One of the best talks at the 2013 Ancestral Health Symposium was by Will Lassek, a retired doctor. Here’s the abstract:
One puzzle is why human males have such a strong preference for women with hourglass figures and low weights that can compromise fertility. The second is why slender young women typically have about one third of their weight in body fat, more than bears starting to hibernate, and why human infants are also very fat. Finally, why do women typically gain another twenty pounds or more during their reproductive years? The answer may lie in the roles that fat plays in providing essential fatty acids needed for the growth of a very large brain and in regulating overall fetal growth.
His answer to the first question was that death during childbirth was a serious danger. Women of lower weight give birth to babies with smaller heads — less likely to cause death. Wider hips means a larger birth canal. Women gain weight after their first birth because their birth canal is wider — the optimal baby size has gone up. A variety of data supported these ideas. Lessek’s answer to the first question is quite different than what evolutionary psychologists have said.