would be even more compatible with the idea of self-organisation on a metal sulfide surface. The key enzyme of this pathway, carbon monoxide dehydrogenase/acetyl-CoA synthase harbours mixed nickel-iron-sulfur clusters in its reaction centers and catalyses the formation of acetyl-CoA (which may be regarded as a modern form of acetyl-thiol) in a single step.

====Bubble Theory====
Waves breaking on the shore create a delicate foam composed of bubbles. Winds sweeping across the ocean have a tendency to drive things to shore, much like driftwood collecting on the beach. It is possible that organic molecules were concentrated on the shorelines in much the same way. Shallow coastal waters also tend to be warmer, further concentrating the molecules through [[evaporation]]. While bubbles composed mostly of water burst quickly, water containing [[amphiphiles]] forms much more stable bubbles, lending more time to the particular bubble to perform these crucial experiments.

Amphiphiles are oily compounds containing a [[hydrophilic]] head on one or both ends of a [[hydrophobic]] molecule. Some amphiphiles have the tendency to spontaneously form membranes in water. A spherically closed membrane contains water and is a hypothetical precursor to the modern cell membrane. If a protein came along that increased the integrity of its parent bubble, then that bubble had an advantage, and was placed at the top of the [[natural selection]] waiting list. Primitive reproduction can be envisioned when the bubbles burst, releasing the results of the experiment into the surrounding medium. Once enough of the 'right stuff' was released into the medium, the development of the first [[prokaryotes]], [[eukaryotes]], and multicellular organisms could be achieved.<ref>''"The Cell: Evolution of the First Organism"'' by Joseph Panno</ref>

Similarly, bubbles formed entirely out of protein-like molecules, called [[microsphere]]s, will form spontaneously under the right conditions. But they are not a likely precursor to the modern cell membrane, as cell membranes are composed primarily of lipid compounds rather than amino-acid compounds (for types of membrane spheres associated with abiogenesis, see [[protobionts]], [[micelle]], [[coacervate]]).

A recent model by Fernando and Rowe<ref>{{cite web|url=http://www.cogs.susx.ac.uk/users/ctf20/dphil_2005/publications.htm|title=www.cogs.susx.ac.uk/users/ctf20/dphil_2005/publications.htm<!--INSERT TITLE-->|accessdate=2007-07-10}}</ref> suggests that the enclosure of an autocatalytic non-enzymatic metabolism within protocells may have been one way of avoiding the side-reaction problem that is typical of metabolism first models.

==Other models==
===Autocatalysis===
[[United Kingdom|British]] [[ethologist]] [[Richard Dawkins]] wrote about [[autocatalysis]] as a potential explanation for the origin of life in his [[2004]] book ''[[The Ancestor's Tale]]''. Autocatalysts are substances which catalyze the production of themselves, and therefore have the property of being a simple molecular replicator. In his book, Dawkins cites experiments performed by [[Julius Rebek]] and his colleagues at the [[Scripps Research Institute]] in [[California]] in which they combined [[amino adenosine]] and [[pentafluorophenyl ester]] with the autocatalyst [[amino adenosine triacid ester]] (AATE). One system from the experiment contained variants of AATE which catalysed the synthesis of themselves. This experiment demonstrated the possibility that autocatalysts could exhibit competition within a population of entities with heredity, which could be interpreted as a rudimentary form of [[natural selection]].

===Clay theory===
A model for the origin of life based on [[clay]] was forwarded by Dr A. [[Graham Cairns-Smith]] of the [[University of Glasgow]] in [[1985]] and adopted as a plausible illustration by several other scientists, including [[Richard Dawkins]]. [[Graham Cairns-Smith#Clay Theory|Clay theory]] postulates that complex organic molecules arose gradually on a pre-existing, non-organic replication platform — silicate crystals in solution. Complexity in companion molecules developed as a function of selection pressures on types of clay crystal is then [[Exaptation|exapted]] to serve the replication of organic molecules independently of their silicate "launch stage". 

Cairns-Smith is a staunch critic of other models of chemical evolution.<ref>''Genetic Takeover: And the Mineral Origins of Life'' ISBN 0-521-23312-7</ref> However, he admits, that like many models of the origin of life, his own also has its shortcomings (Horgan 1991).

In 2007, Kahr and colleagues reported their experiments to examine the idea that crystals can act as 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 from solution. They then examined the distribution of imperfections in the crystal system and found that the imperfections in the mother crystals were indeed reproduced in the daughters. The daughter crystals had many additional imperfections. For a gene-like behavior the additional imperfections should be much less than the parent ones, thus Kahr concludes that the crystals "were not faithful enough to store and transfer information form one generation to the next".<ref>Test of Cairns-Smiths crystals-as-genes hypothesis, Theresa Bullard, John Freudenthal, Serine Avagyan and Bart Kahr, Faraday Discuss., 2007, DOI: 10.1039/b616612c</ref><ref>{{cite news|author=Caroline Moore|title=Crystals as genes?|date=[[16 July]] [[2007]]|publisher=Chemical Science|url=http://www.rsc.org/Publishing/ChemScience/Volume/2007/08/Crystals_as_genes.asp }}</ref>

==="Deep-hot biosphere" model of Gold===
The discovery of [[nanobe]]s (filamental structures that are smaller than bacteria, but that may contain DNA in deep rocks) in the late 1990s has been informally linked with led to a controversial theory put forward by [[Thomas Gold]] in the 1970s that life first developed not on the surface of the Earth, but several kilometers below the surface.<ref name="nanobe">[http://www.microscopy-uk.org.uk/index.html?http://www.microscopy-uk.org.uk/nanobes/nanobes.html Nanobes – Intro] microscopy-uk.org, Retrieved on [[2008-01-14]]</ref> 

<P>It is now reasonably well established that [[microbe|microbial]] life is plentiful at shallow depths in the Earth (up to five kilometers below the surface)<ref name="nanobe"/> in the form of [[archaea | extremophile archaea]], rather than the better-known [[bacterium|eubacteria]] (which live in more accessible conditions). It is claimed that discovery of microbial life below the surface of another body in our [[solar system]] would lend significant credence to this theory. [[Thomas Gold]] also asserted that a trickle of food from a deep, unreachable, source is needed for survival because life arising in a puddle of organic material is likely to consume all of its food and become extinct. Gold's theory is that that flow of food is due to out-gassing of primordial [[methane]] from the [[mantle| Earth's mantle]]; more conventional explanations of the food supply of deep microbes (away from sedimentary carbon compounds) is that the organisms subsist on [[Microbial_metabolism#Hydrogen_oxidation | hydrogen]] released by an interaction between water and (reduced) iron compounds in rocks

==="Primitive" extraterrestrial life===
An alternative to Earthly abiogenesis is the hypothesis that primitive life may have originally formed extraterrestrially, either in space or on a nearby planet (Mars). (Note that ''exogenesis'' is related to, but not the same as, the notion of [[panspermia]]). A supporter of this theory was [[Francis Crick]].

Organic compounds are relatively common in space, especially in the outer solar system where volatiles are not evaporated by solar heating. Comets are encrusted by outer layers of dark material, thought to be a [[tar]]-like substance composed of complex organic material formed from simple carbon compounds after reactions initiated mostly by irradiation by [[ultraviolet]] light. It is supposed that a rain of material from [[comet]]s could have brought significant quantities of such complex organic molecules to Earth.

An alternative but related hypothesis, proposed to explain the presence of life on Earth so soon after the planet had cooled down, with apparently very little time for prebiotic evolution, is that life formed first on early [[Mars (planet)|Mars]]. Due to its smaller size Mars cooled before Earth (a difference of hundreds of millions of years), allowing prebiotic processes there while Earth was still too hot. Life was then transported to the cooled Earth when crustal material was blasted off Mars by asteroid and comet impacts. Mars continued to cool faster and eventually became hostile to the continued evolution or even existence of life (it lost its atmosphere due to low volcanism), Earth is following the same fate as Mars, but at a slower rate.

Neither hypothesis actually answers the question of how life first originated, but merely shifts it to another planet or a comet. However, the advantage of an extraterrestrial origin of primitive life is that life is not required to have evolved on each planet it occurs on, but rather in a single location, and then spread about the galaxy to other star systems via cometary and/or meteorite impact. Evidence to support the plausibility of the concept is scant, but it finds support in recent study of Martian meteorites found in Antarctica and in studies of extremophile microbes.<ref>{{cite web|url=http://www.newscientist.com/channel/life/evolution/dn2844|title=http://www.newscientist.com/channel/life/evolution/dn2844|accessdate=2007-07-10}}</ref> Additional support comes from a recent discovery of a bacterial ecosytem whose energy source is radioactivity.<ref>{{cite journal
|title = Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome
|first = Li-Hung
|last = Lin
|coauthors = Pei-Ling Wang, Douglas Rumble, Johanna Lippmann-Pipke, Erik Boice, Lisa M. Pratt, Barbara Sherwood Lollar, Eoin L. Brodie, Terry C. Hazen, Gary L. Andersen, Todd Z. DeSantis, Duane P. Moser, Dave Kershaw, T. C. Onstott
|journal = Science
|month = October
|year = 2006
|volume = 314
|pages = 479–482
|id = 5798
|doi = 10.1126/science.1127376
|accessdate = 2006-11-12
}}</ref>

=== Lipid World ===
There is a theory that ascribes the first self-replicating object to be lipid-like.<ref>{{cite web|url=http://ool.weizmann.ac.il/|title=ool.weizmann.ac.il/<!--INSERT TITLE-->|accessdate=2007-07-10}}</ref> It is known that phospholipids form bilayers in water while under agitation– the same structure as in cell membranes. These molecules were not present on early earth, however other amphiphilic long chain molecules also form membranes. Furthermore, these bodies may expand (by insertion of additional lipids), and under excessive expansion may undergo spontaneous splitting which preserves the same size and composition of lipids in the two progenies. The main idea in this theory is that the molecular composition of the lipid bodies is the preliminary way for information storage, and evolution led to the appearance of polymer entities such as RNA or DNA that may store information favorably.  Still, no biochemical mechanism has been offered to support the Lipid World theory.

===Polyphosphate model===
The problem with most scenarios of abiogenesis is that the thermodynamic equilibrium of amino acid versus peptides is in the direction of separate amino acids. What has been missing is some force that drives polymerization. The resolution of this problem may well be in the properties of polyphosphates.<ref>{{cite web|url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=528972|title=www.pubmedcentral.nih.gov/articlerender.fcgi?artid=528972<!--INSERT TITLE-->|accessdate=2007-07-10}}</ref><ref>{{cite web|url=http://www.science.siu.edu/microbiology/micr425/425Notes/14-OriginLife.html|title=www.science.siu.edu/microbiology/micr425/425Notes/14-OriginLife.html<!--INSERT TITLE-->|accessdate=2007-07-10}}</ref> Polyphosphates are formed by polymerization of ordinary monophosphate ions PO<sub>4</sub><sup>−3</sup> by ultraviolet light. Polyphosphates cause polymerization of amino acids into peptides. Ample ultraviolet light must have existed in the early oceans. The key issue seems to be that calcium reacts with soluble phosphate to form insoluble [[calcium phosphate]] ([[apatite]]), so some plausible mechanism must be found to keep free calcium ions from solution.

===PAH world hypothesis===
{{Main|PAH world hypothesis}}
Other sources of complex molecules have been postulated, including extraterrestrial stellar or interstellar origin. For example, from spectral analyses, organic molecules are known to be present in comets and meteorites. In [[2004]], a team detected traces of [[Aromatic hydrocarbon#PAHs and the origins of life|polycyclic aromatic hydrocarbons]] (PAH's) in a [[nebula]].<ref>[http://www.aas.org/publications/baas/v35n5/aas203/189.htm Discovery of Blue Fluorescence by Polycyclic Aromatic Hydrocarbon Molecules in the Red Rectangle.] A. N. Witt, et al</ref> Those are the most complex molecules so far found in space. The use of PAH's has also been proposed as a precursor to the RNA world in the [[PAH world hypothesis]].<ref>Battersby, S. (2004). Space molecules point to organic origins. Retrieved [[January 11]], [[2004]] from http://www.newscientist.com/news/news.jsp?id=ns99994552</ref>

===Multiple genesis===

Different forms of life may have appeared quasi-simultaneously in the early history of Earth.<ref>''[http://www.sciam.com/article.cfm?id=are-aliens-among-us&sc=SA_20071119 Are Aliens Among Us? In pursuit of evidence that life arose on Earth more than once, scientists are searching for microbes that are radically different from all known organisms]'' Scientific American. [[19 November]] [[2007]]</ref> The other forms may be extinct, leaving distinctive fossils through their different biochemistry (e.g., [[Alternative biochemistry#Other exotic biochemical elements|using arsenic instead of phosphorus]]), survive as [[extremophiles]], or simply be unnoticed through their being [[Analogy (biology)|analogous]] to organisms of the current life tree.  Hartman<ref>Hartman, Hyman (1998) "Photosynthesis and the Origin of Life" (Origins of Life and Evolution of Biospheres, Volume 28, Numbers 4–6 / October, 1998)</ref> for example combines a number of theories together, by proposing that:
<blockquote>The first organisms were self-replicating iron-rich clays which fixed carbon dioxide into oxalic and other dicarboxylic acids. This system of replicating clays and their metabolic phenotype then evolved into the sulfide rich region of the hotspring acquiring the ability to fix nitrogen. Finally phosphate was incorporated into the evolving system which allowed the synthesis of nucleotides and phospholipids. If biosynthesis recapitulates biopoesis, then the synthesis of amino acids preceded the synthesis of the purine and pyrimidine bases. Furthermore the polymerization of the amino acid thioesters into polypeptides preceded the directed polymerization of amino acid esters by polynucleotides.</blockquote>

==See also==
<div style="-moz-column-count:4; column-count:4;">
* [[List of publications in biology#Origin of life|Important publications in origin of life]]
* [[Astrochemistry]]
* [[Biogenesis]]
* [[Drake equation]]
* [[History of Earth]]
* [[Mimivirus]]
* [[Planetary habitability]]
* [[Rare Earth hypothesis]]
* [[Common descent]]
* [[Zeolite]]s
* [[Origin of the world's oceans]]
* [[Mediocrity principle]]
* [[Autocatalytic reactions and order creation]]
</div>

==References==
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*{{cite book|
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*{{cite journal | author=Fernando CT, Rowe, J| title=Natural selection in chemical evolution. | journal=Journal of Theoretical Biology | year=2007 | volume=247 | pages=152–67}}
*{{cite journal| author=Horgan, J |title=In the beginning |journal=[[Scientific American]]| year=1991 |volume=264 | pages=100–109}} (Cited on p. 108).
*{{cite journal| author=Huber, C. and Wächterhäuser, G., |title=Peptides by activation of amino acids with CO on (Ni,Fe)S surfaces: implications for the origin of life|journal=[[Science (journal)|Science]]| year=1998 |volume=281 | pages=670–672}} (Cited on p. 108).
*{{cite journal| author=Martin, W. and Russell M.J. |title=On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells
|journal=Philosophical Transactions of the Royal Society: Biological sciences| year=2002 |volume=358 | pages=59–85}}
*{{cite journal | author=Russell MJ, Hall AJ, Cairns-Smith AG, Braterman PS | title=Submarine hot springs and the origin of life | journal=Nature | year=1988 | volume=336 | pages=117}}
*{{cite journal | last=Schopf | first=J. W. | coauthors=et al. | title=Laser-Raman imagery of Earth's earliest fossils | journal=Nature | year=2002 | volume=416 | pages=73–76 | pmid=11882894
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*{{cite book|
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*{{cite book|
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*Morowitz, Harold J. (1992) "Beginnings of Cellular Life: Metabolism Recapitulates Biogenesis". Yale University Press. ISBN 0-300-05483-1
*[http://publishing.royalsociety.org/cell-evolution Dedicated issue of ''Philosophical Transactions B'' on Major Steps in Cell Evolution freely available.]
*[http://publishing.royalsociety.org/emergence-of-life Dedicated issue of ''Philosophical Transactions B'' on the Emergence of Life on the Early Earth freely available.]
*{{cite book|
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}}

==External links==
* {{PDFlink|[http://genetics.mgh.harvard.edu/szostakweb/publications/Szostak_pdfs/Hanczyc_and_Szostak_2004_COChemBio.pdf Martin M Hanczyc and Jack W Szostak. Replicating vesicles as models of primitive cell growth and division. Current Opinion in Chemical Biology 2004, 8:660–664.]|192&nbsp;[[Kibibyte|KiB]]<!-- application/pdf, 197274 bytes -->}}
*[http://web.archive.org/web/20061015000732/http://www.santafe.edu/sfi/People/kauffman/sak-peptides.html "SELF-REPLICATION: Even peptides do it"] by [[Stuart Kauffman|Stuart A. Kauffman]] <small>(web archive version as original page no longer accessible)</small>
*[http://www.gla.ac.uk/projects/originoflife/ Origins of Life website including papers, resources, by Dr. Michael Russell at the U. of Glasgow]
* [http://pokey.arc.nasa.gov/~astrochm/LifeImplications.html Possible Connections Between Interstellar Chemistry and the Origin of Life on the Earth]
* [http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=207 Scientists Find Clues That Life Began in Deep Space &mdash; NASA Astrobiology Institute]
*[http://www.pnas.org/cgi/content/full/97/23/12503?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=biochemical+cycles&searchid=1119837712082_3423&stored_search=&FIRSTINDEX=0&journalcode=pnas Self-organizing biochemical cycles &mdash; by Leslie Orgel]
*[http://www.livescience.com/animalworld/060609_life_origin.html How Life Began: New Research Suggests Simple Approach]
*[http://sciam.com/article.cfm?chanID=sa004&articleID=9952573C-E7F2-99DF-32F2928046329479 Primordial Soup's On: Scientists Repeat Evolution's Most Famous Experiment] – an article in Scientific American. [[March 28]], [[2007]]
*[http://www.evolution-textbook.org/content/free/figures/ch04.html Illustrations from ''Evolution'' (textbook)]

== Additional reading ==
* Buehler, Lukas K. (2000–2005) ''The physico-chemical basis of life'', http://www.whatislife.com/about.html accessed [[27 October]] [[2005]].
* Pitsch, S. Krishnamurthy, R. Arrhenius, G. (2000). Concentration of simple aldehydes by sulfite-containing double-layer hydroxide minerals: implications for biopoesis. ''[http://www3.interscience.wiley.com/cgi-bin/abstract/73501648/ABSTRACT Helvetica chimica acta. Sep-Oct. 83(9):2398–411]''. 	
* Hartman,  H.  (1998).  Photosynthesis and the origin of life. ''[http://www.kluweronline.com/article.asp?PIPS=130560&PDF=1 Orig Life Evol Biosph. Oct. 28(4–6):515–21]''.
* ''Things Come to Life'' by Henry Harris (2002) ISBN 0-19-851538-3
* Arrhenius, G.  Sales, B. Mojzsis, S. Lee, T. (1997).  Entropy and charge in molecular evolution — the case of phosphate.   ''[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WMD-45KKS26-30&_coverDate=08%2F21%2F1997&_alid=207952210&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6932&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=b479dc1b44ba7db55790640aaf059006 J Theor Biol. Aug 21. 187(4):503–22''].
* NASA Astrobiology Institute: [http://astrobiology.ucla.edu/pages/res3b.html Earth's Early Environment and Life]
* NASA Specialized Center of Research and Training in Exobiology: [http://exobio.ucsd.edu/arrhenius.htm Gustaf O. Arrhenius]

==Notes==
{{reflist|2}}

{{Origin of life}}

[[Category:Biology theories]]
[[Category:Origin of life]]

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