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Abiogenesis

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This article focuses on the theory of abiogenesis (the spontaneous generation of life from non-living sources) in the history of science. For abiogenesis as a scientific study, see main article, Origin of life

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 sea, and most probably through a number of intermediate steps, such as non-living but self-replicating molecules (biopoiesis).

The Spontaneous Generation hypothesis

Earlier notions of abiogenesis, now more commonly known as spontaneous generation, held that complex, living organisms are generated by decaying organic substances, e.g. that mice spontaneously appear in stored grain or maggots spontaneously appear in meat.

According to Aristotle it was a readily observable truth that aphids arise from the dew which falls on plants, fleas from putrid matter, mice from dirty hay, and so forth. Such was the prestige of the ancient Greek philosophers in general, and of Aristotle in particular, that this was accepted without question until the 17th century. One of the first to question the theory was 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." The indignation which greeted Browne can be judged from the reaction of his contemporary, Alexander Ross: "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 Nylus, to the great calamity of the inhabitants."

Nevertheless, experimental scientists continued to roll back the frontiers within which the spontaneous generation of complex organisms could be observed. The first step was taken by the Italian Francesco Redi, who, in 1668, proved that no maggots 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, spontaneous generation did not occur, but that omne vivum ex ovo - every living thing came from a pre-existing living thing.

Then 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 realm of abiogenesis increased, and it became tempting to hypothesise that while abiogenesis might not take place for creatures visible to the naked eye, there existed a fount at the microscopic level from which living organisms continually arose from inorganic matter.

In 1768 Lazzaro Spallanzani proved that microbes came from the air, and could be killed by boiling. Yet it was not until 1862 that Louis Pasteur performed a series of careful experiments which conclusively proved that a truly sterile medium would remain sterile.

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, that species changed over time. 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.

Pasteur had demonstrated that Spontaneous Generation was wrong, and he also seemed to have demonstrated that any concept involving the generation of living matter from non-living matter was also wrong. Science seemed to be moving in opposing directions. But Pasteur's experiments were limited to a closed sterile system for a very brief (geologically) time period of modern scientific experimentation, and not to time scales on millions or billions of years on the open surface of a planet. The ur-organism implication of Darwin's theories would have occurred in the deep geological past, the dawn of time on this planet, 3.87 billion years ago, and it had a billion years from the beginning of the planet to be formed. What were the conditions like back then? More importantly, how did those conditions differ from those in Pasteur's flasks?

The Primordial Soup hypothesis

File:UreyMillerExperiment.jpeg
The Miller-Urey experiment attempted to recreate the chemical conditions predicted for the primitive Earth in the laboratory, and synthesized a few of the simpler (but racemized) building blocks of life.

In 1936 Aleksandr Ivanovich Oparin, in his "The Origin of Life on Earth", demonstrated 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 "grow" by fusion with other droplets, "reproduce" through fission into daughter droplets, and so have a primitive metabolism in which those factors which promote "cell integrity" survive, and those that don't become extinct. 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 chemists Stanley L. Miller and Harold C. Urey carried out an experiment on the "primeval soup". Within two weeks a few simpler organic amino acids, the basic building blocks of life (but racemized), had formed. While Miller and Urey did not actually create life; they demonstrated that a more complex molecule — a few simpler amino-acids — could emerge spontaneously from simpler chemicals, in the presence of an external energy source in an atmosphere largely devoid of oxygen (the experiment involved shooting a spark, representing lightning, into their flask of supposedly primitive earth-gases) and with careful filtering in place to preserve the results.

Their experiments had different results from Pasteur's because they involved different conditions. 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. . Some involve the formation of self-replicating molecules that make copies of themselves, but are not regarded as living (see definition of life)

See also

Current Research

Publications

  1. Pitsch, S. Krishnamurthy, R. Arrhenius, G. (2000). Concentration of simple aldehydes by sulfite-containing double-layer hydroxide minerals: implications for biopoesis. Helvetica chimica acta. Sep-Oct. 83(9):2398-411.
  2. Hartman, H. (1998). Photosynthesis and the origin of life. Orig Life Evol Biosph. Oct. 28(4-6):515-21.
  3. Arrhenius, G. Sales, B. Mojzsis, S. Lee, T. (1997). Entropy and charge in molecular evolution--the case of phosphate. J Theor Biol. Aug 21. 187(4):503-22.

See also: Hypercycle, RNA world hypothesis, proteinoid, Miller-Urey experiment.

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]).

The principles of Yockey's argument seem to be effective for a variety of creative disciplines, such as the writing of computer software, where FOSS seems to play the social role of an Intelligent Designer while traditional computer programming methods seem to take the role of traditional chemical evolution. (-: Yes, quotes about omitting millions of years/steps in the process are appropriate. :-)

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.

The Second Law of Thermodynamics and the theory

Another common criticism of the theory usually invokes the second "law" of thermodynamics, claiming that order cannot rise out of chaos, and thus some help was needed for life to form. However, this is an incomplete summary of the main criticism. Order can rise from chaos but only if such a reaction produces a negative change in Gibb's free energy. In a system where order increases this can only happen if the overall reaction is exothermic. As the proposed reaction would be massively endothermic the change in Gibb's free energy would be positive and therefore infeasible.

The main problem with this criticism is that the law applies to closed systems and earth is not a closed system, energy is pouring into it from the sun, and this energy is what can cause reactions that would not otherwise occur to happen.

Also, reactions to this criticism typically depend upon an inappropriately close representation of the Second Law, which can also be expressed as "the total entropy of the system and surroundings never decreases" & thus still apply, as it will if expressed in terms of an energy source being insufficient to add useful complexity by itself.


References

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