112 INTRODUCTION TO EVOLUTION 



with the former. Sometimes if an antiserum is very potent a small reac- 

 tion occurs. It may well be that all mammals have some small amount of 

 chemical similarity in their serum proteins. 



The results just described are typical of tests employing antibodies 

 against human serum. Many other species might be and have been in- 

 cluded, but the principles involved are well exemplified by the test as de- 

 scribed. 



We have seen that chimpanzee serum is so like human serum as to be 

 nearly or quite indistinguishable from it (without introducing special re- 

 finements into the test), while baboon serum is less like human serum than 

 chimpanzee serum is. How is the similarity between human serum and 

 chimpanzee serum to be explained? As with morphological similarities 

 (Chap. 3) there are two possible explanations. We may hold that the simi- 

 larity is more or less a coincidence, or occurred because a common chemi- 

 cal pattern for blood proteins was followed in creation. But according to 

 the explanation most satisfactory to modern biologists the similarity is the 

 result of inheritance from common ancestry. Both man and chimpanzee 

 inherited their serum proteins from a form ancestral to both of them, and 

 the proteins have behaved as conservative characteristics, becoming but 

 little altered in the subsequent evolutionary history of man, on the one 

 hand, and of the chimpanzee, on the other. 



The evolutionary explanation for the origin of the similarity between 

 human serum and baboon serum follows similar lines. In the distant past 

 (probably as long ago as the Oligocene period — p. 137) the baboon, an 

 Old World monkey, and man shared a common ancestor. From that an- 

 cestor both inherited a certain pattern of serum structure. But in subse- 

 quent evolutionary history the pattern has been modified so that today 

 there is less similarity between baboon and human serum than between 

 chimpanzee and human serum. It may be that man and the chimpanzee 

 shared a common ancestor more recently than did man and baboon. This 

 possibility would accord well with one school of thought concerning human 

 evolution (Chap. 11). On the other hand, some students of the subject 

 maintain that man is not more closely related to the great apes (including 

 the chimpanzee) than he is to the lower monkeys. Do the serological find- 

 ings contradict this latter view? Such is the usual conclusion, but in all 

 fairness we should point out that it is not the only possible one. We 

 should remember that evolution does not occur at a constant rate in all 

 evolutionary lines. Perhaps for some reason the serum proteins in the line 

 of ancestry leading to the baboon have changed at a more rapid rate 

 than have the serum proteins in the lines leading to man and the great 



