© CLAIRE ARTMAN/CORBISMenopause puzzles evolutionary biologists. After all, avoiding mortality and ensuring reproduction are the two most fundamental tenets of Darwinian natural selection. Theoretically, species should evolve so that individuals live only as long as they reproduce. So why did humans evolve a long adulthood with females becoming nonreproductive about halfway through?
Menopause, quantified as the proportion of expected time in adulthood during which an individual is nonreproductive, is remarkable in humans relative to most other species in the animal kingdom. Among the many hypotheses that have been proposed to explain the origin of menopause, two have garnered substantial attention. According to the most popular explanation, known as the grandmother hypothesis, older females increase their genetic contribution to future generations by assisting in rearing their grandchildren rather than continuing to bear and rear children of their own. This scenario requires that older females become grandmothers twice for each forgone additional opportunity to become mothers themselves, as grandmothers typically share half as many genes with their grandchildren as with their own children. The other leading explanation, called the ...
