Human babies enter the birth canal from the womb in the same way a chimp does but just before the actual birth the skull rotates 90 degrees in order to exit the rounded birth canal that humans have evolved. In Homo Sapiens, evolution reached a compromise that favored even bigger brains at a further cost to birthing and efficient walking. The Homo Erectus pelvis was very narrow. Humans are unique among mammals in the extent to which the brain keeps growing well after birth. The scientific terms for this is secondary altriciality. It involves accelerating the birthing process and arresting the development until after birth. Monkeys and apes are born with brains half as heavy as they will ever be. A chimpanzee brain, for example, will weigh perhaps 7 ounces at birth and about 14 ounces as an adult. Human brains are about a third of their final size in newborns; they more than double in size in the first year after birth. On average, human babies are born with a brain that weighs 14 ounces but reaches 35 ounces in one year. It will continue to grow until it reaches about 45 ounces in size (at age 6 or 7).
Gestation in humans should be about 21 months rather than
the normal 9 we think in terms of. This is the process of accelerating the
birthing process to enable the enlarged brain to escape the birth canal.
Development of the brain then continues external to the womb for well over
the first several years. What this intense development means is that a human
infant is born relatively helpless. A baby can neither stand up or in any
way fend for itself for a long time. Stephen Jay Gould has written our sexual
maturation comes almost absurdly late in a Darwinian world supposedly regulated
by a constant struggle to secure reproductive success and pass more genes
along to future generations....slower development must provide some power
advantage to evolve, in the face of its obvious drawbacks. In fact, must
of what makes us human in the end may stem from this unnaturally long period
of helplessness in the very early part of our lives.