by Angela Saini
At the same time, other scientists have turned their attention to the bigger, broader evolutionary question of why women experience menopause at all. Does it serve a purpose, which has some biological logic to it? Or is it like wrinkling and gray hair—an unavoidable aspect of aging, a disease of deficiency—that mark the body’s inevitable decline? Why then do all women experience it, but some men seem to be able to keep reproducing until they die?
When it happened to my mother, she remained an active, working woman. Menopause didn’t stop her from running a business, teaching yoga, cooking, and babysitting. And her experiences are echoed by people like her the world over. As far as history tells us, it has always been this way. The existence of healthy postmenopausal women poses an enormous dilemma for evolutionary biologists. Why would nature render them infertile when they are still so full of life?
“These old ladies . . . were just dynamos.”
When a phenomenon as important as menopause happens in humans, we almost always find it in other species, too, particularly among our primate relatives, like chimpanzees and the other great apes. But with menopause, that’s not the case. It’s freakishly unusual. In almost every species, females die before they become infertile. Chimpanzees are, like us, fertile for no more than around forty years. The difference is, in the wild they rarely survive beyond this. Elephants live longer but carry on having babies into their sixties. A long postmenopausal life is so rare that, as far as we know, humans share it with only a few distant species, including killer whales, which stop reproducing in their thirties or forties but can survive into their nineties.
The reason for this rarity seems to be that we and all other animals have few physical features that haven’t been pared down by evolution to make them fit their purpose. We are streamlined by nature, having long ago ditched most of what we don’t need and honed what we do. Life span appears to be one of those features. By and large, animals live long enough to have children and maybe see them grow up, and then they die. If you can’t reproduce and your genes are no longer being passed along to another generation, then harsh as it seems, nature generally doesn’t want to know. This logic dictates that nobody should be alive beyond menopausal age. By this ruthless measure, my mother and all postmenopausal women should be dead.
Yet they’re all around us. What’s more, on average, they live even longer than men do, even though men can keep producing sperm well into old age. (Although one study in 2014 did find that semen tends to change after the age of thirty-five, making it slightly less likely a man’s partner will get pregnant after sex. Research published in 2003 also showed that pregnancies from older fathers, especially past the age of fifty-five, are more likely to lead to miscarriage and birth defects.)
Answers to the mystery began with a brief observation made back in 1957 by George Williams, a biologist who was one of the most important evolutionary scientists of the twentieth century and at that time working at Michigan State University. The exact question he was pondering was why women lose the ability to have babies in middle age so abruptly when other parts of aging happen more gradually. He proposed, briefly and without much elaboration, that menopause may have emerged to protect older women from the risks linked to childbirth, keeping them alive long enough to look after the children they already had.
Until fairly recently, having babies was a huge killer of women. In the nineteenth century the number who died from or during childbirth in England and Wales hovered between four and seven for every thousand births, and this didn’t fall significantly until after the Second World War. Having babies into old age would have multiplied the risks for both mothers and their children. “It is improper to regard menopause as a part of the aging syndrome,” concluded Williams. His kernel of an idea came to be known as the “grandmother hypothesis.”
To parents whose own parents are still alive, the grandmother hypothesis makes instinctive sense. For me, sitting at my desk today is a benefit made possible by my mother-in-law. She’s busy taking care of my baby son, leaving me free to do other work or have more babies. And she isn’t alone. Grandmothers (and it has to be said, a few grandfathers, too, these days) are a common sight on the streets where I live in London, pushing buggies in the middle of the morning and carting beloved spawn of spawn back from schools and nurseries in the afternoon. It’s a trend that we nowadays associate with busy working parents and the high cost of child care, but it has far longer roots. Extended families, in which children live with their grandparents, were until recently a common feature around the world. In Africa and Asia, they still are. Research by the US-based organization Child Trends found in 2013 that at least 40 percent of children in Asia live with extended family as well as their parents. This, in essence, could have been the kind of crucible in which the grandmother hypothesis operated.
The focus on grandmothering also casts menopause in a new light, suggesting that it isn’t some biological blip or routine curse of old age, but that it’s there for a distinct evolutionary purpose: to allow women to safely continue caring for their children as they grow older and perhaps also be there for their grandchildren. The old image of the useless crone is replaced by a useful woman. Rather than being a burden on society, retreating into a quieter life, she is front and center. She is propping up her family.
For the sixty years since Williams first shared this thought, researchers have been searching for the evidence to prove it.
“I was just trying to understand what the men were doing,” says Kristen Hawkes, professor of anthropology at the University of Utah. She’s the world’s leading researcher on the grandmother hypothesis, and its strongest advocate.
Hawkes spent the 1980s doing fieldwork with the Aché, nomadic hunter-gatherers in Eastern Paraguay. And she soon realized, like anthropologists before her, that men weren’t providing all the food for their families. Hunting by men alone simply didn’t put enough on the table for women and children to survive. “The things that they were foraging for were the things that went around to everybody. So the fraction that went to their wife and kids was no different from what everybody else got,” she tells me. Meat from a hunt not only had to be divided among many but was also sporadic. Long periods of time could go without a kill.
Trying to uncover more clues about how mothers and babies were surviving, Hawkes went to study Hadza hunter-gatherers in Tanzania. The Hadza are particularly special to anthropologists because they arguably live a life as close to how humans lived before agriculture as anyone is likely to find today. A large portion of the Hadza don’t tend crops or herd animals, and they live south of the Serengeti in a region not far from where fossils have been found of some of our very earliest ancestors. “That was paramount for me going to the Hadza,” she explains.
And it was there that she saw hardworking grandmothers.
“There they were, right in front of us. These old ladies who were just dynamos.” It’s impossible to hear Hawkes talk about her fieldwork and not get carried away by her excitement. Her voice shifts a gear and, to this day, she sounds genuinely surprised by what she found all those decades ago. There was a division of labor between childbearing women and grandmothers, with active older women foraging for food alongside everyone else.
Hawkes discovered that the Hadza grandmothers and other senior women, including aunts, helped daughters raise more and healthier children. They were vital to reproduction even if they weren’t themselves having babies. Grandmothers, she suggested, were also the reason women were able to have shorter intervals between babies. They stepped in to help before other children became independent. Her landmark scientific paper on the subject, published with her colleagues in 1989, was titled “Hardworking Hadza Grandmothers.” More work by Hawkes and her team has since revealed just how industrious they are. Women in their sixties and seventies are described as working long hours in all seasons, bringing back as much food or even more than younger women in their families.
Other anthropologists have seen similar
things. Adrienne Zihlman, who helped develop the idea of woman the gatherer, recounts a particularly vivid example for me, which she read in the New Yorker in 1990. It comes from the writer Elizabeth Marshall Thomas, who lived with nomadic hunter-gatherers in the Kalahari in southern Africa. Thomas describes a group of people who fell ill during an epidemic. One young widow and her two children were too sick to leave with the group when it decided to shift camp in search of more food. “But her mother was there,” she writes. “This small, rather elderly woman took her daughter on her back, her infant grandchild in a sling across her chest, and her four year old grandchild on her hip. She carried them thirty-five miles, to her people’s new camp.” The superhuman efforts of this grandmother meant her daughter and two grandchildren recovered from their illness and weren’t left behind.
A common counterargument to the grandmother hypothesis, known as the “extended-longevity” or “life-span–artifact” hypothesis, is that menopause must be a by-product of our increased life expectancy. We don’t have to go back many generations to know that we’re living on average far longer and healthier lives than our ancestors. In the United Kingdom, life expectancy for women has risen from forty-nine years in 1901 to almost eighty-three in 2015. This is expected to go up by another four or so years by 2032. In the United States, female life expectancy was just over eighty-one years in 2015, according to the Centers for Disease Control and Prevention’s National Center for Health Statistics. So the line of reasoning goes that older women become infertile because, were it not for all the good food they eat, sanitation, and modern medicine, they would be dead too early to experience menopause at all.
In reality, life-expectancy data can be misleading. A large chunk of a population’s average life expectancy will often be decided by infant mortality. If more children die, this drives down the average. This in turn means there’s every likelihood some people were long ago achieving ripe old ages, even if most of the people around them had shorter lives. According to the latest findings, it is almost certain that some women would have experienced menopause in our ancient past. The earliest recorded mention is often attributed to Aristotle in the fourth century BC, when he is supposed to have noted that women stopped giving birth around the ages of forty or fifty.
Research comparing the body weights and body sizes of our primate cousins suggests that a small proportion of our early human ancestors could have lived to between sixty-six and seventy-eight years. Most convincing, scientists studying hunter-gatherers the way Kristen Hawkes has done have noticed that between 20 and 40 percent of adult women are postmenopausal. In other words, older women would always have existed.
In her book Mothers and Others, anthropologist Sarah Blaffer Hrdy suggests, “Fewer than half of Pleistocene mothers would be likely to have had a mother alive or living in the same group when they first gave birth.” So not every child would have had a grandmother alive, but many would have. Grandmothers are the “ideal allomothers,” she adds. “Experienced in child care, sensitive to infant cues, adept at local subsistence tasks, undistracted by babies of their own or even the possibility of having them, and (like old men as well) repositories of useful knowledge, postmenopausal females are also unusually altruistic.”
Hard data, too, have backed up Hawkes’s findings. Studies in Gambia have found that the presence of a grandmother increases a child’s chance of survival. Similar results have been found in historical data from Japan and Germany. One study of three thousand Finnish and Canadian women from the eighteenth and nineteenth centuries found that women had two extra grandchildren for every ten years they survived beyond menopause.
In 2011 evolutionary demographer Rebecca Sear and biomedical scientist David Coall pulled together research from across the world to find out who, other than mothers, has the greatest impact on child survival. They concluded in their paper, published in Population and Development Review, that maternal grandmothers are consistently among the most reliable helpers. “In more than two-thirds of cases her presence improved child survival rates. Paternal grandmothers were also often associated with positive survival outcomes, though somewhat less consistently: in just over half of cases they improved child survival,” they note.
“Very few species have a prolonged period of their life span when they no longer reproduce,” says Darren Croft, a psychologist who studies animal behavior at the University of Exeter in the United Kingdom. Croft has a particular interest in resident killer whales—orcas—one of a few species of whale in which females are known to stop having babies and yet live for many decades afterward, sometimes into their nineties. The males die far younger, in their thirties or forties.
His explanation for this, outlined in a paper published by him and his team in the journal Science in 2012, lies in the power of the enormously tight bond between whale mothers and their sons. “Female killer whales act as lifelong carers for their own offspring, particularly their adult sons,” he explains. A mother killer whale with a son focuses her efforts on him throughout her life. Indeed, such is the connection between them that data have shown that when a mother killer whale dies, her son is more likely to die far younger. Incidentally, this is just a son thing. The link between the life spans of mothers and daughters is weaker.
Croft carried out further research with colleagues at Exeter University, York University, and the Center for Whale Research in the United States, published in Current Biology in 2015, also looking at resident killer whales in the northern Pacific. Watching the whales led them to believe that it’s the wisdom gathered over their lifetimes that makes the older females so invaluable. “They are more likely than the males to lead a group of orcas, especially in times of short food supply,” says Croft. “For killer whales, what’s really crucial is when and where salmon is going to be,” and the older females seem to have this knowledge.
Croft believes that research like his into menopausal whales, unusual though they are, could provide an extra piece of the human menopause puzzle. If this can happen in the wild to another species, then it could have happened to us. “Following old females isn’t unique,” he adds. Elephants, too, have matriarchs who seem to have special information about threats from predators.
Since the grandmother hypothesis has emerged, other theories have added to it. In 2007 Barry Kuhle, in the psychology department at Scranton University, proposed that fathers (more specifically, absent fathers) might also have helped in the evolution of menopause. His idea is that men become less involved parents as mothers get older, partly because they die sooner but also because they are more likely to leave their partners. This supports the grandmother hypothesis, again, because it makes what grandmothers do even more vital. “I simply added an additional factor,” says Kuhle.
Others have added that grandmothers aren’t necessarily heartwarming, selfless babysitters living in harmonious families. Research published in the journal Ecology Letters in 2012 has indicated that what forces some women into caring for their grandchildren is intergenerational conflict rather than the failure to have babies of their own. Evolutionary biologist Virpi Lummaa and her colleagues studied parish-record data in Finland and found that infant survival was drastically reduced when daughters-in-law and mothersin-law had babies at the same time, if there weren’t enough resources for all the children. If a mother-in-law cares for her grandchildren, she benefits because she is genetically related to them. There’s no such benefit the other way round for the daughters-in-law, says Lummaa. Grandmothers are genetically related to their grandchildren, whereas daughters-in-law are not genetically related to their sisters- and brothers-in-law. Grandmothering, then, is just a canny choice when resources are scarce.
“Men, young and old, prefer younger women.”
The grandmother hypothesis hasn’t gone unchallenged.
At least a dozen competing ideas have come along over the years, each with its own drawbacks and merits. They include the follicular depletion hypothesis, which, like the extended longevity hypothesi
s, says that women nowadays outlive their eggs. The problem with this is that you might then expect women with more children to go through menopause later, because they’re not menstruating while pregnant. They don’t. Another hypothesis focuses on reproductive cost, saying that baby making takes such a large physical toll on a woman’s body that menopause evolved to protect her from further damage. But if this were true, we might expect to see women with more children experiencing menopause earlier, and we don’t. Another, the senescence hypothesis, offers up the possibility that menopause is just a natural feature of aging, like wrinkles or loss of hearing. And while other side effects of old age may happen gradually, including male infertility, female fertility just happens to end more abruptly for physical reasons.
In 2010 evolutionary biologist Friederike Kachel and a team of researchers at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, decided to run a test to see if the grandmother hypothesis really is the best explanation for menopause among these alternatives. They ran computer simulations of how humans could have evolved with women living longer after menopause. To the surprise of anthropologist Kristen Hawkes and her team, who by now had amassed many years of evidence in their favor, Kachel’s group found that while helpful grandmothers certainly did raise the survival rates of their grandkids, this effect didn’t appear to be enough to account for why women now live so long.
In 2012, rescuing the hypothesis from news reports that were already questioning it, Hawkes’s team published the results of their own computer simulation, which showed that slowly increasing the proportion of particularly long-lived grandmothers in a population, from 1 percent to 43 percent over the course of thousands of years, could indeed drive up everyone’s life span. She and her colleagues believe that part of the problem with the German mathematical model may have been that it was run for just ten thousand years, when in fact the long sweep of human evolution means that the effects could have taken much longer to appear. They also argue that the model hadn’t accounted for possible costs for men in living longer, such as having to compete with the same number of fertile men for a proportionally smaller pool of fertile women.