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Other models

Autocatalysis

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. 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 exapted 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. 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".

"Deep-hot biosphere" model of Gold

The discovery of nanobes (filamental structures that are smaller than bacteria, but that may contain DNA) in deep rocks, led to a controversial theory put forward by Thomas Gold in the 1990s that life first developed not on the surface of the Earth, but several kilometers below the surface. It is now known that microbial life is plentiful up to five kilometers below the earth's surface in the form of archaea, which are generally considered to have originated either before or around the same time as eubacteria, 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 solar system 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 panspermia). A supporter of this theory is 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 comets 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. 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. Additional support comes from a recent discovery of a bacterial ecosytem whose energy source is radioactivity.

The Lipid World

There is a theory that ascribes the first self-replicating object to be lipid-like. 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.

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. Polyphosphates are formed by polymerization of ordinary monophosphate ions PO₄ 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. Possibly, the answer may be in some stable, non-reactive complex such as calcium citrate.

The Ecopoesis model

The Ecopoesis model proposes that the geochemical cycles of biogenic elements, driven by an early oxygen-rich atmosphere, were the basis of a planetary metabolism that preceded and conditioned the gradual evolution of organismal life.

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 2004, a team detected traces of polycyclic aromatic hydrocarbons (PAH's) in a nebula. 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.

Multiple genesis

Different forms of life may have appeared quasi-simultaneously in the early history of Earth . The other forms may be extinct, leaving distinctive fossils through their different biochemistry (e.g., using arsenic instead of phosphorus), survive as extremophiles, or simply be unnoticed through their being analoguous to organisms of the current life tree.

Criticisms

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

Other scientists have proposed counterpoints to abiogenesis, such as, Harold Urey, Stanley Miller, Francis Crick (a molecular biologist), and Leslie Orgel (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

Sir Fred Hoyle, with Chandra Wickramasinghe, was a critic of abiogenesis. Specifically, Hoyle rejected chemical evolution in explaining the naturalistic origin of life. 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 panspermia as an alternative natural explanation to the origin of life on Earth.

Yockey

Information theorist Hubert Yockey argued that chemical evolutionary research faces the following problem:

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 genesis 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.

In a book he wrote 15 years later, Yockey argued that the idea of abiogenesis from a primordial soup is a failed paradigm:

Although at the beginning the paradigm was worth consideration, now the entire effort in the primeval soup paradigm is self-deception on the ideology of its champions.…

The history of science 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 religion, philosophy and ideology that he must have a set of beliefs, come what may (Hoffer, 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 ignorance. This has been universally the case in the history of science as Kuhn (1970) has discussed in detail. There is no reason that this should be different in the research on the origin of life.

Yockey's publications have become favorites to quote among creationists, though he is not a creationist himself (as noted in this 1995 email).

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 pyrimidine nucleobases cytosine and uracil under prebiotic conditions. Other problems are the difficulty of nucleoside synthesis (from ribose and nucleobase), ligating nucleosides with phosphate to form the RNA backbone, and the short lifetime of the nucleoside molecules, especially cytosine which is prone to hydrolysis. Recent experiments also suggest that the original estimates of the size of an RNA molecule capable of self-replication were most probably vast underestimates. More-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

Another unsolved issue in chemical evolution is 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 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. However, homochirality 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 Murchison meteorite 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 polarised light has the power to destroy one enantiomer within the proto-planetary disk. Once established, chirality would be selected for.

Work performed in 2003 by scientists at Purdue identified the amino acid serine as being a probable root cause of the organic molecules' homochirality. 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

• Astrobiology 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.
• Complex systems

See also

• Important publications in origin of life
• Astrochemistry
• Biogenesis
• Drake equation
• History of Earth
• Mimivirus Giant and very old virus that could have emerged prior to cellular organisms.
• Planetary habitability
• Rare Earth hypothesis
• Common descent
• Zeolites
• Origin of the world's oceans
• Mediocrity principle

Notes

1. Genetic Takeover: And the Mineral Origins of Life ISBN 0-521-23312-7
2. Test of Cairns-Smiths crystals-as-genes hypothesis, Theresa Bullard, John Freudenthal, Serine Avagyan and Bart Kahr, Faraday Discuss., 2007, DOI: 10.1039/b616612c
3. Caroline Moore (16 July 2007). "Crystals as genes?". Chemical Science.
4. "http://www.newscientist.com/channel/life/evolution/dn2844". Retrieved 2007-07-10. {{cite web}}: External link in |title= (help)
5. Lin, Li-Hung (2006). "Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome". Science. 314: 479–482. doi:10.1126/science.1127376. 5798. {{cite journal}}: |access-date= requires |url= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
6. "ool.weizmann.ac.il/". Retrieved 2007-07-10.
7. "www.pubmedcentral.nih.gov/articlerender.fcgi?artid=528972". Retrieved 2007-07-10.
8. "www.science.siu.edu/microbiology/micr425/425Notes/14-OriginLife.html". Retrieved 2007-07-10.
9. "www.ecopoese.bio.br/ingles.htm". Retrieved 2007-07-10.
10. Discovery of Blue Fluorescence by Polycyclic Aromatic Hydrocarbon Molecules in the Red Rectangle. A. N. Witt, et al
11. Battersby, S. (2004). Space molecules point to organic origins. Retrieved January 11, 2004 from http://www.newscientist.com/news/news.jsp?id=ns99994552
12. [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
13. Yockey, 1977. A calculation of the probability of spontaneous biogenesis by information theory, Journal of Theoretical Biology 67:377–398, quotes from pp. 379, 396.
14. Yockey, 1992. Information Theory and Molecular Biology, p. 336, Cambridge University Press, UK, ISBN 0-521-80293-8.
15. L. Orgel, The origin of life on earth. Scientific American. 271 (4) p. 81, 1994.
16. 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)
17. Clark, S. (1999), "Polarised starlight and the handedness of Life" (American Scientist 97, pp336-343)
18. 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.

References

• Brooks, J (1973). Origins and Development of Living Systems. Academic Press. p. 359. ISBN 0-12-135740-6. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• De Duve, Christian (1996). Vital Dust: The Origin and Evolution of Life on Earth. Basic Books. ISBN 0-465-09045-1. {{cite book}}: Cite has empty unknown parameter: |1= (help); Unknown parameter |month= ignored (help)
• Fernando CT, Rowe, J (2007). "Natural selection in chemical evolution". Journal of Theoretical Biology. 247: 152–67.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Horgan, J (1991). "In the beginning". Scientific American. 264: 100–109. (Cited on p. 108).
• 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.{{cite journal}}: CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link) (Cited on p. 108).
• Martin, W. and Russell M.J. (2002). "On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells". Philosophical Transactions of the Royal Society: Biological sciences. 358: 59–85.
• Russell MJ, Hall AJ, Cairns-Smith AG, Braterman PS (1988). "Submarine hot springs and the origin of life". Nature. 336: 117.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• JW Schopf; et al. (2002). "Laser-Raman imagery of Earth's earliest fossils". Nature. 416: 73–76. PMID 11882894. {{cite journal}}: Explicit use of et al. in: |author= (help)
• Maynard Smith, John (2000-03-16). The Origins of Life: From the Birth of Life to the Origin of Language. Oxford Paperbacks. ISBN 0-19-286209-X. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Hazen, Robert M. (2005). Genesis: The Scientific Quest for Life's Origins. Joseph Henry Press. ISBN 0-309-09432-1. {{cite book}}: Unknown parameter |month= ignored (help)
• Morowitz, Harold J. (1992) "Beginnings of Cellular Life: Metabolism Recapitulates Biogenesis". Yale University Press. ISBN 0-300-05483-1

External links

• Template:PDFlink
• "SELF-REPLICATION: Even peptides do it" by Stuart A. Kauffman (web archive version as original page no longer accessible)
• Origins of Life website including papers, resources, by Dr. Michael Russell at the U. of Glasgow
• Possible Connections Between Interstellar Chemistry and the Origin of Life on the Earth
• Scientists Find Clues That Life Began in Deep Space — NASA Astrobiology Institute
• Self-organizing biochemical cycles — by Leslie Orgel
• How Life Began: New Research Suggests Simple Approach
• Primordial Soup's On: Scientists Repeat Evolution's Most Famous Experiment - an article in Scientific American. March 28, 2007

• Evolution
• Metabolism
• Origin of life