Why do grandmothers exist?

The question is not as unfeeling as it sounds.

From the point of view of the selfish gene, creatures are supposed to drop dead as soon as they lose the power to reproduce. A man can make babies his whole life, even if the sperm of his old age lacks vigor and genetic fidelity. A woman outlives her eggs by about 20 years, which almost no other female mammals do. (Only female killer and pilot whales and orcas are known to last as long after the end of their menstrual cycles.)

Besides being classed among the oddities of the animal kingdom, post-menopausal women lack obvious utility. They tend to be weak. They don't have much sex appeal. They eat food working people might make better use of. In Paraguay's Ache tribe, aging women used to listen with terror for the footsteps of the young men whose job it was to sneak up on them with an ax and brain them. Most societies don't actually murder their grannies, but that women manage to attain old age is an evolutionary mystery and requires explanation.

Are senior citizens really "greedy geezers?"

Some people deny that women did live past menopause, whether in the Pleistocene era or the nineteenth century. Before modern hygiene and medicine, the argument goes, people just didn't live very long. But most scientists don't think that anymore. It is true that, in the olden days, fewer people reached their golden years. Children dropped dead with disturbing ease, keeping life-expectancy averages low. But humans still had the capacity to live twice as long as our hominid ancestors. Those who got to 15 had about a 60 percent chance of making it to 45, at which point odds were respectable that they'd reach old age. Many anthropologists and biologists now believe that the bodies of Homo sapiens were designed to last about 72 years.