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.




 Hypotheses

 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.

Scientific Approaches To The Meaning of Life  Entropy and Life  What is Life?   Philosophical views on the meaning of life 

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