It's also worth noting that reproduction is more taxing and more dangerous for human women than for the females of essentially any other mammal (perhaps even an

It's also worth noting that reproduction is more taxing and more dangerous for human women than for the females of essentially any other mammal (perhaps even any other vertebrate?) species. Additionally, we have a longer period of immaturity than virtually any other animal species. A woman who ceases to bear children [let's say at age 45] while she still has 15 years of vigorous healthy life ahead of her is more likely to be alive and healthy to raise her last infant to adulthood because a) she's less likely to die in childbirth and b) she won't be weakened and spend resources on further pregnancies -- which helps ensure that the immense investment she's made in the earlier children will pay off. Now image her counterpart who continues to be fertile after age 45 but will keel over the minute she hits 60. What if her next pregnancy after 45 kills her and leaves all her youngest children (say, ages 1-10) without a mother? That hurts their chances of survival and of the fitness and health that would make them desirable reproductive partners. And if she does survive all her following pregnancies? She's leaving behind several young, motherless children (perhaps even a newborn) who also don't have a great chance in the world. What, their siblings will help raise them? But that decreases the resources available to the elder siblings' children -- the next generation -- and jeopardizes their fitness too. Human reproduction is unusually resource intensive and errs on the side of investing more in a few high quality offspring rather than having hundreds (think tadpoles) of which only a few survive to maturity. Resources in terms of all of the energy and nutrients the mother gives the fetus, and resources in terms of years of teaching and socialization and investment in developing the brain. You basically can't have what we'd consider a functional adult human without that huge investment of resources.