The results suggest that in nonhuman primates, reproductive decline is surpassed by declines in survival, so that very few females run out of reproductive steam before they die. A female baboon, for example, may live to age 19, and continues to reproduce to the end.
But in human females the reproductive system shuts down much more rapidly than the rest of the body. “Half of women experience menopause by the age of 50, and fertility starts to decline about two decades before that,” Alberts said.
What distinguishes a human female from her primate cousins is not that the human biological clock ticks faster, but that mortality is so much lower in humans than in other primates, according to work done by University of Utah anthropologist Kristen Hawkes, who was not an author of this study.
This is not a mystery if you assume that older women tend to be intelligent enough to look after themselves and each other and to stay close to people who will help them. In some places, this is called a family; in others, a retirement residence. That was one of the reasons they got to be older in the first place.
Now for the Darwinian explanation, which assumes that human intelligence plays no role:
It may be that older females who forego future breeding to invest in the survival of their existing children and grandchildren gain a greater evolutionary edge than those who continue to reproduce. Once a baby chimp is weaned it can forage for itself, whereas human infants are nutritionally dependent long after they leave the breast.
“[Human children] can benefit greatly from having mothers and grandmothers who are still alive and not tied up with helpless infants,” Alberts explained.
Another possibility is that mammalian eggs simply have a limited shelf life. According to this idea, we’ve extended our lives to the point where we’ve outlived our egg supply.
It’s purely an accident, to be sure, that women arrange to outlive their egg supply and animals don’t. 😉
You be the judge.