HUMAN EVOLUTION 



Under selective influence of the arboreal habitat, primate 

 vision became predominant and was perfected into a leading 

 sense. If we compare Eocene primates and other early mammals 

 with contemporary primates, we see that the position of the eyes 

 in the head has changed. They have moved from a side posi- 

 tion to a front position. The arrangement with both eyes on 

 the front aspect permits an overlap in the fields of vision. Both 

 eyes inspect a common section of the external world, and thus 

 binocular vision and depth perception is possible. The loss in 

 ability to see out of the sides of the head was more than com- 

 pensated for by the ability to see in depth from the front of a 

 head mounted on a movable neck. It was further compensated 

 for by having several individuals look— as a social group. 



We can understand the real significance of these anatomical 

 shiftings by comparing the optic chiasma of side-looking and 

 forward-looking mammals. In man, about 40 per cent of the 

 nerve fibers from the retina do not cross over at the optic 

 chiasma. In most non-primate mammals, nearly all of the optic 

 fibers cross over. This means that the functional representations 

 of the retinas of both eyes are transmitted to the same cortical 

 hemisphere in man, whereas in many mammals the right visual 

 cortex does not know what the right retina is "seeing," and the 

 left visual cortex does not know what the left retina is "seeing." 

 Seemingly there is an integrative advantage in having the visual 

 representations of both eyes projected to the same area of the 

 brain, or, in fact, to duplicate areas of the brain. 



Visual behavior is one of the key differences between the 

 nocturnal prosimians (lemurs, etc.), who are mostly solitary or 

 go about in pairs, and the diurnal, more social monkeys and 

 apes. This difference between the prosimians and the Anthro- 

 poidea is perhaps the largest gap in non-human primate social 

 behavior. With arboreal, sitting-up, or upright posture, vision 

 in the monkeys and apes gained a fine association with manipu- 

 lation. In a moment we will see it become super-vision— a guide 

 and control of expert manipulation. 



The relationship between the evolution of binocular vision 

 with good depth perception and fine manipulation goes both 

 ways. As Polyak (1957) wrote: ". . . vision itself [in the mon- 

 keys and apes] became more refined and the intellectual ab- 



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