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for menopause, because the puzzle has already been solved. The solution (they say) lies in the physiological mechanism of menopause: a woman’s egg supply is fixed at her birth and not added to later in her life. One or more eggs are lost by ovulation at each menstrual cycle, and far more eggs simply die (termed atresia). By the time a woman is fifty years old, most of her original egg supply has been depleted. Those eggs that remain are half a century old, increasingly unresponsive to pituitary hormones, and too few in number to produce enough estradiol to trigger the release of pituitary hormones.

But there is a fatal counterobjection to this objection. While the objection is not wrong, it is incomplete. Yes, depletion and aging of the egg supply are the immediate causes of human menopause, but why did natural selection program women such that their eggs become depleted or unresponsive in their forties? There is no compelling reason why we could not have evolved twice as large a starting quota of eggs, or eggs that remain responsive after half a century. The eggs of elephants, baleen whales, and possibly albatrosses remain viable for at least sixty years, and the eggs of tortoises are viable for much longer, so human eggs could presumably have evolved the same capability.

The basic reason why the third objection is incomplete is because it confuses proximate mechanisms with ultimate causal explanations. (A proximate mechanism is an immediate direct cause, while an ultimate explanation is the last in the long chain of factors leading up to that immediate cause. For example, the proximate cause of a marriage breakup may be a husband’s discovery of his wife’s extramarital affairs, but the ultimate explanation may be the husband’s chronic insensitivity and the couple’s basic incompatibility that drove the wife to affairs.) Physiologists and molecular biologists regularly fall into the trap of overlooking this distinction, which is fundamental to biology, history, and human behavior. Physiology and molecular biology can do no more than identify proximate mechanisms; only evolutionary biology can provide ultimate causal explanations. As one simple example, the proximate reason why so-called poison-dart frogs are poisonous is that they secrete a lethal chemical named batrachotoxin. But that molecular biological mechanism for the frogs’ poisonousness could be considered an unimportant detail because many other poisonous chemicals would have worked equally well. The ultimate causal explanation is that poison-dart frogs evolved poisonous chemicals because they are small, otherwise defenseless animals that would be easy prey for predators if they were not protected by poison.

We have already seen repeatedly in this book that the big questions about human sexuality are the evolutionary questions about ultimate causal explanation, not the search for proximate physiological mechanisms. Yes, sex is fun for us because women have concealed ovulations and are constantly receptive, but why did they evolve that unusual reproductive physiology? Yes, men have the physiological capacity to produce milk, but why did they not evolve to exploit that capacity? For menopause as well, the easy part of the puzzle is the mundane fact that a woman’s egg supply gets depleted or impaired by around the time she is fifty years old. The challenge is to understand why we evolved that seemingly self-defeating detail of reproductive physiology.

The aging (or senescence, as biologists call it) of the female reproductive tract cannot be profitably considered in isolation from other aging processes. Our eyes, kidneys, heart, and all other organs and tissues also senesce. But that aging of our organs is not physiologically inevitable—or at least it’s not inevitable that they senesce as rapidly as they do in the human species, because the organs of some turtles, clams, and other species remain in good condition much longer than ours do.

Physiologists and many other researchers on aging tend to search for a single all-encompassing explanation of aging. Popular