THE ANIMAL CLIMAX-THE CHORDATES 



353 



have taken place at least 20 million years 

 ago. Some think that among the great apes 

 the gibbons do not appear to be very close 

 relatives of man, whereas others believe 

 they are more closely related than any of 

 the rest. Certainly the skull and general 

 trunk proportions are man-like. However, 

 the gibbon has become so completely 

 adapted to arboreal life that its arms are 

 disproportionately long and the fingers are 

 modified into hook-like structures for grasp- 

 ing limbs, the thumb being reduced to a 

 tiny protuberance (Fig. 13-72). Its long 

 arms allow it to brachiate through the trees 

 with great speed, that is, to move hand 

 over hand along branches in a swift, grace- 

 ful manner. Even in a cage, excited gibbons 

 are a spectacle to observe. 



Brachiation in the gibbon may have some 

 bearing on the achievement of upright pos- 

 ture in man. The gibbon hangs in a perpen- 

 dicular position, with the legs suspended 

 and the back vertical, in other words, in a 

 rnan-like position. It is possible that ances- 

 tral arboreal forms began to assume this 

 position at an early date but before speciali- 

 zation had gone as far as it has in the case 

 with the gibbons, and that at this point they 

 descended to the ground and started their 

 terrestrial existence. The gibbon stayed in 

 the trees and became further adapted to 

 arboreal life. Just why man's precursor, and 

 the baboons too, for that matter, came out 

 of the trees is a matter of speculation. It 

 may have been due to a wasting away of 

 the forested areas, in which case adaptation 

 to life on the surface of the earth had to 

 be made in order to survive. 



The orangutan, another tree-dweller, 

 lives exclusively in Borneo and Sumatra 

 and is more restricted in its range than any 

 of the group. Its name means "man of the 

 forest," which certainly describes this cum- 

 bersome beast. In direct contrast to the 

 swift-moving gibbons, the orangutan moves 

 cautiously and very deliberately among 

 the tree branches. It is well adapted to life 

 in the trees, with its long arms, powerful 



Fig. 13-72. The long-armed gibbon {Hylobates lar) is 

 strictly an arboreal primate, being able to travel 

 more rapidly through the tree tops than many ani- 

 mals can travel on the ground. This specimen has her 

 nursing baby held securely between her legs. 



enough to handle its great weight easily 

 and the hooked hands with very small 

 thumbs like the gibbon. On the ground its 

 small legs serve poorly as locomotor organs. 

 Its brain case has a capacity of about 500 

 cc, which is considerably more than that 

 of the gibbons but less than that of the 

 gorilla. In the latter, the brain case has 

 been known to reach a capacity of well 

 over 600 cc. The orangutan's eyes are close 

 together and its general facial appearance 

 resembles tliat of many people. 



The closest relatives of man are probably 

 the great African apes, the gorilla, and 

 chimpanzees. There seems to have been a 

 tendency among the primates during Mio- 

 cene times to try their luck at terrestrial 

 existence. These apes have gone part of the 

 way. Although they are at home among the 

 trees, particularly the chimpanzee which 

 brachiates very well even today, they still 

 spend a good deal of their time on the 

 ground. The gorilla even seems to prefer 



