Gene Expression

Re the grandmother hypothesis smacking of Lamarckism, I don't see this. As I understand it, the idea is simply that women whose potential life spans extend considerably beyond their fertile period will on average be able to devote more time to help raise their grandchildren, and as a result of such help will on average have more of those grandchildren survive and reproduce.

This increases the grandmothers' own fitness, and thus there is natural selection for genetic changes to shut down female fertility at an earlier age. As that age gets earlier, at some point the benefits to fitness of having more time to help grandchildren will be outweighed by the decrease in fitness due to having less time to have one's own children. Thus the age of menopause will stabilize at some point between the onset of fertility and average age at death.

As for the idea that women's reproductive capabilities simply haven't yet caught up with increased lifespan, I think Razib's point is that this is contradicted by the coordinated nature of the menopausal changes: If there were increases in average lifespan and no selection pressure for early termination of fertility, then we would see a gradual drop-off in fertility extending to the end of women's lives, as we do for men. But this is not the case.

Posted by: dev | August 30, 2007 9:13 AM

Why do humans live to be as old as they do?

Because old people carry cultural memory.

Why do women go through menopause, but not men?

1. Fertility is immensely more costly for women than for men. Not merely childbearing itself, but just carrying around fertile organs has a big cost for women.

2. Speculation: as human bodies decline with age (attrition, entropy, etc.), perhaps the egg-production and sperm-production functions only decline a little, but the carrying-to-term function declines a lot. With modern medicine and nutrition this may have changed, though, just by reducing and repairing attrition.

Posted by: John Emerson | August 30, 2007 1:40 PM

So it would be purely (according to my hypothesis) that older mothers weren't as good an environment for the fetus.

IIRC, sperm are continuously produced and old-men's sperm is less viable. Or maybe I've just assumed that.

Posted by: John Emerson | August 30, 2007 3:07 PM

John: Speculation: as human bodies decline with age (attrition, entropy, etc.), perhaps the egg-production and sperm-production functions only decline a little, but the carrying-to-term function declines a lot.

But simply pointing to more reproductive problems with age does not explain why a menopause should evolve. If old females weren't contributing to anyone else's fitness, they might as well give it one last shot.

Are you familiar with Craig Packer's theory that "menopause" exists in all mammals, and females are only likely to stick around for the time that it takes to raise the last kid successfully? I think it resembles your scenario a bit. He says this is enough to explain why a human female hunter-gatherer is likely to survive to 55 instead of 40, assuming that children are dependent until their late teens. I am skeptical about this: he talks about lions a lot, they might survive for a couple of years after last reproduction, and that seems like a far cry from an actual menopause. And his theory does not rule out humans having a novel adaptation for shutting down the reproduction to be able to concentrate on raising that last kid.

razib: both sperm and eggs become less viable. the risk of down syndrome starts going up at 35, and shoots up at 40. so late children from "old" eggs might be very unfit.

But even if it shoots up (to a few percent?) is the risk so great that reproduction should be avoided altogether? The child mortality rate in the first year might be up to 20% in hunter-gatherers - even assuming every Down baby as extra mortality on top of that, I think they might not be a deal-breaker?

Posted by: windy | August 30, 2007 6:55 PM

The culture-carrier function has to be considered separately from the birthing / childraising function. It wouldn't be limited to childraising -- during all periods women had valuable knowledge in areas like weaving, horticulture, etc.

Posted by: John Emerson | August 30, 2007 10:28 PM