HUMAN EVOLUTION 



4. Carnivorous-omnivorous diet 



5. Cortical control of sexual behavior 



6. Systematic symbolic vocal communication 



7. Expansion of the cerebral cortex 



I do not mean to imply that these seven conditions alone 

 changed a population of apes something like Proconsul into a 

 population of men something like Homo. Compared to the 

 average effect of single gene substitutions, the biological changes 

 exemplified by these conditions are probably to be measured on 

 a larger scale. The conditions do not represent unit mutations, 

 although mutation is the ultimate source of the genetic varia- 

 tion in each condition. Some of the conditions involve change 

 in function and arrangement of pre-existing parts. The order 

 of listing is not strictly chronological nor is it strictly phylo- 

 genetic. Evolutionary changes in some of the seven conditions 

 were interdependent and roughly synchronous. 



We will now consider the seven conditions in the order listed 

 above. At several points, our consideration will be helped by 

 the introduction of comparative material based on observation 

 on the living, higher primates. 



1. Accommodative Vision 



By early Miocene time, twenty-five million years ago, vision 

 had long been established as the primary discriminative sense 

 in the higher primates. Of course, vision has been an impor- 

 tant sense in vertebrates as far back as we know them. It pro- 

 vides data on the position, size, shape, texture, color, and ve- 

 locity of objects at a distance. Vision makes possible the great 

 goal-oriented mobility of vertebrates in pursuit of food and in 

 protection from enemies. The most complex vertebrates, birds 

 and mammals, interact with their external environment pre- 

 dominantly by information which enters via the eyes, goes to 

 visual centers in the brain, and then triggers the action of the 

 motor organs. 



Of course the sense of smell is also an important way of 

 knowing about the external environment. The ability to re- 

 ceive and act upon information about the type and intensity of 

 smell from an external object is part of the explanation for the 



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