Unbound: How Eight Technologies Made Us Human and Brought Our World to the Brink

by Richard L. Currier

Thus, as the tremendous expansion in the size of the hominid brain took place during the past million years, it created increasing difficulties in childbirth for the emerging humans, and this problem became even more acute as the emerging humans evolved into modern humans with truly massive brains. If the birth canal remained fixed in size while the infant’s head and the brain within it was growing progressively larger, how did the evolving hominids continue giving birth to babies with increasingly larger heads? The evolutionary solution to this problem turned out to be that as the size of the brain increased, hominid babies began to be born before their brains were fully developed. In fact, if you apply the normal rules of mammalian fetal development to humans, the “normal” gestation period for humans would be at least twelve months, not nine. But at twelve months the average infant brain would simply be too large to pass through the average human birth canal, making human birth mechanically impossible.