On the Geogenous Evolution of Self-Reproducing Systems and Macromolecules

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ON THE GEOGENOUS EVOLUTION OF SELF-REPRODUCING SYSTEMS AND MACROMOLECULES H. S. ANKER* Many speculations involving the origin of living organisms on earth assume that this event was preceded by the spontaneous generation of macromolecules, protein or ribonucleic acid in nature (i, 2). While such hypotheses are impossible to disprove, the spontaneous formation ofsuch large molecules appears highly improbable. A macromolecule which was able to serve as the basis ofall living things presumably had some sort of particular structure. Its formation by random assembly was therefore an extremely rare event1 which could have occurred only under circumstances where large numbers ofmacromolecules were formed.2 However, in order for the formation oflarge quantities of marcomolecules to continue , their equally rapid degradation was necessary. It is difficult to see how the "able" macromolecule could escape destruction preferentially. Although the probability offormation of an "able" macromolecule is much greater by selection than by random assembly,3 it is equally difficult * Department ofBiochemistry, University of Chicago. 1 It can be shown that the formation of a particular amino acid sequence in a protein of about 10,000 molecular weight and composed of io different amino acids will occur once in io100 times. Since the earth has existed for some 1.5 X io1' seconds and since the earth surface is 4 X iol8cm2, it can be calculated that the random formation ofsuch molecules throughout geological time had to occur at a rate of1.5 X 1064 molecules/cms/sec, or about 2 X 1036 tons/cm2/sec, in order for a given sequence to be formed once. Similar considerations apply to other macromolecules. 2 H. Gaffron in a letter comments to this argument as follows: "I disagree firmly and forever with the implied meaning. You want to show how impossible it is to make it certain that a catalytic, nicely folded protein appears spontaneously. I agree, ofcourse, [that] it is quite impossible and senseless that the appearance oflife may be accomplished by producing enormous amounts ofproteins in succession , but it is a point that an extremely rare event could have happened nevertheless once on this earth, in the way onewins theIrish Sweepstakes, and this would convey uniqueness to life and to ourselves in the entire cosmos. This is the hypothesis ofspecial creation interms ofchemistry. As scientists we cannot work with this [hypothesis], therefore we do not believe in it; but I think [that] unless we have done the proper experiments and shown that living things could be made in the laboratory, the hypothesis ofa unique event (never to be repeated) has not been disproved." 3 The stepwise synthesis ofthe protein offootnote I can be visualized as follows: AU possibledipeptides are first formed spontaneously. This requires the synthesis of100 different molecules. After elimination ofall but one ofthese by selection, 20 different tripeptides can be formed from the surviving dipeptide. Repetition ofthesesteps willthen yieldthe protein moleculeafter a maximumof2000 steps. 86 H. S. Anker · Evolution ofSelf-reproducing Systems Perspectives in Biology and Medicine · Autumn 1961 to imagine the formation ofan "able" molecule by any purely chemical means involving selection, since it is evident that selective forces as such directing the evolution ofmacromolecules could not have existed on the primitive earth. Any "evolution" ofmacromolecules could have occurred only as the result of selection for increasing efficiency and specialization of some function. Therefore it must be concluded that such a function was an attribute ofthe original molecules from which the macromolecules evolved and that, since function is a property which has meaning only within the context of a self-reproducing system, self-reproducing systems had to precede the appearance offunctional macromolecules. Macromolecules accordingly have to be regarded as the products rather than the beginnings oflife on earth. Once it has been postulated that self-reproducing systems consisting entirely of simple molecules did precede the evolution of any specific macromolecule, it is possible to consider the origin and properties ofsuch a system. It is generally agreed that the primitive earth was covered by oceans containing a large amount ofdissolved organic matter (primordial soup [3]) and that in the absence of atmospheric oxygen the radiation reaching the earth from the sun included the energy-rich, ultraviolet part of the spectrum. The...
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