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- 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 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 hypotheses about the chemical origin of life, such as from a primordial sea or in the vicinity of hydrothermal vents, and most probably through a number of intermediate steps, such as non-living but self-replicating molecules (biopoiesis). Abiogenesis remains a hypothesis, meaning it is the working assumption for scientists researching how life began. If it were proven false, then another line of thought would be used to modify or replace abiogenesis as a hypothesis. If test results provide sufficient support for acceptance, then that is the point at which it would become a theory.
Spontaneous generation Edit
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 (literally, from an egg).
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 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.
Primordial soup Edit
- Main article: primeval soup
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 chemist Stanley L. Miller working under Harold C. Urey carried out an experiment on the "primeval soup". Within two weeks a racemic mixture, containing 13 of the 22 amino acids used to synthesize proteins in cells, 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 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. There was careful filtering in place to preserve the results from destruction.
- Main article: 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 (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 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.
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 actually occurring. Hubert Yockey did so by saying that it is closer to theology than science.
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.
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 address his argument, Hoyle never saw chemical evolution as a reasonable explanation. Hoyle preferred panspermia as an alternative natural explanation to the origin of life on Earth.
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.In a book he wrote 15 years later, Yockey argued that the idea of abiogenesis from a primordial soup is a failed paradigm:
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.
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. …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).
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.
The second law of thermodynamicsEdit
The second law of thermodynamics states that entropy (which has commonly been described as relating to "disorder" at a molecular level in an early analogy for the energetic movement of molecules) 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 is done, i.e. energy is transferred from outside the system.
Intelligent design proponent Stuart Pullen challenged the concept of abiogenesis on the grounds that the creation of life from nonlife would violate the above law, since creation of living organisms must be associated with a decrease in the entropy of the system. This challenge has been refuted on the grounds that the Earth is not an isolated system, but an open system receiving energy from the Sun, and 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 under the second law.
- Autocatalytic set
- Origin of life
- RNA world hypothesis
- Miller-Urey experiment
- 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.
- Hartman, H. (1998). Photosynthesis and the origin of life. Orig Life Evol Biosph. Oct. 28(4-6):515-21.
- 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.
- NASA Astrobiology Institute: Earth's Early Environment and Life
- NASA Specialized Center of Research and Training in Exobiology: Gustaf O. Arrhenius
- ↑ Cairns-Smith, Graham, Seven clues on the origin of life, Cambridge University Press, 1985.
- ↑ 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.
- ↑ http://www.theory-of-evolution.net/
- Things Come to Life by Henry Harris (2002) ISBN 0-19-851538-3
- Buehler, Lukas K. (2000-2005) The physico-chemical basis of life, http://www.whatislife.com/about.html accessed 27 October 2005.
- 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
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