Menopause seems like a cruel prank that Mother Nature plays on women. First come the hot flashes, night sweats, vaginal dryness, irregular periods, irritability and weight gain. Then menstruation stops and fertility ends. Why, many women ask, must they suffer through this? Evolutionary biologists, it turns out, ask themselves more or less the same question. How on Earth could such a seemingly maladaptive trait ever evolve?
From a Darwinian point of view, menopause is just weird. In the game of evolution, winning means securing your genetic legacy by having as many children as possible. So it seems counterintuitive that evolution would produce women whose fertility disappears decades before they die.
That’s why it’s not surprising that some researchers think women’s postmenopausal lives are just an artifact of modern society. Now that we’re healthier and living longer, they suggest, women are outlasting the fixed supply of eggs they have from birth.
Yet there’s evidence that menopause goes all the way back to the Stone Age, Daniel Levitis of the University of Southern Denmark and colleagues conclude in the March/April Evolutionary Anthropology.
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The support comes from contemporary hunter-gatherers who lead traditional lifestyles that, presumably, resemble those of our Stone Age ancestors. Among the Hadza and !Kung of sub-Saharan Africa, for example, women enjoy lengthy lives after fertility ends. In fact, women in these populations spend nearly half of their adult lives postmenopausal. If these groups are truly representative of early humans, then menopause could be at least as old as our species.
Given its apparently ancient roots, many scientists would like to believe that menopause must have some hidden evolutionary benefits. A popular explanation involves raising grandchildren. Selection might have favored longevity after childbearing ceased if grandmothers contributed to the survival of their grandchildren, and thus of their own DNA. Observational evidence of the importance of grandmas in historical populations lends some weight to the idea, not to mention a warm and fuzzy feeling.
Other scientists see menopause coming from a much harsher place—the propensity that men sometimes have for throwing off long-term partners in favor of younger ones. A recent computer simulation demonstrates that if men in a population consistently passed over aging women in favor of younger ones, then mutations favoring the loss of fertility in older women would accumulate and spread through a population over generations.
Normally, any mutation that interfered with reproduction would be quickly weeded out of the gene pool. If you can’t have babies, you can’t pass on those harmful mutations. But such mutations would be less of a blow, and thus more likely to be passed on to future generations, if their negative effects arose only after a woman was old enough to have done most of her reproducing.
So if enough generations of men didn’t give older women the time of day, eventually enough late-acting mutations harmful to reproduction would accumulate to produce full-blown menopause, biologists (all men) explained in June in PLOS Computational Biology.One wrinkle: The study doesn’t account for why men would initially prefer younger partners. It’s not necessarily the norm in the animal kingdom. Chimpanzee males, for instance, prefer mating with older, more experienced females.
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In addition to computers, scientists have turned to other animals for help in solving the menopause puzzle. Mounting evidence suggests menopause is not something that we share with our primate cousins. Although there are cases of old zoo apes or aging lab monkeys defying expectations and living long after their procreative capacity stops, these animals are exceptions to the rule. Levitis and colleagues argue that the relatively cushy conditions of captivity allow some individuals to enjoy a postreproductive life.
A new investigation of wild primates supports this belief. Susan Alberts of Duke University and others examined long-term field data from several species representing numerous branches of the primate family tree. The team did detect declines in fertility with age among the primates, but they didn’t find any examples of a definitive end in reproduction, the researchers wrote in July in the Proceedings of the National Academy of Sciences.
Perhaps scientists’ best hope for finding clues to menopause’s origins comes from a surprising source: killer whales. Like people, killer whales are very social, residing with their mothers and maternal relatives their whole lives. They’re also long-lived, going strong into their 90s. And like women, female orcas stop making babies in their 30s and 40s, long before they die.
Menopause and a long postmenopausal life are evolutionary oddities that people and killer whales developed independently. But by comparing the two species’ biology, developmental patterns, social structures and ecologies, scientists may better pinpoint the factors that played a role in how and why menopause evolved.