LONG HUMAN CHILDHOOD

So to build clever humans from such crummy parts, we need a huge interconnected brain with 500 trillion synapses. This takes a lot of space—about 1,200 cubic centimeters (cc). That’s so big that it would not fit through the birth canal. Changes to the pelvis to make a larger birth canal would presumably interfere with upright walking. So the painful solution is to have human babies born with 400-cc brains (about the size of an adult chimpanzee’s brain). Even this size is still a problem—the baby’s head barely fits through the vagina. (In fact, death in childbirth, while common through most of human history, is almost unheard of in other mammals.) Once born, humans undergo an unusually long childhood while that 400-cc brain matures and grows, a process that is not complete until about age twenty. There’s no other animal species in which an eight-year-old cannot live without its parents. Our extra-long human childhoods drive many aspects of human social life, including our dominant mating system of long-term pair bonding, an aspect that is very rare in the mammalian world. Or to put it another way, if neurons could have been optimally redesigned at some point in evolution, we likely wouldn’t have marriage as a dominant cross-cultural institution.