Abiogenesis

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This article focuses on the history of the theory of abiogenesis (the spontaneous generation of life from non-living sources). 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).

Contents

[edit] Theories/Hypotheses

[edit] 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 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. 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 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 Nylus, to the great calamity of the inhabitants."

However, experimental scientists continued to decrease the conditions 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, 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.

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 always 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 hypothesize that while abiogenesis might not take place for creatures visible to the naked eye, at the microscopic level, 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 disproved that organisms such as bacteria and fungi appear in nutrient rich media of their own accord in non-living material and 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, 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, science seemed to be moving in opposing directions. However, Pasteur's experiments were limited to a closed limited 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.

[edit] Primordial Soup

Main article: primeval soup
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 building blocks of life.
Enlarge
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 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 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 chemists Stanley L. Miller and Harold C. Urey carried out an experiment on the "primeval soup". Within two weeks a racemic mixture of a few amino acids, some of the building blocks of life, had formed from the highly reduced mixture of methane, ammonia, water vapor and hydrogen. While Miller and Urey did not actually create life, they demonstrated that a more complex molecule — a few amino-acids — could emerge spontaneously from simpler chemicals. The environment was meant to simulate a primeval earth. It included an external energy source and an atmosphere largely devoid of oxygen. (the specific experiment involved shooting a spark, representing lightning, into their flask) There was careful filtering in place to preserve the results from destruction.

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.

[edit] Alternative Theories

[edit] Panspermia

Main article: Panspermia

Panspermia, a theory that allows life on Earth to have originated elsewhere in the universe, is viewed by some as a criticism of abiogenesis. In its strongest form Panspermia says that life has always existed: and can possibly spread interstellarly, such as to Earth. More common forms, however, simply transfer the origin problem elsewhere. These more common forms have no contention with abiogenesis, although they may mitigate the potential problem of time constraints of life forming on Earth.

[edit] Clay theory

Main article: Graham Cairns-Smith

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. Clay theory 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 inorganical lifeforms to DNA-based organisms was a "genetic takeover".

Cairns-Smith suggests crystals as original naked genes, and in particular clays. Clays can also include other atoms and molecules in their structures, and would have 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.[1]

[edit] Criticisms

The modern concept of abiogenesis has been criticized by scientists throughout the years. 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 theory.

Since it is difficult to prove abiogenesis has occurred, and even more difficult to falsify it, the hypothesis has many such critics: even in the scientific world, unlike many other theories. Nonetheless, research and hypothesizing continues in the hope of putting together the specific building blocks thought to yield abiogenesis.

[edit] Hoyle

Sir Fred Hoyle, with Chandra Wickramasinghe, was a critic of abiogenesis. Specifically Hoyle rejected chemical evolution to explain 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 undermine his argument, Hoyle never saw it as a reasonable explanation. Hoyle preferred Panspermia as an alternative natural explanation to the life on Earth.

[edit] 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. [2]

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. [3]

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).

[edit] The Second Law of Thermodynamics

The second law of thermodynamics is the scientific statement that entropy increases in an isolated system. Often, critics claim that abiogenesis violates this law. The second law of thermodynamics states that entropy (which is, in a sense, disorder) must not decrease in a system as time continues without energy input. However, similar claims can be given against the possibility of life in general. Such claims neglect the observation that the living organisms (and even Earth as a whole) are open systems, which means that energy flows between them and their surrounding. This flow of energy can give rise to symmetry and self-organization. What the second law of thermodynamics states is that entropy can decrease only if work is being done (i.e. energy is consumed), and indeed organisms consume energy for their life; This means that abiogenesis must involve a mechanism in which energy from an energy source is consumed (for example, energy from chemical compounds).

Another common misconception is that the second law of thermodynamics is violated by abiogenesis because it presumably states that every system must reach thermal equilibrium, which clearly does not allow the existence of life; This is not true both because Earth is not a closed system, and because the relevant time scales in which typical systems reach equilibrium can be very long, and clearly Earth has not reached such an equilibrium by now. [4]

[edit] See also

[edit] 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.

[edit] Current Research

[edit] References

  1. ^ Cairns-Smith, Graham, Seven clues on the origin of life, Cambridge University Press, 1985.
  2. ^ 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.
  3. ^ Yockey, 1992. Information Theory and Molecular Biology, p. 336, Cambridge University Press, UK, ISBN 0-521-80293-8.
  4. ^ Another application of the Second Law of Thermodynamics as a criticism to abiogenesis is the fact that amino acids and other vital molecules that form would contain stereoisomers: both right-hand and left-hand isomers of vital molecules.

[edit] External links

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