Ancient Oceans

HOW COULD THE ANCIENT OCEANS HAVE GIVEN RISE TO ORGANIC POLYMERS? Though organic compounds appeared on the primordial earth, most of these molecules are highly perishable under present conditions:  they react with oxygen and become oxidized, or are consumed by microorganisms.  Since the prebiotic atmosphere contained virtually no free oxygen, and there were no organisms, neither oxidation nor decay would have destroyed the organic molecules; they could have accumulated in the seas over hundreds of millions of years.  No such accumulation would be possible today.

Suppose, then, that a variety of hydrocarbons, fatty acids, amino acids, purine and pyrimidine bases, simple sugars, and other relatively small organic compounds slowly accumulated in the ancient seas.  This combination would still not be a sufficient basis for life, which also needs macromolecules, particularly polypeptides and nucleic acids.  How could these polymers have formed from the mixture of building-block substances in the "soup" of the ancient oceans?  This question is not easy to answer; and several hypotheses are currently being investigated.

Some think that the concentration of organic material in the seas was high enough for chance bondings between simpler molecules to give rise in time to considerable quantities of macromolecules.  They point out that, even though each such polymerization reaction is rather unlikely in the absence of protein enzymes, on a vast time scale enough rare and unlikely events would probably occur to produce, collectively, a major change.

Other investigators suggest that concentration mechanisms must have speeded up chemical reactions.  One such mechanism might have been adsorption of the building-block compounds on the surface of clay minerals.  Another might have involved the accumulation of small amounts of dilute solutions of building-block compounds in puddles on the beaches of lagoons and ponds.  The heat of the sun would have evaporated most of the water, thus concentrating the organic chemicals, and providing energy for polymerization reactions.  The resulting polymers might then have been washed back into the pond.  Such a process could slowly have built up a supply of macromolecules in the pond.  Sidney W. Fox has shown that if a nearly dry mixture of amino acids is heated, polypeptide molecules are rapidly synthesized (particularly if phosphates are present).  Alternatively, after condensation by evaporation, the energy for polymerization reactions in the puddles might have come from UV radiation rather than heat. 

From Gould & Keeton, Biological Science, 6th edition.

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