LECTURES IN BIOLOGICAL SCIENCES 



sorption and mental utilization more complete and lasting, as 

 the skilled movements became more complex and more efficient." 

 This system of two-or-more-way causation is of general impor- 

 tance. Upright, bipedal locomotion freed the hands for manip- 

 ulation and helped to establish full eye-hand coordination. 



2. Bipedal Locomotion 



We do not know for certain whether man's ancestors were 

 brachiators before they became bipedal. The question is still 

 open. We lack the needed fossil evidence from the Miocene 

 and Pliocene periods. The presumptive evidence from compari- 

 son of living forms can never be conclusive on this problem, as 

 the evidence is without time depth. 



I agree with Gregory, Washburn, and others that brachiation 

 was most probably a feature of man's biological history. Those 

 who oppose this view tend to put too much stress on the often 

 misunderstood "rule" that evolution is irreversible. This rule 

 is properly applied when a paleontologist asserts he can tell a 

 fossil whale, or a fossil marine reptile, from a fossil fish, although 

 all of them are streamlined for marine life; it is misapplied 

 when a comparative anatomist asserts man could not be de- 

 scended from a brachiator because, for example, the long flexor 

 tendon of the thumb is present in man but absent in certain 

 brachiating pongids. 



On the most elementary level, all morphological variation 

 is explainable (the environments being specified) in terms of 

 gene mutations. The presence and absence of certain muscles 

 and their tendons have been shown to be under fairly simple 

 gene control in human populations (Spuhler, 1951). The phe- 

 nomenon of reverse mutation is well established in experimental 

 organisms. I see no theoretical difficulty in the assumption that 

 a population of Miocene or Pliocene apes under a set of selec- 

 tive pressures where 99 per cent lacked a long flexor tendon 

 of the thumb could be ancestral to a later population of homi- 

 noids under a different set of selective pressures where 99 per 

 cent possessed a long flexor tendon of the thumb. The same 

 holds for other relatively minor variations in the skeletons, mus- 

 culature, or dentition of primates. 



74 



