HUMAN EVOLUTION 



80 per cent of the actual value. Thus modern man has a larger 

 brain than is typical of primates in general by about one-fifth. 



We also have physiological evidence that man has a remark- 

 ably spacious brain. Crile (1941) discovered a close relationship 

 for animals in general between metabolism and brain weight. 

 He found 1 gram of brain is required to produce by oxidation 

 12.12 calories of heat in 24 hours. This relationship holds for 

 a wide range of animals from grasshoppers to elephants; of the 

 hundreds of animals examined by Crile, only man and the 

 chimpanzee were way off the curve. In terms of Crile's physio- 

 logical criterion, man's brain is indeed huge. A man of 150 

 pounds has a basal metabolism of about 1650 calories a day. 

 He would require only 135 grams of brain to execute his basal 

 metabolism. Since the average brain weight for modern man 

 is 1345 grams, he has a brain mass nearly ten times larger than 

 would be required for basal metabolism alone. An 84-pound 

 chimpanzee had a basal metabolism of 1090 calories and an 

 estimated brain weight of 430 grams. Thus the chimpanzee 

 brain is about five times larger than required for a day's pro- 

 duction of basal energy by oxidation. In the sheep, an animal 

 not noted for intelligence, the brain weight-metabolism factor 

 is only 1.11. (The raw data for chimpanzee and sheep are from 

 Brody, 1945.) 



We should neither overemphasize nor underemphasize the 

 importance of relative brain size for intelligent behavior. Over- 

 all brain size is a poor predictor of intellectual ability within 

 our own species (excluding, of course, the lower, or micro- 

 cephalic, limits). We know of extreme cases where men of 

 demonstrated high intellectual ability differ in brain volume 

 by a factor of two. Still, when we consider differences between 

 species of living animals, relative brain size is the best available 

 single predictor of capacity for intelligent behavior. It is the 

 internal organization of the brain that counts. The volume of 

 the neocortex in Homo is about 58 per cent of total brain vol- 

 ume compared to 46 per cent in the lower primates (Harman, 

 1957). The ratio of the volume of nerve cells to the total volume 

 of the gray matter in the visual cortex is 50 per cent higher in 

 man than in the chimpanzee (Haug, 1958). 



Brains of living primates differ in their surface character- 



