HUMAN EVOLUTION 



(I am oversimplifying the actual situation in order to show 

 some comparative differences in homologous areas in the cortical 

 maps of different species. In addition to the pre- and postcentral 

 projection areas, there are well-organized supplementary motor 

 areas in the anterior cortex and second somatic sensory, visual, 

 and auditory areas in the posterior cortex. Further, the sensory 

 areas are not completely sensory, and the motor areas are not 

 entirely motor.) 



Maps showing the localization in the four somatotopically 

 organized areas of the Rolandic region are available, in roughly 

 comparable detail, for the rat, rabbit, cat, marmoset, macaque, 

 chimpanzee, and man (Woolsey, 1958). The size of the motor 

 area which controls various muscles relates to the skill in using 

 the muscle and not to the size of the muscle. In the rat, rabbit, 

 and cat the cortical centers for the various body parts are ar- 

 ranged (in orientation, proportion, and relation of part to part) 

 much as they exist in the actual somatic animal. As one might 

 expect from a knowledge of the history and habits of the pri- 

 mates, the hand achieves a high degree of corticalization. In 

 the chimpanzee and man the cortical representation of the hand 

 in both pre- and postcentral fields has increased to such an ex- 

 tent that the cortical centers for the face lose continuity with 

 the center for the back of the head and neck. In the cortical 

 map, the great expansion of the hand in the hominoids has 

 severed the face from the occiput. The cortical topography of 

 the macaque shows an intermediate situation in that the separa- 

 tion of face from occiput occurs in the postcentral area but not 

 in the precentral. And, to add to the nicety of this graded series, 

 the New World marmoset shows a transitional status in the post- 

 central area and a basic mammalian, or rat-rabbit-cat-like status, 

 in the precentral gyrus. 



Each of the first six of our seven biological conditions which 

 are preadaptive for man's capacity to symbol and to have culture 

 is reflected in the structure of the cerebral cortex. Especially 

 in the motor projection areas we see evidence that man's long 

 history of tool use and shorter history of language are both a 

 cause and an effect of cortical expansion. The cortical motor 

 areas which are most involved in tool use and in speech are 

 much expanded in man's brain. These morphological changes, 

 alone: with others we have mentioned, illustrate some features 



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