Grandmothers are an evolutionary mystery.
Well, not grandmothers exactly: rather, women who have passed menopause. Human men can sire children as long as they live, but human women can’t have children after they go through menopause. But why do we have menopause at all—why stop having babies? Isn’t it always better, evolutionarily, to have more babies?
The mystery is far from solved, but we have some good clues.