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			==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:'''377–398, 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> Recent experiments also suggest that the original estimates of the size of an RNA molecule capable of self-replication were most probably vast underestimates.{{Fact\|date=August 2007}} 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===
			{{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==		==References==
	*{{cite book\|		*{{cite book\|

Revision as of 00:03, 20 March 2008

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

Main article: Homochirality

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

Notes

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.
Yockey, 1992. Information Theory and Molecular Biology, p. 336, Cambridge University Press, UK, ISBN 0-521-80293-8.
L. Orgel, The origin of life on earth. Scientific American. 271 (4) p. 81, 1994.
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)
Clark, S. (1999), "Polarised starlight and the handedness of Life" (American Scientist 97, pp336-343)
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

v t e Origin of life
History of research	Panspermia (5th century BC) Spontaneous generation (4th century BC) Primordial soup (19th century)
Prebiotic synthesis	Pseudo-panspermia Miller–Urey experiment Formamide-based prebiotic chemistry Alternative abiogenesis scenarios
Protocells	GADV-protein world Clay hypothesis Iron–sulfur world Primordial sandwich PAH world Peptide-RNA world Quasispecies model RNA world
Earliest organisms	Earliest known life forms Last universal common ancestor (LUCA)
Research	Astrobiology Paleobiology

v t e Branches of biology
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See also	History of biology Nobel Prize in Physiology or Medicine Timeline of biology and organic chemistry

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