THE HUMAN SPECIES 



533 



movement had to be on foot; mountains and rivers, swamps and deserts 

 formed effective barriers to contact between populations, to say nothing 

 of the oceans. We may therefore conclude that throughout the Paleolithic 

 cultural era evolutionary trends in man were determined by low popula- 

 tion densities, the existence of many small and more or less isolated breed- 

 ing groups, little environmental control by man and hence effective 

 selection on both individual and group levels, and a totality of conditions 

 favorable for the occurrence of genetic drift. In other words things were 

 much the same from the evolutionary standpoint as they had been before 

 man acquired the basic elements of culture. 



SJa^ ^ 



IRAN 

 ARABIA 



Fig. 31.19. The "Fertile Crescent" (shaded) as extended in the light of modern studies. Here 

 agriculture was born and the Neolithic revolution began. The location of the oldest known 

 farm villages is shown. (Redrawn from Braidwood, Prehistoric Men, by permission Chicago 

 Natural History Museum.) 



Under such conditions we should expect increasing divergence between 

 isolates in the human population. According to the geological record that 

 is what actually occurred. Various species of men developed prior to or 

 during the Pleistocene, among them Homo sapiens. Long before the close 

 of the epoch our own species had differentiated into at least three or four 

 great groups or subspecies, each with several or many geographically 

 separated races. We may be sure that none of these were ever "pure" 

 races in the sense that their individuals were all alike genetically or 

 morphologically ; but there was probably less variation within them than 

 there is in most groups today, simply because there was less chance for 

 gene interchange between different groups. 



Post-Neolithic cultures and evolutionary trends. Sometime between 

 8000 and 5000 B.C., or let us say about 8 milleniums ago, agriculture and 



