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{{Mergefrom|Abiogenesis|date=January 2008}} #REDIRECT]
{{r mentioned in hatnote}}
{{dablink|For the definition, see ]. This article focuses on modern scientific research on the origin of life. For alternate, non-scientific ], including ], see ]. For religious beliefs about the creation of life, see ]. For the observed evolution of life on earth, see the ]. }}

] ] in the Siyeh Formation, ]. In 2002, William Schopf of ] published a controversial paper in the ] '']'' arguing that geological formations such as this possess 3.5 billion year old ] ] 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 ], ], the question of the '''origin of life''', is the study of how ] might have emerged from ]. Scientific consensus is that abiogenesis occurred sometime between 4.4 ] years ago, when ] 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 ] of ] (] and ] ), ] and ] 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 ].<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 ] and other exogenic theories regarding possible extra-planetary or extra-terrestrial origins of life, thought to have possibly occurred sometime over the last 13.7 billion years in the ] of the ] since the ].<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 ] 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 ] (metabolism first) and the ] (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 ==
]

Until the early ] people frequently believed in ] of life from non-living matter.

=== Darwin and Pasteur ===

By the middle of the 19th century ] and others had 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 ] framework. In a letter to ] on February 1, 1871,{{fact}} ] 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 ], ed. 1887. ''The life and letters of Charles Darwin, including an autobiographical chapter.'' London: John Murray. Volume 3. p. </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 ] 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'', Oparin argued that a "]" 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 ] droplet. These droplets would "]" by fusion with other droplets, and "]" through fission into daughter droplets, and so have a primitive ] 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 ] 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 ] or ], the process of living matter evolving from self-replicating but nonliving molecules.

== Early conditions ==
Morse and MacKenzie <ref>Morse, J.W. and MacKenzie, F.T. (1998) "Hadean Ocean Carbonate chemistry" in ''"Aquatic Geochemistry"''' 4 pp.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) ] environment, and that the ] 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 ] crystals found in the metamorphosed ] of ] in Western Australia, previously thought to be 4.1-4.2 billion years old, to 4.404 billion years. This means that oceans and ] existed within 150 million years of Earth's formation. Despite this, the ] 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 begins to decrease but the cloud base would still be elevated probably for the next thousand years after which at low altitude it starts to rain. 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 ] 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.

Evidence of the early appearance of life comes from the ] supercrustal belt in Western Greenland and from similar formations in nearby the ]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 ] into a 7000 gene filamentous ] would have required only 7 million years.<ref>Lazcano A, and S.L. Miller (1996) "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 ]s (]s) of life, such as ]s. This was demonstrated in the ] by ] and ] in 1953.
# ]s (of an appropriate length) can spontaneously form ]s, a basic component of the ].
# The ]ization of ]s into random ] molecules might have resulted in self-replicating '']s'' ('']'').
# ] pressures for catalytic efficiency and diversity result in ribozymes which catalyse ] (hence formation of small proteins), since oligopeptides complex with RNA to form better catalysts. Thus the first ] is born, and protein synthesis becomes more prevalent.
# ] outcompete ribozymes in catalytic ability, and therefore become the dominant biopolymer. Nucleic acids are restricted to predominantly ] use.

The origin of the basic ]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:-
#] (CH<sub>4</sub>),
#] (NH<sub>3</sub>),
#] (H<sub>2</sub>O),
#] (H<sub>2</sub>S),
#] (CO<sub>2</sub>) or ] (CO), and
#] (PO<sub>4</sub><sup>3-</sup>).
Molecular ] (O<sub>2</sub>) and ] (O<sub>3</sub>) were either rare or absent.

As of ], 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 ] at ] and ] at ]. Others have argued that a ''"top-down approach"'' is more feasible. One such approach, attempted by ] and others at ], 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 ], coined the term ] 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 ]
*Stage 2: The origin of biological ]
*Stage 3: The evolution from molecules to cell

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

=== Origin of organic molecules ===
]

==== Miller's experiments ====
{{Main|Miller experiment}}
In 1953 a graduate student, ], and his professor, ], performed an experiment that proved organic molecules could have spontaneously formed on ] from inorganic precursors. The now-famous “]” used a highly reduced mixture of gases - ], ] and ] – to form basic organic ]s, such as amino acids. Whether the mixture of gases used in the Miller-Urey experiment truly reflects the atmospheric content of ] 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 ].

Simple organic molecules are, of course, a long way from a fully functional ] life form. But in an environment with no pre-existing life these molecules may have accumulated and provided a rich environment for ] ("]"). On the other hand, the spontaneous formation of complex ]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 ] 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.

==== Fox's experiments ====
In the 1950s and 1960s ], studied the spontaneous formation of ] 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. Fox described these formations as ], protein spheres that could grow and reproduce.<ref name="foxexp"> 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 ] and ] of the ]. They examined the transient stages between the molecular chaos and a self replicating ] 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 ] (possibly ]) produces an ], 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 ], 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 ]s, capable of catalyzing their own chemical reactions.<ref name="eigen"> 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}}
]]]
Another possible answer to this polymerization conundrum was provided in 1980s by ], in his ]. 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, ]s of ] and other minerals (e.g. pyrite). The energy released from ] reactions of these metal sulfides is not only available for the synthesis of organic molecules, but also for the formation of ]s and ]s. It is therefore hypothesized that such systems may be able to evolve into ] of self-replicating, metabolically active entities that would predate the life forms known today.

The experiment produced a relatively small yield of ] (0.4% to 12.4%) and a smaller yield of ]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>

===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 ("]s-first") whereas others postulate the evolution of biochemical reactions and pathways first ("]-first"). Recently, trends are emerging to create hybrid models that combine aspects of both.

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

] suggests that relatively short ] 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 ]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 ] substrates or on the surface of ] 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 ]); its many important roles as an intermediate in the expression and maintenance of the genetic information (in the form of ]) 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, Science 292, 1319 (2001)</ref>

A slightly different version of this hypothesis is that a different type of ], such as ], ] or ], 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. (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 ] with ]'s notion of primitive self-replicating ] which predated the discovery of the structure of DNA. More recent variants in the 1980s and 1990s include ]'s ] and models introduced by ] based on the chemistry of ]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 ] in the early 1980s and ]'s notion of collectively ]s, discussed later in that decade.

However, the idea that a closed metabolic cycle, such as the reductive ], could form spontaneously (proposed by Günter Wächtershäuser) remains unsupported. According to ], 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, ] ], 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" ] 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 ]. While bubbles composed mostly of water burst quickly, water containing ] forms much more stable bubbles, lending more time to the particular bubble to perform these crucial experiments.

Amphiphiles are oily compounds containing a ] head on one or both ends of a ] 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 ] 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 ], ], 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 ]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 ], ], ]).


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.

==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 extreme tidal processes from a much closer moon, may have concentrated radioactive grains of uranium and other radioactive elements common in the young earth 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 desposit of such radioactive materials could experience the same self-sustaining nuclear reaction as that found in the Oklo uranium ore seam in Gabon. Adam has found that such radioactive beach sand provides sufficient energy to generate biological molecules in water, such as acetonitrile, which can produce amino acids and sugars when irradiated. Radioactive monazite also releases soluble phosphate into regions between sand-grains, making it biologically "accessible" in the water. Thus amino acids, sugars and soluble phosphates can all be simultaneously produced in the radioactive beach environment, according to Adam. Radioactive actinates, then in greater concentrations, could have formed part of organo-metalic complexes, made up of a metalic ion and organic molecule. 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.

==Other models==
===Autocatalysis===
] ] ] wrote about ] as a potential explanation for the origin of life in his ] book '']''. 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 ] and his colleagues at the ] in ] in which they combined ] and ] with the autocatalyst ] (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 ].

===Clay theory===
A model for the origin of life based on ] was forwarded by Dr A. ] of the ] in ] and adopted as a plausible illustration by several other scientists, including ]. ] 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 ] to serve the replication of organic molecules independently of their silicate "launch stage". It is, truly, "life from a rock."

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 ]. "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 ]s (filamental structures that are smaller than bacteria, but that may contain DNA) in deep rocks, led to a controversial theory put forward by ] in the 1990s that life first developed not on the surface of the Earth, but several kilometers below the surface.<ref name="nanobe"> microscopy-uk.org, Retrieved on- 2008-01-14</ref> It is now known that ] life is plentiful up to five kilometers below the earth's surface <ref name="nanobe"/> in the form of ], which are generally considered to have originated either before or around the same time as ], most of which live on the surface including the oceans. It is claimed that discovery of microbial life below the surface of another body in our ] would lend significant credence to this theory. He also noted 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.

==="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 ]). A supporter of this theory is ].

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 ]-like substance composed of complex organic material formed from simple carbon compounds after reactions initiated mostly by irradiation by ] light. It is supposed that a rain of material from ]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 ]. 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>

=== The 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 spontaneously form bilayers in water - 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.

===The 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 ] (]), so some plausible mechanism must be found to keep free calcium ions from solution. Possibly, the answer may be in some stable, non-reactive complex such as calcium citrate.



===PAH world hypothesis===
{{Main|PAH world hypothesis}}
Other sources of complex molecules have been postulated, including extra-terrestrial stellar or interstellar origin. For example, from spectral analyses, organic molecules are known to be present in comets and meteorites. In ], a team detected traces of ] (PAH's) in a ].<ref> 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 ]<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., ]), survive as ], or simply be unnoticed through their being ] to organisms of the current life tree.

==Criticisms==
The modern concept of ] has been criticized by scientists throughout the years. Astronomer ] did so based on the probability of abiogenesis randomly occurring. Physicist ] did so by saying that it is closer to theology than science.

Other scientists have proposed counterpoints to abiogenesis, such as, ], ], ] (a molecular biologist), and ] (through the Directed Panspermia hypothesis).

Beyond making the trivial observation that life exists, it is difficult to prove or falsify abiogenesis; therefore, the hypothesis has many such critics, both in the scientific and nonscientific communities. Nonetheless, research and hypothesizing continue in the hope of developing a satisfactory theoretical mechanism of abiogenesis.

===Hoyle===
], with ], was a critic of abiogenesis. Specifically, Hoyle rejected ] in explaining the ] ]. His argument was mainly based on the improbability of what were thought to be the necessary components coming together for chemical evolution. Though modern theories address his argument, Hoyle never saw chemical evolution as a reasonable explanation, preferring ] as an alternative natural explanation to the origin of life on ].

===Yockey===
Information theorist ] argued that chemical evolutionary research faces the following problem:
<blockquote>
Research on the origin of life seems to be unique in that the conclusion has already been authoritatively accepted…. What remains to be done is to find the scenarios which describe the detailed mechanisms and processes by which this happened.

One must conclude that, contrary to the established and current wisdom a scenario describing the ] of life on earth by chance and natural causes which can be accepted on the basis of fact and not faith has not yet been written.<ref>Yockey, 1977. A calculation of the probability of spontaneous biogenesis by information theory, ''Journal of Theoretical Biology'' '''67:'''377&ndash;398, quotes from pp. 379, 396.</ref>
</blockquote>
In a book he wrote 15 years later, Yockey argued that the idea of abiogenesis from a primordial soup is a failed ]:
<blockquote>
Although at the beginning the paradigm was worth consideration, now the entire effort in the primeval soup paradigm is self-deception on the ] of its champions.…

The ] shows that a paradigm, once it has achieved the status of acceptance (and is incorporated in textbooks) and regardless of its failures, is declared invalid only when a new paradigm is available to replace it. Nevertheless, in order to make progress in science, it is necessary to clear the decks, so to speak, of failed paradigms. This must be done even if this leaves the decks entirely clear and no paradigms survive. It is a characteristic of the true believer in ], philosophy and ideology that he must have a set of beliefs, come what may (], 1951). Belief in a primeval soup on the grounds that no other paradigm is available is an example of the logical fallacy of the false alternative. In science it is a virtue to acknowledge ]. This has been universally the case in the history of science as ] (1970) has discussed in detail. There is no reason that this should be different in the research on the origin of life.<ref>Yockey, 1992. ''Information Theory and Molecular Biology'', p. 336, ] Press, UK, ISBN 0-521-80293-8.</ref>
</blockquote>
Yockey's publications have become favorites to ] among ]s, though he is not a creationist himself (as noted in ).

===Abiogenic synthesis of key chemicals===
A number of problems with the RNA world hypothesis remain. There are no known chemical pathways for the abiogenic synthesis of nucleotides from ] nucleobases ] and ] under prebiotic conditions.<ref>L. Orgel, The origin of life on earth. Scientific American. 271 (4) p. 81, 1994.</ref> Other problems are the difficulty of ] synthesis (from ] and ]), ligating nucleosides with ] to form the RNA backbone, and the short lifetime of the nucleoside molecules, especially cytosine which is prone to hydrolysis.<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>
Several modern forms of the RNA World theory propose that a simpler molecule was capable of self-replication (that other "World" then evolved over time to produce the RNA World). At this time however, the various hypotheses have incomplete evidence supporting them. Many of them can be simulated and tested in the lab, but a lack of undisturbed sedimentary rock from that early in Earth's history leaves few opportunities to test this hypothesis robustly.

===Homochirality Problem===
{{Main|Homochirality}}
Another unsolved issue in chemical evolution is the origin of ], i.e. all building blocks in living organisms having the same "handedness" (] being left-handed, nucleic acid sugars (] and ]) being right-handed, and chiral ]). 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 ]. This is called a ] mixture. However, ] is essential for the formation of functional ribozymes and proteins. Proper formation is impeded by the very presence of right-handed amino acids and/or left-handed sugars in that they create malformed structures.

Clark has suggested that homochirality may have started in space, as the studies of the amino acids on the ] showed L-analine 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 ] has the power to destroy one ] within the ]. Once established, chirality would be selected for<ref>Clark, S. (1999), "Polarised starlight and the handedness of Life" (American Scientist 97, pp336-343)</ref>.

Work performed in 2003 by scientists at ] identified the amino acid ] as being a probable root cause of the organic molecules' homochirality.<ref>Nanita, Sergio C.; Cooks, R. Graham,"Serine Octamers: Cluster Formation, Reactions, and Implications for Biomolecule Homochirality",Angewandte Chemie International Edition, 2006,45(4),554-569,doi: 10.1002/anie.200501328.</ref> Serine forms particularly strong bonds with amino acids of the same chirality, resulting in a cluster of eight molecules that must be all right-handed or left-handed. This property stands in contrast with other amino acids which are able to form weak bonds with amino acids of opposite chirality. Although the mystery of why left-handed serine became dominant is still unsolved, this result suggests an answer to the question of chiral transmission: how organic molecules of one chirality maintain dominance once asymmetry is established.

==Relevant fields==
* ] is a field that may shed light on the nature of life in general, instead of just life as we know it on Earth, and may give clues as to how life originates.
* ]s

==See also==
<div style="-moz-column-count:4; column-count:4;">
* ]
* ]
* ]
* ]
* ]
* ] Giant and very old virus that could have emerged prior to cellular organisms.
* ]
* ]
* ]
* ]s
* ]
* ]
</div>

==Notes==
{{reflist}}

==References==
*{{cite book|
title=Origins and Development of Living Systems.|
last=Brooks|
first=J|
coauthors=Shaw, G.|
year=1973|
publisher=]|
id=ISBN 0-12-135740-6|
pages=359
}}
*{{cite book|
title=Vital Dust: The Origin and Evolution of Life on Earth|
last=De Duve |
first=Christian|
authorlink=Christian de Duve|
year=1996|
month=Jan|
publisher=]|
id=ISBN 0-465-09045-1|
}}
*{{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=]| 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=]| 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 | author=JW Schopf et al. | title=Laser-Raman imagery of Earth's earliest fossils. | journal=Nature | year=2002 | volume=416 | pages=73-76 | id=PMID 11882894}}
*{{cite book|
title=The Origins of Life: From the Birth of Life to the Origin of Language|
last=Maynard Smith|
first=John|
authorlink=John Maynard Smith|
coauthors=Szathmary, Eors|date=2000-03-16|
publisher=Oxford Paperbacks|
id=ISBN 0-19-286209-X
}}
*{{cite book|
last=Hazen|
first=Robert M.|
publisher=Joseph Henry Press|
id=ISBN 0-309-09432-1|
year=2005|
month=Dec|
title=Genesis: The Scientific Quest for Life's Origins|
url=http://newton.nap.edu/books/0309094321/html
}}
*Morowitz, Harold J. (1992) "Beginnings of Cellular Life: Metabolism Recapitulates Biogenesis". Yale University Press. ISBN 0-300-05483-1
==External links==
* {{PDFlink||192&nbsp;]<!-- application/pdf, 197274 bytes -->}}
* by ] <small>(web archive version as original page no longer accessible)</small>
*
*
*
*
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* - an article in Scientific American. March 28, 2007

=== Podcasts, videos ===
*
* - lecture by Harold Morowitz, ]. April 04, 2007.

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