Table LS: Summary oj African great apes :5.4-:i.(> 



"extra neurons" in hominoid Australopithecines 3.9-4.5 



tarn {in billions) H. habilis 5.2-5.4 



H. erectus 5.7-8.4 



H. sapiens (various populations) 8.4-8.9 



tus could likewise be differentiated from the australopithecines on the one 

 hand and from H. sapiens on the other: Jerison's figures for H. erectus were 

 based on 2 specimens, the first of 900 c.c. and the other of 1000 c.c, which 

 gave him values of 6.4 and 7.0, respectively. I have added the smallest 

 H. erectus capacity thus far determined (750 c.c.) and a range for H. erectus 

 from 750 to 1225 c.c, giving values ranging from 5.7 to 8.4 billion "extra 

 neurons." 



Similarly, the value for H. sapiens is given by Jerison as 8.5 billion; 

 I have calculated outer limits of 8.4 to 8.9 billion for population means. 

 The numbers of "excess neurons" are summarized in Table 18. It is of in- 

 terest to note that the values for the type and paratype of H. habilis range 

 from about halfway between those of the australopithecines and those of 

 H. erectus to a point close to those of H. erectus (Tobias 1964, 1965c). 



Holloway (1966, 1968a) has criticized Jerison's method, pointing out that 

 his "mathematical extrapolations rest upon assumptions of cortical volume 

 and neuronal density, and overlook the fact that in primate evolution, the 

 cortex has undergone reorganization" (Holloway 1968a, p. 125). Further, 

 Holloway has pointed out in the same place that Jerison's figures do not 

 agree with the empirical counts made by Shariff (1953). On the other hand, 

 Holloway has himself indicated elsewhere in the same paper (p. 142) that, 

 "Actually, as Sholl (1956) has pointed out, Shariff's value for the volume 

 of human cortex is somewhat low. . . ." Since Shariff's calculations of the 

 number of neurons are based on neuronal densities and on volume oj cortex, 

 it is clear that if his estimates of the volume of the cortex are in error, so 

 must be his computation of the number of neurons. Hence, the mere fact 

 that Jerison's computation of the numbers of neurons differs from that of 

 Shariff does not per se invalidate either Jerison's method or his results. 



However, it is accepted that much more work needs to be done on 

 the problems of neuronal densities, neuron size, length and complexity 

 of branching, glia/neuron ratios— in different areas of the brain and at dif- 

 ferent ages, and in different living hominoids as well as lower Primates— 

 and the results of such regional and other variations need to be taken into 

 account in Jerison's approach before really significant and valid results may 

 be deemed to have flowed from his method. 



% 112 



