The Evolution of Man | 283 



but not all, of their time on the ground. Remaining in the arboreal 

 habitat is an array of forms including such diverse primates as orang- 

 utans and gibbons, as well as monkeys, marmosets, tarsiers, and 

 tree shrews. 



It is important that our early ancestors lived in the trees, but it is 

 also very important that they left them. It is difficult to see how a 

 culture even vaguely resembling ours could have been developed 

 by tree-dwelling organisms, if for no other reason than that there 

 was an almost complete lack of raw materials for even primitive 

 technology in the arboreal habitat. Although the reasons why our 

 ancestors left the trees are obscure, perhaps one was that some of the 

 larger primates found competition from smaller more agile ones too 

 severe. Because of their increased size and intelligence, they were 

 better able than their ancestors to cope with the problems of ter- 

 restrial living. Almost certainly the return was gradual, and at one 

 time our ancestors must have lived much as do modern-day chim- 

 panzees, foraging on the ground in the daytime but retreating to 

 the trees at night. It seems likely that the efficient bipedal posture so 

 characteristic of man was achieved after early prehominids left 

 the shelter of the trees and began to forage in bands out on grassy 

 savannahs. Fossil evidence indicates that an abundance of food was 

 available in the game animals that roamed the open spaces, and 

 selection probably favored any protohuman genotypes permitting, 

 by whatever means, utilization of this food resource. An upright 

 posture, providing reasonably rapid locomotion while at the same 

 time freeing the hands to grip stones or staves, would have been at 

 a premium. Intelligence and social organization would also have 

 had their reward in food. 



It is important to remember, when one is casually discussing our 

 family tree in this manner, that the evolutionary processes discussed 

 are no different in principle from those accounting for bandless 

 water snakes or banded snails. To say that our ancestors moved 

 out of the trees to escape the competition of more agile foragers is 

 merely a shorthand for the following: At one point in our evolution- 

 ary history any recombinant that had the slightest behavioral 

 tendency to descend from the trees and forage on the ground had a 

 better chance of contributing to the gene pool of the following 

 generations than other genotypes lacking this tendency. The fre- 

 quency of the kind of genetic information producing this sort of 

 behavior therefore increased in the populations concerned and the 

 behavioral norm was slowly shifted. A great many generations after 

 the first pioneer individuals foraged briefly on the ground, the be- 

 havior of all individuals in the populations concerned became 

 terrestrial. 



