52 Life: Its Nature and Origin 



primeval organisms were undoubtedly in the form of cells. They 

 must have been much simpler chemically and probably physically, 

 for just as highly specialized composite flowers arose by small 

 steps from a simple buttercup-like type, so existing cells must be 

 the result of many changes from a much simpler progenitor. 



If this supposition is true, at least three general facets of evolu- 

 tionary change can be postulated: elaboration of chemical com- 

 pounds, increase in complexity of chemical organization, and evolu- 

 tion of sex. 



Elaboration of Chemical Compounds 



The frequency of occurrence of chemical families of compounds 

 suggests a great deal of channelling of chemical evolution, presum- 

 ably conditioned by the inheritance of precursor substances. All 

 authors of chemical evolution are agreed on this point. Blum ( 1951 ) 

 gives examples of familial tetrapyrole structures in the blood pig- 

 ments of animals and certain photosynthetic pigments in plants 

 (Fig. 22). Several investigators have pointed out that analyses of 

 proteins bring out strong familial relations in a chemical sense. 

 For example, Fox (1956) reported that in proteins of soy bean, 

 corn, wheat, and rye only the amino acid radical lysine occupies 

 the "N" end of the protein molecules in significant and substantial 

 proportions; the radicle arginine predominated in the penultimate 

 position on these protein chains. Alanine occupies the same "N" 

 end position in three proteins of chick egg ( Fraenkel-Conrat, and 

 Porter, 1952); and the protamines in the sperm nuclei of five fish 

 species bear proline (Felix, 1955). Fox concluded that the actual 

 number of protein types in cells is relatively small, only an infini- 

 tesimal fraction of the theoretically possible number of amino acid 

 combinations which would satisfy the empirical fomiula of the 

 proteins. 



These and many other examples suggest that in organisms the 

 evolution of chemical compounds has followed to a remarkable de- 

 gree an elaboration of several early basic types. 



Complexity of Organization 



Oparin (1938) and others have expressed the opinion that the 

 earliest cell contained only a small number of macromolecules, 

 presumably nucleoproteins for the most part and that this ag- 

 gregation absorbed and fed on macromolecules only slightly less 

 complex. Under these conditions, only a few steps in chemical 



