G.D., a coworker, introduced me to a radical new idea today.
I had been under the impression that evolution cares little for individuals who are too old to reproduce. Their survival is irrelevant to the propagation of their genes. So they die before long. That is, a species doesn't evolve for greater longevity, beyond child-rearing years. There's no evolutionary advantage to it.
A consequence of this, or so it seemed, would be that natural selection is not working very hard in the rich world these days. Most people in America and Western Europe live long past child-raising age. Natural selection requires inferior genes to go out of circulation through death. No death, no evolution. (Evolution is still happening all the time, but mainly through other mechanisms: sexual selection, for example.)
But this view makes the longevity of humans, modest though it may be, something of a mystery. Especially for women, who typically reach menopause in their forties. (A healthy 50-year-old American woman still has, on average, 30 years ahead of her.) Nature wastes nothing. It seems implausible that people would continue living for decades after evolution no longer had any use for them.
It turns out that women and men whose mothers are alive and nearby start having kids sooner and have more children on average than people whose mothers aren't there. And their children are more likely to survive to adulthood.
Grandmother's footsteps - A much better article. Expensive subscription required.
Kicker: This means natural selection is hard at work even today. According to studies of 18th- and 19th-century records, women who die at the tender age of 50 have on average four fewer grandchildren than women who survive to 70. Even after your kids grow up and move out, evolution is still watching.