Let's return to Sarah Hrdy's marvelous paper "The Past, Present, and Future
of the Human Family," and examine two passages. One:

Looked at comparatively, rates of teenage pregnancy (which happen to be higher in the United States than in any other developed nation) have less to do with moral decline than with changes in the nutritional status of human beings over the last tens of thousands and hundreds of years. Teenage pregnancy, then, is very much a human-made problem, a human-solvable public health issue, not a moral one.

What is this brilliant sociobiologist getting at?

As in all apes, human ovaries evolved to factor how much fat a woman’s body had stored. For a still partially dependent girl living among nomadic hunter-gatherers, this indicator of nutritional status would have been synonymous with how much social support she had. Among nomadic foragers, where youngsters depend on shared nutritional subsidies from other group members, a young girl’s fat reserves provided a fairly good indicator of how much social support she could expect from parents, grandparents, boyfriends, her mate perhaps, as well as other group members.

By and large, the plumper a girl is, the sooner she matures. Girls growing up in nomadic foraging society on the African savanna remained active, intermittently fed, and very lean, menstruating for the first time closer to sixteen than twelve, the average age of girls today in sedentary, hypernourished Western societies.

I will now leave you with another passage (this time by the primatologist Richard Wrangham—I highly recommend his book Catching Fire: How Cooking Made Us Human) that I think connects well with Hrdy's insights:

I was impressed to learn that raw-foodists are thin compared to those eating cooked diets, given that in most cases they are eating domesticated foods with lots of nutrients, are processing them in machines like electric blenders, and of course, living as most do in the developed world, never suffering through seasonal food shortage. Yet despite all these advantages over anyone who might try eating wild foods raw, the average woman on a 100% raw diet did not have a functioning menstrual cycle. About 50% of women entirely stopped menstruating! When a raw-foodist’s reproductive system does not allow her to have a baby even when her diet is composed of processed, high-quality, agricultural foods, the obvious explanation is that she is not getting enough calories.