HUMAN EVOLUTION 



From general principles and the evidence of comparative 

 anatomy alone, it is very hard to derive a man-like shoulder from 

 a quadrupedal monkey. It is easier to derive it from the shoul- 

 der of a brachiating pongid. Since man shows a whole complex 

 of traits which, considered together, go with brachiation, there 

 is presumptive evidence of a brachiation stage in human phylo- 

 genetic history. The quadrupedal monkeys have a small acro- 

 mial process of the scapula, a short clavicle, a high attachment 

 of a small deltoid to the humerus, and no costo-clavicular liga- 

 ment. Man and the pongid brachiators have a large acromial 

 process, a long clavicle, a low attachment of a large deltoid, 

 and a strong costoclavicular ligament. 



The most economical historical explanation of man's arms, 

 shoulder, and thorax is that his ancestors were brachiators in 

 the sense of use of the forelimbs as a means of grasping hand- 

 holds above the head and of swinging the body forward in 

 arboreal progression. There is no reason to suppose that they 

 were specialized brachiators, such as the contemporary gibbons 

 with long arms and hook-like hands. 



This assumption allows an explanation of the broad, rela- 

 tively shallow thorax of man and the complex anatomy of his 

 shoulder. It happens that use of the arms for manipulation of 

 tools would favor selectively a broad thorax with widely spaced 

 shoulders. But since the earliest evidence of tool use is late in 

 hominoid history, I would prefer to explain the anatomy of the 

 thorax and shoulder as a heritage from brachiation rather than 

 a convergent resultant of tool use. 



But let us return to what we know more directly about the 

 mode of locomotion in fossil hominoids. The fore-limb of 

 Proconsul is beautifully preserved in the fossil record (Napier 

 and Davis, 1959). The "arm" and shoulder of this Miocene 

 primate show both quadrupedal and brachiating features. It 

 was not a specialized brachiator like the modern gibbon, but it 

 was structurally capable of arboreal travel using overhead, grasp- 

 ing arm movements. It was also capable of ground locomotion 

 in a quadrupedal manner. 



It would be an extraordinary bit of good luck should the 

 only Miocene hominoid whose skull and fore-limb has been re- 

 covered be a member of the phylum directly ancestral to man. 



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