Is Aging an Accident?
The grandmother effect
Posted Feb 28, 2020
For most of us, aging feels inevitable, a more or less guaranteed downhill slide that we sometimes dread even more than death itself.
But one of the most fascinating things I discovered in the course of writing my new book on aging and exercise (Exercise is Medicine: How Physical Activity Boosts Health and Slows Aging) is that human aging is actually a kind of accident.
And while most of us think we know what aging is—the sagging skin, the shaky knees, the hunched back—I also discovered that to evolutionary biologists and other scientists who study aging, the process of aging is actually one of the deepest mysteries of the universe.
After all, what’s the point? Once you’ve passed your genes on to the next generation, why stick around? Why take up space and use food and other scarce resources? It’s the young who need those things to live to reproductive age. So why do old animals even exist? Or old people?
Evolution has no reason to favor long life, Steven Austad, a former lion tamer and now a bio-gerontologist at the University of Alabama at Birmingham, told me. Quite the contrary, he says: “Evolution favors early, copious reproduction at the expense of later life survival.”
S. Jay Olshansky, a biodemographer at the University of Illinois, Chicago, puts it this way: “We age because Mother Nature turns her back on us once we’re in the post-reproductive region of the lifespan. Natural selection didn’t build in a program to make us fall apart later in life.”
And yet here we are, a world with growing numbers of old people, even very, very old people. Was it “supposed” to be this way? Come to think of it, why is there even such a thing as menopause? Why do women live 30, 40, 50 years past reproduction? (There’s less of a question about old men—some can produce viable sperm until the day they die, though getting it to its proper destination can get iffy).
Perhaps, as one school of thought suggests, aging exists because old animals—especially females—provide evolutionary advantages, not to the old animal herself, but to her offspring and genetic relatives. It’s the so-called “grandmother effect.”
“If you are a human female, and you are taking care of your grandchild… your act of taking care of your grandchild is a reproductive act,” at least in the eyes of evolutionary biologists, Michael Rose, a professor of ecology and evolutionary biology at the University of California, Irvine, told me.
Actually, grandfathers are sometimes just as important as grandmothers, because they, too, share food, and sharing is evolutionarily crucial, says Harvard University evolutionary biologist Daniel Lieberman.
A hunter-gatherer mother requires enough calories to sustain not just her own body, but the bodies of her children as well, Lieberman notes, which means that hunter-gatherer females who are lactating and caring for young children struggle to get enough energy.
“But grandmothers and grandfathers are unencumbered—they can produce a surplus,” he says. “And that energy goes toward the family. As soon as humans started sharing, there was a strong selective pressure for longevity.” (Longevity, as we’ll soon see, is also fostered by “nice” environments, with lots of food around.)
Among other things, older animals are handy because they know where to find long-forgotten watering holes or food supplies. Female post-menopausal killer whales (orcas) are terrific resources for their tribe—they know where scarce salmon are and are often the ones who lead others to food when supplies are low.
Old female elephants are great resources, too. In fact, elephant societies are famously matriarchal, with older females helping their herds survive droughts, food scarcity, poachers, and, of course, lions.
Some traditional human societies with long-lived men and women show a similar pattern, as Jared Diamond writes. Jared is a professor of geography at the University of California, Los Angeles, who, for more than 50 years, has studied New Guinea farming societies.
To be sure, he says, nomadic hunter-gatherer societies aren’t always kind to old people, who may be cast out to die or simply left behind if they can’t keep up with the group’s wandering.
But in traditional sedentary societies, old folks are valuable. “In traditional societies without writing, older people are the repositories of information,” Diamond says. “It’s their knowledge that spells the difference between survival and death for their whole society in a time of crisis caused by rare events for which only the oldest people alive have had experience.”
That’s exactly what happened in 1993 when an outbreak of the hantavirus triggered a spate of deaths on the Navajo reservation in the Four Corners area of the American Southwest. The virus, originally, and unfairly, dubbed the “Navajo flu” and since renamed the Sin Nombre (No Name) virus, is carried by deer mice. When the feces of deer mice dry up and become aerosolized, humans can unknowingly breathe the contaminated dust and come down with often-fatal pulmonary infections.
When the outbreak hit, scientists from New Mexico’s health department and the federal Centers for Disease Control and Prevention flocked to the Navajo reservation to study the virus. But it was their wise decision to talk with Navajo elders that cracked the case.
The Navajo elders remembered that twice before in the 20th century, they had observed a connection between increases in rainfall, a booster crop of pinyon nuts, and a surge in the deer mouse population. In 1993, those exact conditions pertained—unusually heavy rains, lots of snowmelt, huge quantities of pinyon nuts, a surge in deer mice, and a not-new-after-all epidemic deciphered by the oldest folks around.
Clearly, then, there are some benefits for the group as a whole to have older animals or people around. But, as we’ll see in the next blog, that can’t be the whole story.
To be continued…