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.
According to proponents of this view, menopause could not have been a frequent occurrence for most of the several million years of human evolution, because (supposedly) almost no women or men used to survive past the age of 45 or 50. Of course the female reproductive tract was programmed to shut down by age 50, since it would not have had the opportunity to operate thereafter anyway. The increase in human life span, these critics believe, has occurred much too recently in our evolutionary history for the female reproductive tract to have had time to adjust.
What this view overlooks, however, is that the human male reproductive tract and every other biological function of both women and men continue to function in most people for decades after age 50. If all other biological functions adjusted quickly to our new long life span, why was female reproduction uniquely incapable of doing so?
Furthermore, the claim that in the past few women survived until the age of menopause is based solely on paleodemography, which attempts to estimate age at time of death in ancient skeletons. Those estimates rest on unproven, implausible assumptions, such as that the recovered skeletons represent an unbiased sample of an entire ancient population, or that ancient adult skeletons’ age of death can accurately be determined. While there’s no question that paleodemographers can distinguish an ancient skeleton of a 10-year-old from that of a 25-year-old, they have never demonstrated that they can distinguish an ancient 40-year-old from a 55- year-old. One can hardly reason by comparison with skeletons of modern people, whose bones surely age at different rates from bones of ancients with different life-styles, diets, and diseases.
A second objection acknowledges that human female menopause may be an ancient phenomenon but denies that it is unique to humans. Many wild animals undergo a decline in fertility with age. Some elderly individuals of many wild mammal and bird species are found to be infertile. Among animals in laboratory cages or zoos, with their lives considerably extended over expected spans in the wild by a gourmet diet, superb medical care, and protection from enemies, many elderly female rhesus monkeys and individuals of several strains of laboratory mice do become infertile. Hence some biologists object that human female menopause is merely part of a widespread phenomenon of animal menopause, not something peculiar to humans.
However, one swallow does not make a summer, nor does one sterile female constitute menopause. Establishing the existence of menopause as a biologically significant phenomenon in the wild requires far more than just coming upon the occasional sterile elderly individual in the wild or observing regular sterility in caged animals with artificially extended life spans. It requires finding a wild animal population in which a substantial proportion of females become sterile and spend a significant fraction of their life spans after the end of their fertility.
The human species does fulfill that definition, but only one wild animal species is known to do so: the short-finned pilot whale. One-quarter of all adult females killed by whalers prove to be postmenopausal, as judged by the condition of their ovaries. Female pilot whales enter menopause at the age of 30 or 40 years, have a mean survival of at least 14 years after menopause, and may live for over 60 years. Menopause as a biologically significant phenomenon is thus not strictly unique to humans, being shared at least with that one species of whale.
But human female menopause remains sufficiently unusual in the animal world that its evolution requires explanation. We certainly did not inherit it from pilot whales, from whose ancestors our own ancestors parted company over 50 million years ago. In fact, we must have evolved it after we separated from the apes just 7 million to 5 million years ago, because we undergo menopause whereas chimps and gorillas appear not to (or at least not regularly).
The third and last objection acknowledges human menopause as an ancient phenomenon that is indeed unusual among animals. But these critics say that we need not seek an explanation for menopause, because the puzzle has already been solved. The solution, they say, is the physiological mechanism of menopause: the senescence and exhaustion of a woman’s egg supply, fixed at birth and not added to after birth. An egg is lost at each menstrual cycle. By the time a woman is 50 years old, most of that original egg supply has been depleted. The remaining eggs are half a century old and increasingly unresponsive to hormones.
But there is a fatal counterobjection to this objection. While the objection is not wrong, it is incomplete. Yes, exhaustion and aging of the egg supply are the immediate cause of human menopause, but why did natural selection program women so that their eggs become exhausted or aged in their forties? There is no obvious reason we had to evolve eggs that degenerate by the end of half a century. Eggs of elephants, baleen whales, and tortoises remain viable for at least 60 years. A mutation only slightly altering how eggs degenerate might have sufficed for women to remain fertile until age 60 or 75.
The easy part of the menopause puzzle is identifying the physiological mechanism by which a woman’s egg supply becomes depleted or impaired by the time she is around 50 years old. The challenging problem is understanding why we evolved that seemingly self-defeating detail of reproductive physiology. Apparently there was nothing physiologically inevitable about human female menopause, and there was nothing evolutionarily inevitable about it from the perspective of mammals in general. Instead the human female, but not the human male, was programmed by natural selection, at some time within the last few million years, to shut down reproduction prematurely. That premature senescence is all the more surprising because it goes against an overwhelming trend: in other respects, we humans have evolved to age more slowly, not more rapidly, than most other animals.
Any theory of menopause evolution must explain how a woman’s apparently counterproductive evolutionary strategy of making fewer babies could actually result in her making more. Evidently, as a woman ages, she can do more to increase the number of people bearing her genes by devoting herself to her existing children, her potential grandchildren, and her other relatives than by producing yet another child.
That evolutionary chain of reasoning rests on several cruel facts. One is that the human child depends on its parents for an extraordinarily long time, longer than in any other animal species. A baby chimpanzee, as soon as it starts to be weaned, begins gathering its own food, mostly with its own hands. (Chimpanzee use of tools, such as fishing for termites with blades of grass or cracking nuts with stones, is of great interest to human scientists but of only limited dietary significance to chimpanzees.) The baby chimpanzee also prepares its food with its own hands. But human hunter-gatherers acquire most food with tools (digging sticks, nets, spears), prepare it with other tools (knives, pounders, huskers), and then cook it in a fire made by still other tools. Furthermore, they use tools to protect themselves against dangerous predators, unlike other prey animals, which use teeth and strong muscles. Making and wielding all those tools are completely beyond the manual dexterity and mental ability of young children. Tool use and toolmaking are transmitted not just by imitation but also by language, which takes over a decade for a child to master.
As a result, human children in most societies do not become capable of economic independence until their teens or twenties. Before that, they remain dependent on their parents, especially on the mother, because mothers tend to provide more child care than do fathers. Parents not only bring food and teach toolmaking but also provide protection and status within the tribe. In traditional societies, early death of either parent endangers a child’s life even if the surviving parent remarries, because of possible conflicts with the stepparent’s genetic interests. A young orphan who is not adopted has even worse chances of surviving.
Hence a hunter-gatherer mother who already has several children risks losing her genetic investment in them if she does not survive until the youngest is at least a teenager. That’s one cruel fact underlying human female menopause. Another is that the birth of each successive child immediately jeopardizes a mother’s previous children because the mother risks dying in childbirth. In most other animal species that risk is very low. For example, in one study of 401 rhesus monkey pregnancies, only three mothers died in childbirth. For humans in traditional societies, the risk is much higher and increases with age. Even in affluent twentieth-century Western societies, the risk of dying in childbirth is seven times higher for a mother over the age of 40 than for a 20-year-old. But each new child puts the mother’s life at risk not only because of the immediate risk of death in childbirth but also because of the delayed risk of death related to exhaustion by lactation, carrying a young child, and working harder to feed more mouths.
Infants of older mothers are themselves increasingly unlikely to survive or be healthy, because the risks of abortion, stillbirth, low birth weight, and genetic defects rise as the mother grows older. For instance, the risk of a fetus’s carrying the genetic condition known as Down syndrome increases from one in 2,000 births for a mother under 30, one in 300 for a mother between the ages of 35 and 39, and one in 50 for a 43-year-old mother to the grim odds of one in 10 for a mother in her late forties.
Thus, as a woman gets older, she is likely to have accumulated more children, and she has been caring for them longer, so she is putting a bigger investment at risk with each successive pregnancy. But her chances of dying in or after childbirth, and the chances that the infant will die, also increase. In effect, the older mother is risking more for less potential gain. That’s one set of factors that would tend to favor human female menopause and that would paradoxically result in a woman’s having more surviving children by giving birth to fewer children.
But a hypothetical nonmenopausal older woman who died in childbirth, or while caring for an infant, would thereby be throwing away even more than her investment in her previous children. That is because a woman’s children eventually begin producing children of their own, and those children count as part of the woman’s prior investment. Especially in traditional societies, a woman’s survival is important not only to her children but also to her grandchildren.
That extended role of postmenopausal women has been explored by anthropologists Kristen Hawkes, James O’Connell, and Nicholas Blurton Jones, who studied foraging by women of different ages among the Hadza hunter-gatherers of Tanzania. The women who devoted the most time to gathering food (especially roots, honey, and fruit) were postmenopausal women. Those hardworking Hadza grandmothers put in an impressive seven hours per day, compared with a mere three hours for girls not yet pregnant and four and a half hours for women of childbearing age. As one might expect, foraging returns (measured in pounds of food gathered per hour) increased with age and experience, so that mature women achieved higher returns than teenagers. Interestingly, the grandmothers’ returns were still as high as women in their prime. The combination of putting in more foraging hours and maintaining an unchanged foraging efficiency meant that the postmenopausal grandmothers brought in more food per day than women of any of the younger groups, even though their large harvests were greatly in excess of their own personal needs and they no longer had dependent young children of their own to feed.
Observations indicated that the Hadza grandmothers were sharing their excess food harvest with close relatives, such as their grandchildren and grown children. As a strategy for transforming food calories into pounds of baby, it’s more efficient for an older woman to donate the calories to grandchildren and grown children than to infants of her own, because her fertility decreases with age anyway, while her children are young adults at peak fertility. Naturally, menopausal grandmothers in traditional societies contribute more to their offspring than just food. They also act as baby-sitters for grandchildren, thereby helping their adult children churn out more babies bearing Grandma’s genes. And though they work hard for their grandchildren, they’re less likely to die as a result of exhaustion than if they were nursing infants as well as caring for them.
But menopause has another virtue, one that has received little attention. That is the importance of old people to their entire tribe in preliterate societies, which means every human society in the world from the time of human origins until the rise of writing in Mesopotamia around 3300 B.C.
A common genetics argument is that natural selection cannot weed out mutations that do not damage people until they are old, because old people are supposedly postreproductive. I believe that such statements overlook an essential fact distinguishing humans from most animal species. No humans, except hermits, are ever truly postreproductive, in the sense of being unable to aid in the survival and reproduction of other people bearing their genes. Yes, I grant that if any orangutans lived long enough in the wild to become sterile, they would count as postreproductive, since orangutans (other than mothers with one young offspring) tend to be solitary. I also grant that the contributions of very old people to modern literate societies tend to decrease with age. That new phenomenon of modern societies is at the root of the enormous problems that old age now poses, both for the elderly themselves and for the rest of society. But we moderns get most of our information through writing, television, or radio. We find it impossible to conceive of the overwhelming importance of elderly people in preliterate societies as repositories of information and experience.
Here is an example of that role. During my field studies of bird ecology on New Guinea and adjacent southwestern Pacific islands, I live among people who traditionally were without writing, depended on stone tools, and subsisted by farming and fishing supplemented by hunting and gathering. I am constantly asking villagers to tell me the names of local birds, animals, and plants in their language, and to tell me what they know about each species. New Guineans and Pacific islanders possess an enormous fund of biological knowledge, including names for a thousand or more species, plus information about where each species occurs, its behavior, its ecology, and its usefulness to humans. All that information is important because wild plants and animals furnish much of the people’s food and all their building materials, medicines, and decorations.
Again and again, when I ask about some rare bird, only the older hunters know the answer, and eventually I ask a question that stumps even them. The hunters reply, We have to ask the old man [or the old woman]. They take me to a hut where we find an old man or woman, blind with cataracts and toothless, able to eat food only after someone else has chewed it. But that old person is the tribe’s library. Because the society traditionally lacked writing, that old person knows more about the local environment than anyone else and is the sole person with accurate knowledge of events that happened long ago. Out comes the rare bird’s name, and a description of it.
The accumulated experience that the elderly remember is important for the whole tribe’s survival. In 1976, for instance, I visited Rennell Island, one of the Solomon Islands, lying in the southwestern Pacific’s cyclone belt. When I asked about wild fruits and seeds that birds ate, my Rennellese informants named dozens of plant species by Rennell language names, named for each plant species all the bird and bat species that eat its fruit, and said whether the fruit is edible for people. They ranked fruits in three categories: those that people never eat; those that people regularly eat; and those that people eat only in famine times, such as after--and here I kept hearing a Rennell term initially unfamiliar to me-- the hungi kengi.
Those words proved to be the Rennell name for the most destructive cyclone to have hit the island in living memory--apparently around 1910, based on people’s references to datable events of the European colonial administration. The hungi kengi blew down most of Rennell’s forest, destroyed gardens, and drove people to the brink of starvation. Islanders survived by eating fruits of wild plant species that were normally not eaten. But doing so required detailed knowledge about which plants are poisonous, which are not poisonous, and whether and how the poison can be removed by some technique of food preparation.
When I began pestering my middle-aged Rennellese informants with questions about fruit edibility, I was brought into a hut. There, once my eyes had become accustomed to the dim light, I saw the inevitable frail old woman. She was the last living person with direct experience of which plants were found safe and nutritious to eat after the hungi kengi, until people’s gardens began producing again. The old woman explained that she had been a child not quite of marriageable age at the time of the hungi kengi. Since my visit to Rennell was in 1976, and since the cyclone had struck 66 years before, the woman was probably in her early eighties. Her survival after the 1910 cyclone had depended on information remembered by aged survivors of the last big cyclone before the hungi kengi. Now her people’s ability to survive another cyclone would depend on her own memories, which were fortunately very detailed.
Such anecdotes could be multiplied indefinitely. Traditional human societies face frequent minor risks that threaten a few individuals, and also face rare natural catastrophes or intertribal wars that threaten the lives of everybody in the society. But virtually everyone in a small traditional society is related to one another. Hence old people in a traditional society are essential to the survival not only of their children and grandchildren but also of hundreds of other people who share their genes. In preliterate societies, no one is ever postreproductive.
Any preliterate human societies that included individuals old enough to remember the last hungi kengi had a much better chance of surviving the next one than did societies without such old people. The old men were not at risk from childbirth or from exhausting responsibilities of lactation and child care, so they did not evolve protection by menopause. But old women who did not undergo menopause tended to be eliminated from the human gene pool because they remained exposed to the risk of childbirth and the burden of child care. At times of crises, such as a hungi kengi, the prior death of such an older woman also tended to eliminate all the woman’s relatives from the gene pool--a huge genetic price to pay just for the dubious privilege of continuing to produce another baby or two against lengthening odds. That’s what I see as a major driving force behind the evolution of human female menopause. Similar considerations may have led to the evolution of menopause in female pilot whales. Like us, whales are long-lived, involved in complex social relationships and lifelong family ties, and capable of sophisticated communication and learning.
If one were playing God and deciding whether to make older women undergo menopause, one would do a balance sheet, adding up the benefits of menopause in one column for comparison with its costs in another column. The costs of menopause are the potential children of a woman’s old age that she forgoes. The potential benefits include avoiding the increased risk of death due to childbirth and parenting at an advanced age, and thereby gaining the benefit of improved survival for one’s grandchildren, prior children, and more distant relatives. The sizes of those benefits depend on many details: for example, how large the risk of death is in and after childbirth, how much that risk increases with age, how rapidly fertility decreases with age before menopause, and how rapidly it would continue to decrease in an aging woman who did not undergo menopause. All those factors are bound to differ between societies and are not easy for anthropologists to estimate. But natural selection is a more skilled mathematician because it has had millions of years in which to do the calculation. It concluded that menopause’s benefits outweigh its costs, and that women can make more by making less.