BIOCHEMICAL EVOLUTION 



337 



has enabled us to obtain a clearer view of the heteromorphic aspects of 

 proteins which are more or less isologous and new horizons have been 

 opened up. 



The molecular weights of Vertebrate haemoglobins are situated around 

 65,000-68,000. The molecules are apparently made up of two systems 

 of three peptide chains. The N-terminal amino acid sequence of a number 

 of haemoglobins is given in Table XXIV. There would appear to be a com- 

 mon basic chain in all cases. 



Table XXIV 



(Osawa and Satake 1955) 



Species 



Horse, pig 



Dog 



Bull, goat, sheep 



Cobaya 



Rabbit, Snake 



Hen 



N-Terminal sequences 



Val. Leu. 

 Val. Leu. 

 Val. Leu. 

 Val. Leu. 

 Val. Leu. 

 Val. Leu. 



It would be extremely satisfying to be able to follow in homologous 

 tissues, in every phyletic line, the stages in the biochemical evolution of 

 each type of macromolecule in the cell. But our knowledge of the field of 

 comparative biochemistry is still too sparse for such a task. On the other 

 hand in the field of phylogeny, the gaps left in the explanation of the kinship 

 between one class and the next are enormous, and living beings as we know 

 them appear to possess such a continuity that certain writers have been 

 led to deny the idea of evolution altogether. Frequently biochemical 

 constituents are found in one group without it being possible to study 

 biochemically a group that can be said, with certainty, to be the immediate 

 predecessor in the phyletic series. The general outlines of phylogeny can 

 however serve to define the more or less primitive or specialized character 

 of a biochemical constituent. 



Let us consider, for instance, the distribution of porphyropsin and 

 rhodopsin (p. 290) in the visual organs of animals. A diagram systematizing 

 the observations of different workers and particularly of George Wald and 

 his school (Fig. 90b) shows, in the phylogenic series represented here, a 

 definite chemical evolution in which porphyropsin appears to be more 

 primitive than rhodopsin. At the present time it is generally agreed that 

 Vertebrates originated in fresh water, and it is conceivable therefore that 

 the early Vertebrates already possessed the porphyropsin system. This, 

 of course, is not a matter open to direct investigation but we may notice 

 that the African Lungfish Protopterus, which is believed to have descended 



