122 INTRODUCTION TO EVOLUTION 



members of group O. Knowledge of these blood groups has contributed 

 greatly to the safety of blood transfusions, as well as to tests used in courts 

 of law in cases of disputed paternity of children. 



From the standpoint of evolution antigens A and B are of interest be- 

 cause of their distribution. In the first place we note that no human racial 

 or ethnic group is characterized by the presence or absence of A or B. The 

 nearest approach to an exception to this statement is afforded by the 

 American Indians, some of whom show a high percentage (98 percent) of 

 group O (i.e., lack of both A and B). Antigens A and B do occur in a 

 small proportion of the Indian population, however (see p. 251 ), and evi- 

 dence from the mummies of the cliff-dwelling Indians of the Southwest 

 indicates that the antigens were present in Indians living in prehistoric 

 times. 



While A and B are both present in members of all races, there are in- 

 teresting differences in their proportional distribution. For example, the 

 distribution of blood groups among white Americans or western Europeans 

 is approximately as follows: group O, 47 percent; group A, 43 percent; 

 group B, 7 percent; group AB, 3 percent. Proportions vary, since no sam- 

 ple of population tested is ever completely homogeneous, but the per- 

 centages given are sufficiently exact for our purpose. Thus, about 46 per- 

 cent of our population possesses A, alone or combined with B, while only 

 about 10 percent of our population possesses B, alone or combined with A. 

 Among Chinese the percentages are about as follows: group O, 30 percent; 

 group A, 25 percent; group B, 35 percent; group AB, 10 percent. In con- 

 trast to western Europeans, therefore, only about 35 percent of Chinese 

 possesses A, alone or combined with B, whereas about 45 percent possesses 

 B, alone or combined with A. Populations distributed between the two 

 geographic extremes on the continent of Eurasia show intermediate condi- 

 tions. The latter are distributed in such a way that a gradient is followed by 

 each antigen. Thus as one travels eastward across Eurasia the proportion 

 of substance A encountered in the populations decreases while the propor- 

 tion of B increases. The significance of this striking trend is in doubt, but 

 the trend is probably connected with the past migrations of peoples, what- 

 ever may have been the origin of the differences between the geographic 

 extremes. 



Data concerning blood groups have been collected for great numbers of 

 ethnic and national groups (see pp. 25 1-252; also Wiener, 1943, and Boyd, 

 1950). In some cases the nature of the blood group proportions in a given 

 population has aided anthropologists in determining the relationships of 

 that population to others (see also discussion of human races, pp. 250- 

 255). 



