Few human cultures do not ponder the question of the origin of life and try to come up with an answer. The scientific culture is no exception and the search for an explanation has spanned centuries.
Up to the end ofthe 17th Century, there was no reason to question the occurrence of spontaneous generation as an explanation of departures from like begetting like in biological reproduction, exemplified by the origin ofparasitic worms such as the fluke inside the human body. This view was strengthened by the mechanical philosophy of Rene Descartes who reduced natural science to matter and motion, such that agitation of decomposed matter could create an organism. Francesco Redi, however, described experiments proving that maggots were not generated by meat in a state ofputrefaction but solely by the flies coming to lay their eggs. Georges Cuvier likewise found spontaneous generation unacceptable and declared, "Life has always arisen from life. We see it being transmitted and never being produced." Finally, spontaneous generation was laid to rest when Louis Pasteur showed that sterilization by heat abolished the appearance of bacterial spoilage.
Yet disproving spontaneous generation of worms and bacteria over days or years is not tantamount to disproving spontaneous generation of life over eons. The origin of life on primitive Earth was clearly spontaneous generation of a different order, a more fitting expression of the philosophy of Descartes with embodiment of matter in atoms and molecules and motion in the kinetic energies they need to enter into chemical evolution. In 1828 F. Wohler had accomplished the synthesis of urea, a bodily constituent, from cyanate and ammonium salt, thus breaking down the conceptual barrier between inorganic and organic compounds. In early 20th Century, Archibald Macallum proposed that life originated from particulate matter: "When we seek to explain the origin of life, we do not require to postulate a highly complex organism...as being the primal parent ofall, but rather one which consists ofa few molecules only and of such a size that it is beyond the limits ofvision with the highest power ofthe microscope," and the process would go through countless molecular combinations, such that "one giving the right composition resulted in ultramicroscopic particles endowed with the chemical properties of ultramicroscopic organisms."
Thus prebiotic evolution and natural selection were called upon to deliver life. This was followed by W. Lob's synthesis of simple amino acids such as glycine by exposing wet formamide to silent electrical discharge and to ultraviolet light and E.C. Baly's production of formaldehyde and sugar from the action of ultraviolet light on carbon dioxide and water. A. Oparin formulated in 1924 a prebiotic scenario where organic compounds built from inorganics were organized first into coazervate droplets; "as soon as these droplets became separated from the surrounding medium by a more or less definite border, they at once acquired a certain degree of individuality competition in growth velocity" and at the end "simplest primary organisms have emerged." JBS Haldane could not be more enthusiastic over the potential ofprebiotic synthesis of organic compounds that "must have accumulated till the primitive oceans reached the consistency of dilute soup," thus giving birth to the legendary 'primordial soup'.6-8
In 1953, Stanley Miller, stimulated by Harold Urey's suggestion of the importance of a reducing atmosphere to prebiotic synthesis, proposed and carried out prebiotic synthesis by passing electric discharge through a simulated primitive Earth atmosphere of methane, ammonia, hydrogen and water. The gases were electrified to produce amino acids and his results electrified the world.9 Three centuries after the initiation ofscientific debate on spontaneous generation, prebiotic evolution has become a frontier fueled by advances in biochemistry, molecular biology, genomics and space exploration. Few areas ofscientific enquiry may be expected to yield as many fundamental discoveries in the 21st Century as prebiotic evolution and astrobiology.
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