Rate of CNS Development

Before birth both chimpanzees and humans have relatively fast rates of brain growth, which remain rapid for about eighteen months after birth. However, the total amount of growth in human brain mass increases much more rapidly than in chimpanzees. At birth, human brain weight averages 366 g and chimpanzees average 136 g; at 18 months, human brains weigh more than 1,000 g and chimpanzee brains average about 300 g. After 18 months of age in both species, the rate of brain growth declines, but more so in the chimpanzee. During the early phase of the human childhood stage (age 3–5 years), the rate and the amount of brain growth exceed those of the chimpanzee. Chimpanzee brain growth ends by 5 years of age acheiving an average adult weight of 406 g for males and 368 g for females. Human beings continue a slow but significant rate of brain growth for several more years acheiving a mean adult weight of 1404 for males and 1281 g for females. The sex differences in both species are due to differences in total body mass. In sum, the greater amount of human brain growth before and after birth and the continuation of rapid “fetal-like” rates of brain growth during infancy and early childhood account for the large size of the human brain.