How Much Did Grandmothers Influence Human Evolution?

The seeds of an idea were planted as Kristen Hawkes watched older women collecting vegetables.

Hawkes, a professor of anthropology at the University of Utah, has extensively studied the Hadza, a group of hunter-gatherers in Tanzania who eat a lot of wild foods such as berries and tubers. While young children can pick berries themselves, older women in the community are the ones pulling up the bulbous root vegetables, which would be difficult for young kids.

Hawkes found a correlation between how well children grew and their mother’s foraging work, until the mother had another kid. Then, their growth correlated with “grandmother’s work,” she says. “There were the data right in front of us.”

These observations, which Hawkes and collaborators began in the 1980s, have helped fuel the Grandmother Hypothesis, the idea that grandmothers step in to feed young children and perform other motherly duties so that mothers can focus their own energy and resources on having more children at shorter intervals. The result is that a grandmother enables the birth of more descendants, leaving more copies of her genes in subsequent generations. In prehistoric times, the theory goes, grandmothering led to the spread of genes corresponding to slower aging in women relative to their predecessors, which increased expected lifespans in general.

Combining those observations with models of variation in life history in other organisms, from mice to elephants, Hawkes and colleagues have become convinced that human grandmothers have played a central role in the life history of Homo sapiens. They argue that grandmothers are a driving force behind the increased longevity of our species compared to other primates.

Longevity is also highly correlated with brain size across the mammalian kingdom—the bigger the brain, the longer the lifespan—and the best predictor of brain size is the duration of brain development. “If you’re going to make a bigger brain, it takes longer to make it,” explains Barbara Finlay, professor emerita of psychology at Cornell University, who has collaborated with Hawkes.

Through a combination of anthropological fieldwork, mathematical modeling and comparative primatology, Hawkes and collaborators make the case that a prehistoric division of labor—in which grandmothers take on responsibilities for nourishing grandchildren while mothers pop out more babies—has led to the long lives and big brains we have today.

“All of these pieces start to be connected to this puzzle of us, coming back to this life history shift to this increasing longevity, with older females subsidizing the fertility of younger ones,” Hawkes says.

It’s heartwarming to think of grandmothers as evolutionary heroines, especially in the face of an alternative narrative: that postmenopausal women merely represent evolution’s failure to sustain fertility throughout a woman’s entire life. But to skeptics, the Grandmother Hypothesis remains a “just so” story, a tidy narrative that can’t truly be proven or disproven, which is the burden of science. Nonetheless, the Grandmother Hypothesis hints at broader mysteries and controversies about the human lifespan, women’s bodies and to what extent health declines as a result of menopause.

Evolutionary biologists and anthropologists have spent decades trying to figure out why female humans outlive their fertile period when few other animals do. (The Grandmother Hypothesis originated with a 1957 paper by the late ecologist George C. Williams, but more recently Hawkes has carried the torch for grandmothers’ role in evolution.) Chimpanzees, our closest mammalian relatives, and other great apes do not typically live past menopause—and most don’t even experience it. The killer whale and pilot whale are rare examples of mammalian species in which females continue living after they can no longer reproduce. A 2019 study shows that grandmother whales increase the survival of their aquatic grandkids; for example, grandmothers can lead the group to food when there are fewer salmon around.

While everyone agrees grandmothers can provide welcome childcare support and resources for their children raising new babies, one debate about the Grandmother Hypothesis surrounds which is a more relevant metric: how long people lived on average or, instead, how long people could live.

Donna Holmes Parks, associate clinical professor of biology at the University of Idaho, argues that long lifespans among humans is a modern phenomenon. Ancient humans rarely lived beyond 50 years, and prior to the industrial revolution most people tended to die by 35, the age at which fertility starts to decline in both sexes, Parks writes in the book The Arc of Life, which she edited with Grazyna Jasienska and Diana Sherry.

Life expectancy from birth in the U.S. in 1900 was 45; over the course of the 20th century, as modern medicine entered the scene, it climbed to 78 to 80 years old, she writes. But Hawkes and others counter that in previous eras, many more babies and juveniles died young, lowering the average age of death. They point to the difference between life expectancy and life span potential, of which the latter is much longer.

And then a related question arises—how old is menopause? If menopause is a recent phenomenon, then scientists have a harder time arguing that postmenopausal grandmothers have so strongly shaped human evolution.

If menopause is ancient, anthropologists expect to find commonalities in the symptoms that women experience regardless of their ethnicity. Given that all humans descended from a single African ancestor, population variations observed today are associated with evolution in more recent eras, more like 5,000 to 50,000 years ago, according to a 2020 study in BMC Women’s Health. And this new study does find modest differences between ethnic groups in both self-reported menopausal symptoms and associated hormones, arguing that menopause is relatively recent in human history.

But Lynette Sievert, biological anthropologist at the University of Massachusetts, Amherst, is skeptical. She has done fieldwork on menopause in many communities worldwide, from Bangladesh to Mexico to Hawaii. Her work has found that while women in some cultures may say that they do not experience hot flashes, monitoring devices on volunteers in those groups show that actually hot flashes are common—these women just don’t talk about them. Sievert says the universalities of the menopausal experience across the world suggest a shared experience of estrogen decline at midlife. Hot flashes may have ancient roots.

While no one can observe the hot flashes of Homo erectus, Sievert and others say humans and their ancestors have gone through menopause for at least 1 million, even up to 1.8 million years—even before anatomically modern Homo sapiens.

Even if menopause is truly ancient, some detractors of the Grandmother Hypothesis point to the health perils of a woman’s post-fertile years to argue that this stage of life is not adaptive—maintaining that postmenopausal womanhood did not result from the selection of inherited beneficial traits over time. Many of the common health problems of aging can be traced to physiological phenomena that are beneficial in younger people, Parks says. Estrogen is essential for fertility, but in later years the decline in estrogen puts women at risk for osteoporosis, on top of the unpleasant and sometimes serious symptoms of menopause itself. Decreases in estrogen may also contribute to the risk of heart disease and dementia. “If grandmas are so important to their relatives, why isn't health in general stable from menopause onward?” Parks writes in an e-mail.

Sievert agrees that grandmothers have played important roles in helping their children and grandchildren, but for her, it’s not the answer to why women live beyond their fertile years and past menopause. What set up our species to have post-fertile grandmothers around in the first place is that women are born with all the eggs they will ever have. Other animals like fish, amphibians and most reptiles produce eggs throughout their lives, so their females will never experience menopause or live in a postmenopausal state.

Human females start out with about 1 million eggs, of which about 300,000 remain when puberty begins. But these tiny cells aren’t only for reproduction. In concert with hormones circulating during fertile years, eggs also produce estrogen and serve other functions besides combining with sperm.

Looking across species, scientists have found an intriguing correlation between the number of eggs the typical female produces and the expected length of life, Sievert says. Mammals produce all their eggs at once, in a greater quantity than they could possibly use. The number of eggs quickly declines around birth, but drops more slowly before the onset of fertility. Then, eggs die off even more slowly until fertility ends.

Sievert argues that as the human lifespan potential became longer and longer, the female body did not simultaneously evolve to make enough eggs to keep up. The maximum potential lifespan, therefore, grew to outpace egg production.

And so women got to experience older ages, even after their eggs ran out. For mammals, ovaries stop working by age 50, setting the stage for a post-reproductive life that can include grandmotherly childcare duties. “I don’t think that becoming a grandmother selected for menopause and post-reproductive life,” Sievert says. “But I think that having post-reproductive life opened the space for effectiveness of grandmothers.”

And grandmothers aren’t the only potential helpers. Aunts and other relatives and community members can provide the “stuff and knowledge” that grandmothers are known for disseminating, says Finlay, the emerita psychology professor at Cornell. The more ancient communal, multigenerational living situation contrasts with the stereotypical American nuclear family in which parents and kids live apart from other relatives—although in reality, many variations exist in households today.

Grandfathers can provide food resources to offspring and grandchildren, too—something corroborated in the fieldwork of Hillard Kaplan, who was Hawkes’ graduate student, and colleagues studying the Tsimané, an indigenous group in Bolivian Amazonia that lives off of hunting, foraging and cultivation.

That’s not surprising to Hawkes, as local environmental and social factors shape different groups and the way they face tradeoffs involving resources and childrearing, she says. But she maintains the bulk of evidence—the economics of foraging for foods among groups such as the Hadza, and mathematical models of grandmotherly effects on lifespans—supports grandmothers as the ancient secondary providers for kids, which shaped human longevity.

While this idea remains controversial, the general importance of grandmothers in the lives of many families is not. If you can, call yours today.

#Nature | https://sciencespies.com/nature/how-much-did-grandmothers-influence-human-evolution/