Homo Mysterious: Evolutionary Puzzles of Human Nature (11 page)

Perhaps menopause is a result of those darned, new-fangled, increased life spans. Thus, what if menopause itself is an aberration rather than an adaptation, a consequence of the fact that our reproductive biology—including the number of eggs built into a newborn baby girl—was tuned to prehistoric times, when people simply didn’t live as long as they do today? After all, if most of our
ancestors were dead before age 50, it is no surprise that our bodies haven’t evolved with an eye toward making productive use of those additional, tacked-on decades. There is, however, a big problem with this explanation: Even in prehistoric times, many people lived into their sixth, seventh, and eighth decades.
Average
life span was shorter in those times, but because infant and early childhood mortality was very high, not because there were hardly any old people. If, for example, one newborn baby died for every person who survived to fourscore, the “average life span” would be 40.

Although life expectancy at birth for contemporary hunter-gatherers is only 30 to 35 years, this, too, is due to high infant and juvenile mortalities, leaving plenty of opportunity for natural selection to act upon the seniors. Having reached age 20, life expectancy for today’s nontechnological foraging people extends about 20 years beyond menopause. So why don’t they—and we—keep reproducing?

Maybe an older female body simply isn’t up to the rigors of pregnancy and lactation. But assisted reproductive techniques have proven to a no-longer-surprised world that women in their 50s and even 60s can sustain pregnancy and bear healthy babies. So clearly the female “infrastructure” can be up to the task. Basic statistics plus the “magic of compound interest” suggest that selection should have favored those individuals who gave it a try, if only because some would have succeeded—and these would have contributed disproportionately to the current population. Natural selection should have favored women who attempted to bear just one more child, no matter how old they were and even if most of them died trying, because any who succeeded would be a step ahead of the competition.

Finally, if menopause is simply the biologically mandated consequence of increased age, an accidental consequence of modern technology keeping our bodies going beyond their usual and allotted life span, then why is it merely the female reproductive system that poops out? Why isn’t it the kidneys, heart, or liver that throws in the towel? Looking simply at the debility of old age, evolutionary theory strongly suggests that all systems should fall apart at about the same time, since as soon as any one began to malfunction first, selection would no longer operate to maintain the others.

Here is a metaphor that might help: It is said that Henry Ford once commissioned an engineer to examine junked Model Ts to determine which parts wore out and which were still functional. He learned that the kingpins remained in good shape, even when the pistons, driveshafts, and so forth showed substantial wear. Ford then ordered that the kingpins be manufactured to
lower
specifications.

It isn’t that all human reproduction ceases at menopause, but specifically human
female
reproduction. (“Male menopause” is a myth; certainly it does not exist with anything like the clear-cut biological specificity of its female counterpart. Although male fertility declines with age, it does so pretty much at the same pace as the aging of other organ systems, whereas female fertility and only female fertility stops abruptly.)

One explanation—ingenious but unlikely—has been dubbed the patriarch hypothesis.
¹⁷
It is the menopausal equivalent of the by-product hypothesis for female orgasm: namely, that the trait in question (this time, menopause) is a tag-along trait, present in women merely because its counterpart has been selected for in men. The idea is that since human beings are primitively harem forming, a small number of highly successful men have long been able to reproduce disproportionately, often into middle age and beyond, ceasing only when they are felled by illness or injury. Selection would therefore have favored extending male lives accordingly. If any such genetically based longevity factors were present on the Y chromosome, they would be unable to influence female biology, since females are XX. However, if genes for extended life span appeared on any of the other, “autosomal” chromosomes, they would be inherited by women, too, whose enhanced life expectancy would have been “dragged along” by its fitness payoff in men.

Not so fast. First, no such longevity genes appear to exist. And second, if this hypothesis were valid and women’s life spans have been “artificially” enhanced because of a breeding payoff to elderly patriarchs among whom selection would presumably have favored their extended sperm production as well, then why weren’t women similarly selected for continued reproduction, too?

This should help bring our attention back where it belongs, to women.

Women get pregnant, not men. Women give birth, not men. And women lactate, not men. All of these are demanding, biologically crucial activities, the costs of which fall entirely upon women. Maternal mortality can be distressingly high. Paternal mortality? The phrase doesn’t even exist. Add to this the regrettable reality of senescence, the fact that as bodies grow older they become less viable and more breakable than their younger counterparts, and it isn’t surprising that at some point as they grow older, women stop bearing children.

But it isn’t quite that simple. Why should it matter to evolution if women kept reproducing, or trying to do so, until they died in the process? After all, this is precisely what happens among nearly all other species, which typically breed until the bitter end. It must somehow be the case that women who, having reached a certain age, desisted from reproduction actually ended up leaving more genetic descendants than did those who kept on keeping on.

But how?

What follows are some ideas about why natural selection might have favored early termination of women’s fertility. First comes simple prudence, as negatively modeled by Jane Goodall’s famous chimpanzee matriarch, Flo. Chimps do not undergo menopause, and Flo—well known to those who have followed Dr. Goodall’s detailed studies of free-living chimpanzees in the Gombe Stream Reserve of Tanzania—kept breeding into advanced old age. It must have seemed a near miracle when Flo became pregnant for what turned out to be the last time, since she was obviously dilapidated in every respect. Here is the sad story, reported by the Jane Goodall Institute:

Flo gave birth to at least five offspring: Faben, Figan, Fifi, Flint, and Flame. She was a wonderful, supportive, affectionate and playful mother to the first three. But she looked very old when the time came to wean young Flint, and she had not fully succeeded in weaning him when she gave birth to Flame. By this time she seemed exhausted and unable to cope with the aggressive demands and tantrums of Flint, who wanted to ride on her back and sleep with her even after the birth of his new sister. She still had not weaned Flint when Flame died at the age of six months, and at this point stopped even trying to push
Flint to independence. Flint therefore became abnormally dependent on his old mother. When Flo died in 1972, he was unable to cope without her. He stopped eating and interacting with others and showed signs of clinical depression. Soon thereafter, Flint’s immune system became too weak to keep him alive. He died at the age of eight and a half, within one month of losing his mother Flo.
¹⁸

Flo wasn’t really a failure, since she produced at least three flourishing offspring. But the likelihood is that if she had been just a wee bit more reproductively prudent—if she had refrained from that last breeding attempt—she wouldn’t merely have survived longer (which, after all, isn’t an evolutionary payoff in itself), but so, too, would the unfortunate Flame and perhaps Flint as well. Maybe, therefore, we shouldn’t speak of Flo’s failure, but rather, her folly. And maybe human menopause is a way that evolution has outfitted our own matriarchs with a way to avoid Flo’s folly, by forcing them to be reproductively prudent and getting their bodies to “just say no.”

Even in medically sophisticated societies, a 40-year-old woman faces seven times more risk of dying in childbirth than does a 20-year-old. Hard-nosed evolutionary biologists might be nonetheless unimpressed, however, pointing out that as with the payoff of reproducing despite the increased risk of genetic anomalies, selection would still favor a woman who tried, even if she failed, simply because it would favor any who succeeded. But this omits another important consideration, somewhat valid for chimpanzees but more so for human beings: the extent to which offspring survival (and thus parental fitness) depends on parental investment in those offspring. Since children depend so heavily on their parents, human parents may well have been under especially strong selection pressure to be prudent rather than go with the Flo.

In the poker game of breeding—in which maximizing your fitness substitutes for maximizing your pile of chips—just as there is a payoff for betting successfully on one’s breeding potential, there is a cost to betting too high, like Flo did. Similarly, it would be suboptimal to be too prudent and bet too low—that is, to quit breeding too soon—thereby underplaying one’s hand. As in Kenny Rogers’s song
The Gambler
, “you got to know when to hold ’em, know when to fold ’em, know when to walk away and know when
to run.” The prudent mother hypothesis is that menopause tells women when to fold ’em. But it’s not the only game in town.

Another is called the grandmother hypothesis.

The basic idea is simple enough, although hidden within is a crucially important revision in our current understanding of evolution and, indeed, of the very nature of living things. First, consider that a woman begins to experience menopause at about the time when her own children, born perhaps two decades or so earlier, are themselves likely to become parents. That is, she may well be—or is about to become—a grandmother. The grandmother hypothesis, then, is that by foregoing reproduction, especially at a time when the cost of reproductive “imprudence” is rising—higher risk of morbidity and mortality during pregnancy and childbirth along with increased prospects of genetic anomalies in any offspring actually produced—a middle-aged woman might be freeing herself to contribute to the eventual success of her grandchildren. By doing so, she is actually being genetically selfish as much as altruistic, since the beneficiaries of her personal reproductive restraint include not only the grandchildren themselves but also their genes—which is to say, the grandmother’s, too.

It could be mere coincidence or—more likely—part of evolution’s design that around the world, grandparents in general and grandmothers in particular pitch in and help out. Not only that, but those who do so typically end up with more grandchildren than those who don’t. The grandmother hypothesis does not preclude the hypothesis of prudent mothering, however, since once a mother is no longer encumbered with dependent children, it makes social as well as biological sense that she would be inclined to help out with her kids’ kids.

Once again, our species’ unusually long period of profound juvenile dependency may also be involved, insofar as such neediness would confer a special benefit to assistance rendered by others beyond the parental pair. Consistent with this, Sarah Hrdy has proposed that humanity may well have evolved in the context of extensive cooperative parenting.
¹⁹
And who would be more
qualified and also better positioned to gain biologically as a result, than grandmothers?

Detailed studies by anthropologist Kristen Hawkes and her colleagues have found that among the Hadza, modern hunter-gatherers of Tanzania, the men hunt while the women gather and forage and (ta da!) the most energetic and productive foragers of all are postmenopausal women.
²⁰
A young mother, no matter how healthy and hard-working, is necessarily constrained while burdened with a baby, making such assistance nothing to sneeze at. Sure enough, Hadza grandmothers give their bounty to their children and grandchildren, whose body weights vary directly with their grandmothers’ food-gathering efforts. It is doubtless significant that among the Hadza studied by Hawkes and her colleagues, every nursing mother had a postmenopausal helper.

The technical term for cooperative breeding is “allo-parenting” (
allo
= “other”) and its likely importance in our evolution should give pause when we consider the extent to which modern Western societies—with their assumption of the “nuclear family”—make it impossible for grandparents to make the kind of social and biological contribution that might well have been crucial for 99% of our biological past. Although multigenerational households can certainly introduce their own forms of stress, it can hardly be denied that children, parents, and grandparents (perhaps grandmothers in particular) have also gained greatly from the interaction.

How much, we cannot tell. But the basic pattern, in which hard-working grandmothers contribute significantly to the success of their grandchildren, has been confirmed by other anthropologists studying other human groups.
²¹
All of this makes it increasingly likely that grandmothers owe their nonreproductive status to the payoffs that—at least in the past—they were able to convey, and the genetic recompense they received as a result.

An interesting twist to the grandmother hypothesis also merits our attention. It is deservedly popular to point to “win–win solutions” by which everyone in a competition—better yet, an interaction—comes out ahead. The sad reality, however, is that life is often a zero-sum game in which benefit to one participant necessitates some cost to another. The simple act of reproducing, and more to the point successfully rearing one’s offspring, is often zero sum, especially when resources are scarce. So perhaps we should
consider the role of menopause as a way of minimizing reproductive competition, something particularly relevant when one individual’s baby making can depress that of another.