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Abiogenesis (Greek a-bio-genesis, "non biological origins") is, in its most general sense, the generation of life from non-living matter. Today the term is primarily used to refer to theories about the chemical origin of life, such as from a primordial soup. Earlier notions of abiogenesis, now more commonly known as spontaneous generation, held that living organisms are generated by decaying organic substances, e.g. that mice spontaneously appear in stored grain or maggots spontaneously appear in meat. (That idea, which has long been known to be incorrect, will be called "Aristotelian abiogenesis" in this article.) History of abiogenesis hypotheses Aristotelian abiogenesis, also known as spontaneous generation, (and, in older texts, Generatio aequivoca, Generatio primaria, archegenesis, autogenesis, and archebiosis), was the theory according to which fully formed living organisms sometimes arise from not-living matter. Aristotle explicitly taught this form of abiogenesis, and laid it down as an observed fact that some animals spring from putrid matter, that aphids arise from the dew which falls on plants, that fleas are developed from putrid matter, that mice come from dirty hay, and so forth. Alexander Ross, in commenting on Sir Thomas Browne's doubt as to "whether mice may be bred by putrefaction", gives a clear statement of the common opinion on abiogenesis held until about two centuries ago. Ross wrote: So may he (Sir Thomas Browne) doubt whether in cheese and timber worms are generated; or if beetles and wasps in cows' dung; or if butterflies, locusts, grasshoppers, shellfish, snails, eels, and such like, be procreated of putrefied matter, which is apt to receive the form of that creature to which it is by formative power disposed. To question this 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 Nylus, to the great calamity of the inhabitants. The first step in the scientific refutation of the theory of Aristotelian abiogenesis was taken by the Italian Francesco Redi, who, in 1668, proved that no maggots were bred in meat on which flies were prevented by wire screens from laying their eggs. From the seventeenth century onwards it was gradually shown that, at least in the case of all the higher and readily visible organisms, spontaneous generation did not occur, but that omne vivum ex ovo, every living thing came from a pre-existing living thing. The invention of the microscope carried the refutation further. In 1683 Antoni van Leeuwenhoek discovered bacteria, and it was soon found that however carefully organic matter might be protected by screens, or by being placed in stoppered receptacles, putrefaction set in, and was invariably accompanied by the appearance of myriad bacteria and other low organisms. As knowledge of microscopic forms of life increased, so the apparent possibilities of abiogenesis increased, and it became a tempting hypothesis that whilst the higher forms of life arose only by generation from their kind, there was a perpetual abiogenetic fount by which the first steps in the evolution of living organisms continued to arise, under suitable conditions, from inorganic matter. This was mostly disproved by Lazzaro Spallanzani, who, in 1768, proved that microbes came from the air, and could be killed by boiling. His work paved the way for Louis Pasteur. It was due chiefly to Louis Pasteur that the occurrence of abiogenesis in the microscopic world was disproved as much as its occurrence in the macroscopic world. If organic matter were first sterilized and then prevented from contamination from without, putrefaction did not occur, and the matter remained free from microbes. The nature of sterilization, and the difficulties in securing it, as well as the extreme delicacy of the manipulations necessary, made it possible for a very long time to be doubtful as to the application of the phrase omne vivum ex vivo to the microscopic world, and there still remain a few belated supporters of abiogenesis. Subjection to the temperature of boiling water for, say, half an hour seemed an efficient mode of sterilization, until it was discovered that the spores of bacteria are so involved in heat-resisting membranes, that only prolonged exposure to dry, baking heat can be recognized as an efficient process of sterilization. Moreover, the presence of bacteria, or their spores, is so universal that only extreme precautions guard against a re-infection of the sterilized material. It was thus concluded definitely that all known living organisms arise only from pre-existing living organisms. Chemical evolution
The experiments of Louis Pasteur disproved Aristotelian abiogenesis, but they say nothing about chemical evolution, which is assumed to happen under totally different conditions and far longer timespans. Many scientists, such as T. H. Huxley, postulate a "primordial archebiosis", in which the living organisms observed in the present world had originally arisen in a series of stages from non-living matter. Unlike Aristotelian abiogenisis, chemical evolution does not propose the spontaneous creation of complex lifeforms out of anorganic substances, but a complex process with several stages. The core of any chemical evolution are self-catalytic molecules.
Critics The modern concept of abiogenesis has been criticised by scientists such as Sir Fred Hoyle and Hubert Yockey; who were not, however, biologists. Leading biologists point to fundamental assumptions in their arguments which have little to no bearing on abiogenesis theories or research. Francis Crick should here be mentioned as an exception. Yockey Information theorist Hubert Yockey argued that chemical evolutionary research raises the question: 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. (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.) 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, 1992. Information Theory and Molecular Biology, p. 336, Cambridge University Press, UK, ISBN 0-521-80293-8). 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 among creationists, though he is not a creationist himself (as noted in this 1995 email [1]). Panspermia advocates Panspermia, the idea that life came to Earth from elsewhere in the universe, is viewed by some as a criticism of abiogenesis. However, panspermia hypotheses simply transfer the origin problem elsewhere without offering a solution, so it does not necessarily address or criticize abiogenesis. Crick Francis Crick, molecular biologist and neuroscientist, most noted for being one of the co-discoverers of the structure of the DNA molecule, and chemist Leslie Orgel co-proposed Directed Panspermia as the mechanism through which life started on Earth. Hoyle Sir Fred Hoyle, with Chandra Wickramasinghe, was a proponent of Panspermia, first proposed by the Greek philosopher Anaxagoras. Hoyle became a staunch critic of chemical evolution to explain the naturalistic origin of life. Critics have shown that Hoyle's understanding of evolution is radically out of touch with modern biology. Although the hypothesis of panspermia is not in conflict with the idea of abiogenesis, Hoyle's interpretation of panspermia is in conflict. References
Links Spontaneous Generation and the Origin of Life — an article part of the Talk.Origins FAQ Probability of Abiogenesis Calculations — part of the Talk.Origins FAQ Retrieved from "http://en.wikipedia.org "
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