Homo sapiens 



Homo erectus 



Homo habilis 



Australopithecines 

 African great apes 



H 1 ■ — i- 



4 5 6 



"Extra" neurones (in thousands of millions) 

 Figure 32: Numbers of "excess neurons" in hominoid brains, computed by 

 Jerison's (1963) method. Small as are the samples available for australo- 

 pithecines, Homo habilis and Homo erectus, the series shows a remark- 

 able stepwise progression. Only the accumulation of more specimens 

 and the refinement of the methods of analysis will verify whether the 

 diagram represents a real evolutionary trend. 



the environment through a wider range of brain-behavior mechanisms; that 

 is, for intelligent adjustment. 



On the basis of cell counts in a variety of Primates and, given certain 

 assumptions, ferison claims that it is possible to estimate the number of 

 cortical nerve cells, not only in the brain as a whole but in each of the 2 

 components. He has developed a series of equations for the calculation of 

 these neuronal values, given the size of the brain and the size of the body. 

 By applying these formulae, he has been able to compute the number of 

 "extra" neurons regarded as being available for brain-behavior adaptive 

 mechanisms. With this second parameter, the number of "excess neurons," 

 he has found it possible to differentiate various Primates, and especially the 

 family of man, on the basis of the numbers of extra neurons. Modern man 

 has far more excess neurons than, say, the chimpanzee and gorilla. While 

 the African great apes can be shown by Jerison's equations to possess 2.4 

 to 3.6 billion excess neurons, modern man has 8 billion (Figure 32). 



109 ^ 



