Most apes give birth to babies whose brain is half-grown; human babies’ brains are one-third grown at birth, and they spend far less time in the womb than is normal for a mammal, given the longevity of man. The reason is obvious: Were the hole in the pelvis through which we are born (the birth canal) commensurately larger, our mothers would be unable to walk at all. The width of human hips reached a certain point and could go no further; as brains continued to grow bigger, earlier birth was the only option left to the species.
