{{Mergeto|Origin of life|date=January 2008}}
{{dablink|This article focuses on the history of thought regarding abiogenesis (the spontaneous generation of life from non-living sources). For abiogenesis as a scientific study, see [[Origin of life]]. For the petroleum generation theory, see [[Abiogenic petroleum origin]].}}

'''Abiogenesis''' ([[Greek language|Greek]] ''a-bio-genesis'', "non biological origins") is the formation of life from non-living matter. Today the term is primarily used to refer to the chemical [[origin of life]], such as from a 'primordial soup' or in the vicinity of [[hydrothermal vent]]s, and most probably through a number of intermediate steps, such as non-living but self-replicating molecules (biopoiesis). The current models of abiogenesis are still being [[scientific method|scientifically]] tested.

== Hypotheses ==

=== Primordial soup ===

In 1936 [[Aleksandr Ivanovich Oparin]], in his "The Origin of Life on Earth", suggested that organic molecules could be created in an oxygen-less atmosphere, through the action of sunlight. These molecules, he suggested, combine in ever-more complex fashion until they are dissolved into a [[coacervate]] droplet. These droplets could then fuse with other droplets and break apart into two replicas of the original. This could be viewed as a primitive form of reproduction and metabolism. Favorable attributes such as increased durability in the structure would survive more often than nonfavorable attributes.

Around the same time [[J. B. S. Haldane]] 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.

In 1953, taking their cue from Oparin and Haldane, the chemist [[Stanley L. Miller]] working under [[Harold C. Urey]] carried out the famous [[Miller-Urey experiment]] on the "[[primeval soup]]". Within two weeks a [[racemic]] mixture, containing 13 of the 21 [[amino acid]]s used to synthesize proteins in cells, had formed from the highly [[reduced gas|reduced]] mixture of [[methane]], [[ammonia]], water vapor and [[hydrogen]]. While Miller and Urey did not actually create life, they demonstrated that more complex molecules could emerge spontaneously from simpler chemicals. The environment simulated atmospheric conditions as the researchers understood them to have been on the primeval earth, including an external energy source in the form of a spark, representing lightning, and an atmosphere largely devoid of oxygen. Since that time there have been other experiments that continue to look into possible ways for life to have formed from non-living chemicals, e.g. the experiments conducted by [[Joan Oró]] in [[1961]].

=== Spontaneous generation ===

Classical notions of abiogenesis, now more precisely known as '''spontaneous generation''', held that complex, living organisms 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|aphids]] arise from the dew which falls on plants, [[flea]]s from putrid matter, mice from dirty hay, alligators 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."

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.

The first step was taken by the [[Italy|Italian]] [[Francesco Redi]], who, in 1688, proved that no [[maggot]]s appeared in meat when flies were prevented from laying eggs. From the seventeenth 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]].

Three years earlier, Darwin's [[On the Origin of Species by Means of Natural Selection]] (published in 1859), had presented an argument that modern organisms had evolved, over immense periods of time, from simpler ancestral forms, and that species changed over time in accordance with cell theory. Darwin himself declined to speculate on some implications of his theory - that at some point there may have existed an ur-organism with no prior ancestor and that such an organism may have come into existence, formed from non-living molecules.

Although Pasteur had demonstrated that modern organisms do not generate spontaneously in nonliving nutrients, his experiments were limited to a smaller system, and for a shorter time, than the open surface of the planet over millions or billions of years. The ur-organism implied by Darwin's theories would have occurred in the deep geological past, 3.87 billion years ago, and it had a billion years from the beginning of the planet to be formed.

=== Panspermia ===

{{main|Panspermia}}

Panspermia, a hypothesis that allows [[life on Earth]] to have originated elsewhere in the universe, is viewed by some as an alternative to abiogenesis. All forms of the theory posit that life has spread through space to Earth, perhaps from other star systems. In its strongest form, Panspermia says that life has always existed. More common forms, however, simply transfer the ''origin problem'' elsewhere, and as such have no contention with abiogenesis; indeed they mitigate the potential problem of time constraints on abiogenesis occurring on Earth.

=== Clay hypothesis ===

{{main|Graham Cairns-Smith}}
Clay hypothesis (sometimes called clay theory) has been presented by [[Graham Cairns-Smith]] as a possible solution of the problem of origin of life from inorganic non-living matter. It is based on the assumption that original living organisms were low-complexity "naked genes", whose shape and chemical properties influenced their survival chances; the transition from inorganic lifeforms to DNA-based organisms was a "genetic takeover".

Cairns-Smith suggests crystals as original naked genes, and in particular [[clay]]s. Clays can also include other atoms and molecules in their structures, and perhaps evolved including more and more complex structures, until DNA-related molecules would have taken control of the organism, becoming the genetic driver of its life.<ref>Cairns-Smith, Graham, ''Seven clues on the origin of life'', Cambridge University Press, 1985.</ref>

== Criticisms ==

The modern concept of abiogenesis has been criticized by scientists throughout the years. Astronomer [[Fred Hoyle|Sir Fred Hoyle]] did so based on the probability of abiogenesis actually occurring. [[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]]'s 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 non-scientific communities. Nonetheless, research and hypothesizing continue in the hope of developing a satisfactory theoretical mechanism of abiogenesis.

Creationists are intense critics of abiogenesis, arguing that it is extremely improbable. Their objections typically attack a [[strawman]] version of abiogenesis, calculating the odds of chemicals forming a modern protein, and assuming a fixed number of proteins. <ref>[http://www.talkorigins.org/faqs/abioprob/abioprob.html Lies, Damned Lies, Statistics, and Probability of Abiogenesis Calculations] Ian Musgrave. Talk Origins. 21 Dec 1998. Accessed 24 Oct 2007.</ref>

=== Hoyle ===

[[Fred Hoyle|Sir Fred Hoyle]], with [[Chandra Wickramasinghe]], was a critic of Earth-bound chemical evolution. Specifically Hoyle rejected [[Fred Hoyle#Rejection of chemical evolution|chemical evolution]] to explain the [[Naturalism (Philosophy)|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. Critics sometimes refer to his arguments on this topic as [[Hoyle's Fallacy]]. Though modern theories address his argument, Hoyle never saw chemical evolution limited to Earth as a reasonable explanation. Hoyle preferred [[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:
<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 [[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.<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 [[paradigm]]:
<blockquote>
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 ([[Eric Hoffer|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 [[Thomas Samuel Kuhn|Kuhn]] (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, [[University of Cambridge|Cambridge University]] Press, UK, ISBN 0-521-80293-8.</ref>
</blockquote>
Yockey, in general, possesses a highly critical attitude toward people who give credence toward natural origins of life, often invoking words like "faith" and "ideology". Yockey's publications have become favorites to [[quote mining|quote]] among [[creationist]]s, though he is not a creationist himself (as noted in [http://www.asa3.org/archive/evolution/199602/0125.html this 1995 email]).

=== The second law of thermodynamics ===

The [[second law of thermodynamics]] states that [[entropy]] (dispersal of energy) will tend to increase in an [[isolated system]] as time continues and differences in temperature, pressure and density tend to even out. More strictly, the entropy of a system can decrease only if [[Work (thermodynamics)|work]] is done, ''i.e.'' energy is transferred from outside the system.

J. Rosenhouse points out that [[Earth]] is not an isolated system, but an [[open system]] receiving energy from the Sun. Furthermore, he notes that the time scales in which such large systems reach equilibrium can be very long, during which time local fluctuations in entropy are perfectly feasible, and may be observed all the time. <ref> "The fact is that natural forces routinely lead to local decreases in entropy. Water freezes into ice and fertilised eggs turn into babies. Plants use sunlight to convert carbon dioxide and water into sugar and oxygen, but [we do] not invoke divine intervention to explain the process." {{cite journal|url=http://www.math.jmu.edu/~rosenhjd/sewell.pdf|title=How Anti-Evolutionists Abuse Mathematics |date=2001|last=Rosenhouse|first=J|journal=The Mathematical Intelligencer|volume=23|issue=4|pages=3-8|accessdate=2007-03-26}}</ref> Furthermore, the concept of entropy in thermodynamics is not identical to the common notion of "disorder". For example, a thermodynamically closed system of certain solutions will eventually transform from a cloudy liquid to a clear solution containing large "orderly" crystals. Most people would characterize the former state as having "more disorder" than the latter state. However, in a purely thermodynamic sense, the entropy has increased in this system, not decreased. The units of measure of entropy in thermodynamics are "units of energy per unit of temperature". Whether a human perceives one state of a system as "more orderly" than another has no bearing on the calculation of this quantity. The common notion that entropy in thermodynamics is equivalent to a popular conception of "disorder" has caused many non-physicists to completely misinterpret what the second law of thermodynamics is really about.

== See also ==

* [[Autocatalytic set]]
* [[Autopoiesis]]
* [[Thioester]]
* [[Origin of life]]
* [[Hypercycle]]
* [[RNA world hypothesis]]
* [[proteinoid]]
* [[Miller-Urey experiment]]
* [[Protobiont]]

== References ==

{{reflist|2}}

== 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]

== External links ==
*[http://www.talkorigins.org/faqs/abioprob/ Abiogenesis FAQs Articles on the Origin of Life] &mdash; an article part of the [[Talk.Origins]] [[FAQ]]
*[http://www.talkorigins.org/faqs/abioprob/abioprob.html Lies, Damned Lies, Statistics, and Probability of Abiogenesis Calculations]
*[http://www.talkorigins.org/faqs/abioprob/borelfaq.html Borel's Law and the Origin of Many Creationist Probability Assertions]
*[http://www.talkorigins.org/faqs/abioprob/spontaneous-generation.html Spontaneous Generation and the Origin of Life]
*[http://www.talkorigins.org/faqs/abioprob/originoflife.html The Origin of Life]
*[http://www.astroscu.unam.mx/~angel/tsb/Rebek.htm Rebek] &mdash; Autocatalytic, selfreplicating molecules with ability to make errors and improvements.
*Willey, Sherwood, Woolverton. ''Prescott, Harley, and Cline's Microbiology.''Boston: McGraw Hill, 2008.

{{Origin of life}}

[[Category:Origin of life]]

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[[da:Abiogenese]]
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[[es:Teoría de la generación espontánea]]
[[eo:Abiogenezo]]
[[fr:Abiogénèse]]
[[gl:Abioxénese]]
[[ko:자연발생설]]
[[ia:Abiogenese]]
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[[he:בריאה ספונטנית]]
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[[ja:自然発生説]]
[[no:Spontan generasjon]]
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[[sr:Абиогенеза]]
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