Fertility Is Kaput, but Life Goes On

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.

I like the idea, but it's difficult for me to swallow the group selection pill. If everyone else in my tribe has this population control gene but me then I benefit greatly because I'll have many more children without the tribe becoming overpopulated. Eventually my gene out competes the communal gene and the entire tribe overpopulates and collapses. Why doesn't this happen more often? What mechanism exists to prevent me from having this mutation? If I have this mutation am I essentially a cancer on the tribe from which the tribe cannot recover? This doesn't seem like the dynamics we observe in nature.

I don't know anything about the subject but still feel like it's a strange explanation.

To me it seems like it would be a great advantage for the species if you could reproduce past the normal reproduction age, if you made it this far it probably means that you are perfectly adapted to your environment.

What about the depletion of resources? I imagine it must have been pretty rare for an animal to be able to survive past the end of his reproductive years, and unless you live in an island I had the impression resources shouldn't be an insurmountable problem.

And if it is, then I guess a big part of your population die, but the other recovers as there are less mouths to feed, and it's still an advantage to be able to reproduce when you're old.

Again, I don't know anything about the subject, and like the author, I'm not that convinced about the Grandma Hypothesis either.

I think it's unlikely given the length of time people actually lived before civilization. More likely there's just no evolutionary advantage in female fertility after age 40 - she starts having children at 14 or 15 and has one a year until she dies in childbirth. She never makes it to middle age.

Why accept there's no advantage in other age-related breakdown (like arthritis or failing eyesight) and expect there's some grand evolutionary plan in fertility loss?

This is interesting. I have wondered for quite some time now if the millennial generation will be subject to mass infertility in their 30's due to constant cell phone radiation near the genitals. Obviously this won't be the end of the world, but it may certainly cause a drop in planned pregnancy. Fertility drugs are typically seen as rolling the dice, so maybe adoption will become the way of the future - who knows?