History Module: The Expansion of the Hominid Brain

The expansion of the hominid brain appears to have only really begun with the genus Homo. The brain of the earlier hominid genus Australopithecus had a volume of about 400 cubic centimetres, not much larger than that of the great apes. But between 2 million and 700 000 years ago, the size of the brain of Homo erectus actually doubled.

The other major increase in brain volume occurred between 500 000 and 100 000 years ago, in Homo sapiens, and the human brain today has a volume of 1 350 cubic centimetres. In less than 4 million years, a relatively short time in evolutionary terms, the hominid brain thus grew to three times the size it had achieved in 60 million years of primate evolution.

1. Chimpanzee 2. A. africanus 3. H. habilis 4. KNM-ER 1470 5. Java Man 6. Peking Man 7. H. saldensis 8. H. saldensis
9. “Broken Hill” Man 10. Neanderthal Man 11. H. sapiens sapiens

Scientists have offered many hypotheses about the forces that may have been working together to cause such spectacular growth. For instance, humans may have needed larger brains in order to: 1. develop the precise motor skills, memory, and planning abilities required to manufacture tools; 2. develop the greater memory capacity that would make it easier to track and predict the location of the animals they hunted; 3. assimilate and follow the complex rules of their societies.

Humans may also have needed this larger brain in order to develop language. Many scientists think that language was an adaptation that occurred very early in hominids, and Broca’s area, a part of the brain closely associated with language, can even be seen in endocasts from Australopithecus. Wernicke’s area appears a bit later, in Homo habilis. But the problem is that the expansion of the brain occurred still later, in Homo erectus. For that matter, the same caveat applies to the other hypotheses just mentioned: the first tools, the first hunts, and the first organized societies all seem to have predated the expansion of the human cerebral cortex.

Other scientists have focused on the likely influence of the assumption of an erect posture on the expansion of the cortex. Bipedalism freed the hands and caused the descent of the larynx, but it also caused the shortening of the female pelvis, so that the pelvic channel became too narrow for the size of an infant's skull. This caused women to give birth prematurely, before the baby's brain had fully matured. In fact, humans’ exceptional learning ability is attributable to the immaturity of their brains at birth.

Thus, the growth of the human brain compared with that of our primate ancestors is an established fact, but what caused it is still the subject of much debate.