Brain development and the female pelvis

Why did the australopithecines go for at least two million years and evolve in various other ways, but never show a significant increase in brain size? Perhaps survival did not make it necessary to them. But perhaps also an increase in brain size was anatomically difficult. It seems that the two physical differences which we notice between hominids and other animals - the upright posture and the larger brain - might at first have been not entirely compatible with one another. It may have been precisely his bipedality - or rather her bipedality - which made it difficult for the australopithecine to develop a bigger brain.

The story of brain development is also the story of the development of the pelvis. When the australopithecines adopted the upright posture, the pelvis changed, to allow for the new centre of gravity of the body and for the necessary bone and muscle modifications required for walking vertically on the back legs alone.

But the pelvis is also a girdle through which the infant head must pass. The picture above, taken from the Cambridge Encyclopedia of Human Evolution (M H Day, P88), shows (left) the pelvis of a female chimpanzee, (centre) that of "Lucy" - a fossil australopithecine - and (right) that of a modern woman. In each case, the infant's head is shown emerging at the time of birth. It shows how relatively more difficult it was for the infant australopithecine's head to pass through his mother's pelvic girdle. The change in pelvis shape needed for upright walking, running and self-defence had made it difficult to give birth to a big-headed child.

Lucy's successors (of the new genus Homo) probably evolved two answers to it. One was for the infant to be born at an earlier and more helpless stage of foetal development, when the head was smaller. Jennifer Worth, in her autobiographical Call the Midwife, comments on this:

The helplessness of the newborn human infant has always made an impression on me. All other animals have a certain amount of autonomy at birth. Many animals , within an hour or two of birth, are up on their feet and running. Others, at the very least, can find the nipple and suck. But the human baby can't even do that. If the nipple or teat is not actually placed in the baby's mouth and sucking encouraged, the baby would die of starvation. I have a theory that all human babies are born prematurely. Given the human life span - three score years and ten - to be comparable with other animals of the similar longevity, human gestation should be about two years. But the human head is so large at two that no woman could deliver it. So our babies are born prematurely, in a state of utter helplessness.

Jennifer Worth: Call the Midwife

The other solution to the problem was a modification in the shape of the female pelvis. This modification must have had implications for walking, running, fighting and climbing trees. Reduced female speed and agility might have meant a greater dependence on the protection of the male. In most other animals, females (whilst often not as aggressive as males) are well able to defend themselves against predators. By contrast, human females have historically tended to depend on male protection at times of physical danger. Perhaps greater brain size may have been balanced by a progressive tendency for the infant to need more protection from the parents and the mother more from the father.

And unlike other animals, hominid mothers (like modern women, but unlike other animals) may now have had difficult births, for which they needed a midwife. Thus they were also more dependent on the older, more experienced females of their group. Perhaps this was a price they had to pay for the size of their brains: but it was also a step in the direction of larger human social groups, in which the skills and experience of older surviving members were valued.

Much research on this very important subject has been done by Wenda Trevathan and Karen Rosenberg. See their article in Scientific American v 285/5 (Nov 2001) pp 61-5 (Internet link) and Trevathan's book Human Birth: An Evolutionary Perspective (1987).

It may be worth commenting that the human reproductive system, which necessitates the birth canal passing through the pelvic girdle, gives an additional proof, if one were needed, of our quadrupedal ancestry. It's an unsatisfactory arrangement for a biped.