It is as if the large nonhuman primates have hit a ceiling that limits brain size below a certain level. These facts suggest that we should look for human characteristics conducive to brain growth outside the womb.Ĭomparing absolute brain weight with body weight, we find that the former increases rapidly with the latter for small primates, but that for the larger primates a large increase in body weight corresponds to only a small increase in the size of the brain. However, by adulthood the human brain is about twice as large in proportion to body size as is an ape's (1.8% vs. 'A variety of sources place the neonatal brain/body ratio at about 12% in both pongids and humans, as in other primates' (Frost 1).

This human distinction becomes apparent only during the course of postnatal development. The human brain is large not only in absolute terms, but also in proportion to body size. Is there then some other explanation? As it turns out, yes, there is. But, as has already been shown, many other facts are consistent with the idea that a hybridization of this sort actually did occur. Clearly, an explanation in terms of hybrid intermediacy, then, would be unsatisfactory. But, of course, it is not: the average cranial capacity of a human being is about 1,350 cc. If humans are derived from crossing between chimpanzees and pigs, the initial expectation is that the human brain should be of an intermediate size. Why would a cross between a chimpanzee and a pig yield a hybrid with a big brain? The average cranial capacity of a chimpanzee is about 375 cubic centimeters that of a pig, just 150. And upon first consideration of the idea that humans might have originated via hybridization between pig and chimpanzee, this particular trait seems to pose a major objection to the hypothesis.

From ancient times, the large size of the human brain has been regarded as the crowning distinction of Homo sapiens.

Gorilla skull mastoid process