This is actually something that I had wondered about.
Since the human birth canal puts a limit on the size of babies' heads, could the availability of Cesarean sections cause humans to evolve to have larger brains?
Turns out the answer might be yes:
Babies have very big heads that squeeze with only great difficulty through a relatively narrow pelvis, so the relationship in size between head diameter and the diameter of the pelvic opening has been a limitation on human evolution. We know this had to be a factor in our evolution: the average newborn mammal has a cranial capacity that is roughly 50% of the adult size, chimpanzee babies have heads about 40% of the adult size, but human babies have crania that are only 23% of what they will be in adults. While our brains have gotten larger over evolutionary time, they have not gotten proportionally larger in utero, because large-headed babies increase the difficulty of labor and cause increased mortality in childbirth. If childbirth could bypass the pelvic bottleneck, that would allow for fetal heads to grow larger without increasing the risk of killing mother and/or child.
Read the rest of PZ Myers's post on Will the availability of C-sections give humans bigger brains?
