THE SUMMING UP 395 



slowly have evolved — backwards. Once organisms could synthesize 

 their substance from inorganic compounds, life was no longer dependent 

 upon the primordial organic broth, and food cycles became possible. 



Eventually the conversion of oxygen into ozone would have formed a 

 layer high in the atmosphere which is nearly impervious to the heavy 

 ultraviolet radiation from the sun. In this and many other ways condi- 

 tions favorable for the origin of life disappeared. It seems unlikely that 

 life is repeatedly arising spontaneously on this planet nowadays; but even 

 if it were, it would be almost immediately destroyed by existing organ- 

 isms. Whether life has independently arisen elsewhere in the universe is 

 a fascinating question which may be answered now in terms of faith or 

 speculation, but which really requires more information than we have at 

 present. We need especially to know about the possibility of alternative 

 forms of life. 



Now let us inquire cautiously into that explosive question: What is 

 life? Physiologists interested in the origin of life have emphasized the 

 requirements of energy availability and structural organization, and gen- 

 eticists have stressed the requirement that the system must replicate. 

 It seems evident that both criteria are essential. At the symposium held 

 in Moscow in 1957, "The Origin of Life on the Earth," many aspects 

 of current research and speculation were presented; the reader is referred 

 to the proceedings, published in English, for details. 



An essential feature of the first living unit was its ability to multiply 

 by a copying process, giving rise to more of the same units. If nucleic 

 acids are the only kinds of molecules able to replicate, then it follows 

 that nucleic acids must have been an essential constituent of the first 

 living unit. Pushing this reasoning to its logical extreme, Horowitz has 

 proposed that a molecule of nucleic acid was the first living thing on 

 earth. 



The weak point of this argument is the assumption that only nucleic 

 acids can replicate. The problem which we have encountered repeatedly 

 in this book, the recognition of hereditary determinants other than 

 chromosomal genes, involves the same unknowns. If replicating cell con- 

 stituents other than nucleic acids, such as the supertemplates discussed 

 in Chapter 9, are found, then clearly they must be considered in relation 

 to the origin of life. Even if it is shown that only nucleic acids replicate 

 in organisms living today, the question must still be investigated whether 

 some simpler replicating system might have existed under the conditions 

 of early life. 



In the 1920' s genes were described as units capable of autocatalysis 

 and heterocatalysis. Today we speak of replication, decoding, and 

 regulation. In terms of evolution, we consider it likely that replication 



