Last fall, a 96-year-old man named Ramajit Raghav became a father. No woman could become a mother at 96, or even 76. That’s because women typically lose the capacity to have children around the age 50–not because they become decrepit, not because civilization has poisoned them, but because they undergo a distinct biological transition, known as menopause.
Scientists have debated for years about why menopause exists. Some have argued that it’s a trait that evolved through natural selection in our ancestors. Women who stopped reproducing ended up with more descendants than women who didn’t. Some scientists proposed that older mothers were better off putting all their effort into caring for their children who were already born, rather than having new ones. As their limited supply of eggs deteriorated, they faced a higher risk of miscarriages and even death during childbirth. (In terms of reproduction, men have it easy by comparison: they can make new sperm through their whole life and don’t have to suffer any of the risks of pregnancy.)
But some studies raise questions about this hypothesis. The risks that childbirth poses to women later in life may not be big enough to make menopause much of an evolutionary benefit. Some scientists have come up with a different explanation: they argue that menopause provides the opportunity for women to help raise their grandchildren. Researchers who studied population records from Finland before the Industrial Revolution found that children were more likely to survive till adulthood if their grandmothers were still alive. Menopause might therefore be a winning evolutionary strategy because it leads to more grandchildren who can carry on Grandma’s genes.
But is it even necessary to think of menopause as a special adaptation in humans? Some scientists don’t think so. They argue that what happens to women as they get older is not terribly different from what happens to females of other species. In many species, females are born with a supply of eggs that then gradually deteriorate over their lifetime. They can invest energy into repairing the eggs, but if they invest too much, they have less energy for other tasks. This evolutionary balance leads females to eventually run out of viable eggs. Whether a female survives beyond that point or not simply has to do with how well her body is equipped to resist aging. There’s nothing special, then, about the fact that females in many species, including rats and elephants, can live past their reproductive years.
In the latest issue of Evolutionary Anthropology, three scientists take a closer look at the nature of human menopause. Daniel Levitis of the University of Southern Denmark, Oskar Burger of the Max Planck Institute for Demographic Research, and Laurie Bingaman Lackey of the International Species Information System started by comparing human biology to that of our primate relatives. Reviewing records from 66 species of primates, they found that in every case females could lived well beyond their last birth. Their post-reproductive life ranged between 25% and 95% of their breeding years.
Taken on its own, this result might suggest that human menopause isn’t anything special. But Levitis and his colleagues caution their readers to take it with a gorilla-sized grain of salt. Most of the records of longevity and births come from zoos, not surprisingly, where primates are well-fed, enjoy the attention of vets, and don’t face a daily threat from predators. Data on wild primates are a lot more sparse, understandably, but the picture that emerges from them is pretty brutal: only a tiny fraction of female primates survive to post-reproductive years.
Humans are different. A substantial portion of the women in any population are post-menopausal. This pattern is not limited to affluent societies. Take the Hadza, a group of people in Tanzania who survive by gathering fruit and killing game. A typical Hadza woman can expect to spend almost half her adult life in a post-fertile state. The slaves of Trinidad experienced some of the most brutal conditions ever recorded–so brutal, in fact, that their population was continually shrinking due to early deaths. And yet even among Trinidad’s slaves, a third of a woman’s adult life, on average, came after her last child.
It seems, then, that there really is something remarkable about the lives of human females compared to other primates. But is menopause what makes them remarkable, or is it just the side effect of something else that evolved in our ancestors? Humans have big brains, for example, and the bigger a primate’s brains, the longer its lifespan tends to be. This link may be due to the fact that big-brained babies demand a huge amount of energy and effort, both during pregnancy and afterwards. Those demands impose a slower pace of life on big-brained primates. So this pattern naturally raises the possibility that big brains in humans led to menopause.
Levitis and his colleagues don’t think so. They analyzed primate females, humans included, comparing their brain size to the age at which they stopped reproducing and the age at which they died. Even taking into account our gigantic brains, women are still odd. Compared to other primates, women stop reproducing sooner than you’d predict, and then go on living much longer than you’d predict.
So Levitis and his colleagues are left with the fact that human females typically enjoy a vastly longer post-reproductive life than their fellow primates, a life that can’t be explained away by other factors such as the size of their brains. Through human history, it appears, at least a third of women were post-fertile at any moment–a percentage that’s ten times higher than among the most pampered zoo-dwelling primates. There seems to be something special, something worth explaining, about menopause.
To explain this remarkable turn of evolutionary events, the scientists offer up a hypothesis. Six million years ago, our early hominin female ancestors were like other female primates. They could potentially live beyond their last childbirth, but they almost never did because most were dead by then.
And then hominins took an unusual evolutionary course. They evolved big brains and acquired new capacities–for making more versatile tools, for example, and for communicating with language. These factors may have allowed hominins to live longer. As a result, more females lived beyond their reproductive years.
Now the benefits of life after menopause could emerge. Any genes that enabled women to live longer would be favored by natural selection, because older women could raise the odds of their descendants surviving. Over many generations, women evolved a life in which they spent a dramatically larger part of it not having children.
In this new hypothesis, human menopause becomes at once special and yet not unique. In many species, females have the capacity to live beyond reproduction, but they rarely do, depriving evolution of the opportunity to expand that stage of life. But if other animals get that chance–for whatever reason–they may evolve to be menopausal too. Even insects can benefit from menopause. In species known as the Japanese gall aphid, females stop reproducing midway through their lives. Now that their abdomens are no longer dedicated to growing eggs, they can use that space to manufacture a sticky chemical. When a predator attacks the aphid colony, the menopausal females rush forward and glue themselves to its body. The predator is swamped by the heroic females, which die in the process. The evolutionary forces behind menopause may differ between humans and aphids, but the outcome is the same.
Originally published April 18, 2013. Copyright 2013 Carl Zimmer.