3. Volcanic outgassing from the interior created a
new atmosphere for the planet, made up of reduced compounds: H2,
CH4, N2,
NH3, H2O,
and H2S. In the absence of today's
high-altitude ozone shield, ultraviolet radiation from the sun penetrated
all the way to the surface of the Earth. This radiation, lightning
discharges, volcanic heat, and natural radioactivity all provided
energy sources for the spontaneous reaction of the atmospheric gases
to form more complex molecules: HCN and aldehydes, nitriles, organic
acids and bases, simple carbohydrates and amino acids. These were
leached into the oceans by rain, where they slowly built up a thin
"Haldane soup", which was stable for long periods of time
in a reducing atmosphere.
4. Life evolved from this soup, perhaps through intermediate stages
of localized but nonreproducing chemical systems, protected by simple
barrier membranes. Catalytic proteins, or crude enzymes, developed
from random polymers of amino acids, sometimes in association with
metal ions and organic molecules. Energy for chemical syntheses
was provided by the breakdown of polyphosphates, or later by molecules
such as ATP, both formed originally by nonbiological means. As competition
depleted the natural supply of many necessary substances in their
surroundings, the more successful "protobionts" developed
the ability to synthesize these substances from more plentiful molecules.
Those primitive chemical systems that also developed a machinery
for duplicating all of this chemistry in daughter systems, crossed
the threshold of what we would define as life. The primitive information-transfer
system need have borne little resemblance to the elaborate DNA-RNA-ribosome
system of today, but the function would have been the same.
5. As the natural supply of polyphosphates and ATP ran short, some
protocells evolved glycolysis as a means of degrading organic molecules
and saving the energy as homemade ATP. This pattern of metabolism
became so advantageous that only those organisms that possessed
it survived to the present. Glycolysis and gluconeogenesis developed,
with the necessary enzymes floating freely in the cell fluid. The
stage of the fermenting bacteria was reached. Even with this metabolic
capability, the amount of life on the planet was strictly limited
by the available supply of nonbiologically produced organic molecules
for use as energy sources.
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