Most wild animals remain fertile until they die. So do human males: although some may eventually become less fertile, men in general experience no shutdown of fertility, and indeed there are innumerable well- attested cases of old men, including a 94-year-old, fathering children.
But for women the situation is different. Human females undergo a steep decline in fertility from around the age of 40 and within a decade or so can no longer produce children. While some women continue to have regular menstrual cycles up to the age of 54 or 55, conception after the age of 50 was almost unknown until the recent advent of hormone therapy and artificial fertilization.
Human female menopause thus appears to be an inevitable fact of life, albeit sometimes a painful one. But to an evolutionary biologist, it is a paradoxical aberration in the animal world. The essence of natural selection is that it promotes genes for traits that increase one’s number of descendants bearing those genes. How could natural selection possibly result in every female member of a species carrying genes that throttle her ability to leave more descendants? Of course, evolutionary biologists (including me) are not implying that a woman’s only proper role is to stay home and care for babies and to forget about other fulfilling experiences. Instead I am using standard evolutionary reasoning to try to understand how men’s and women’s bodies came to be the way they are. That reasoning tends to regard menopause as among the most bizarre features of human sexuality. But it is also among the most important. Along with the big brains and upright posture that every text of human evolution emphasizes, I consider menopause to be among the biological traits essential for making us distinctively human--something qualitatively different from, and more than, an ape.
Not everyone agrees with me about the evolutionary importance of human female menopause. Many biologists see no need to discuss it further, since they don’t think it poses an unsolved problem. Their objections are of three types. First, some dismiss it as a result of a recent increase in human expected life span. That increase stems not just from public health measures developed within the last century but possibly also from the rise of agriculture 10,000 years ago, and even more likely from evolutionary changes leading to increased human survival skills within the last 40,000 years.
