By Alison Gopnik
Why do I exist?
This isn't a philosophical cri de coeur; it's an evolutionary riddle. At 58 I'm well past menopause, yet I'll soldier, on, with luck, for many years more. The riddle grows more vivid when you realize th
at human beings (and killer whales) are the only species where females outlive their fertility. Our closest primate relatives - chimpanzees, for example - usually die before their 50s, when they are still fertile.
This isn't just a miracle of modern medicine. Our human life expectancy is much longer than it used to be, but that's because far fewer children die in infancy. Anthropologists have looked at life spans in hunter-gatherer and forager societies, which are similar to the societies we evolved in. If you make it past childhood, you have a good chance of living into your 60s or 70s.
It turns out that my existence may actually be the key to human nature. This isn't a megalomaniacal boast, but a new biological theory: the "grandmother hypothesis." Twenty years ago, anthropologist Kristen Hawkes at the University of Utah went to study the Hadza, a forager group in Africa, thinking she would uncover the origins of hunting. But then she noticed the many wiry old women who dug roots and cooked dinners and took care of babies (much like me, though my root-digging skills are restricted to dividing the irises).
It turned out these old women, as much as the strapping young hunters, played a key role in providing nutrition for the group. What's more, the old women provided a crucial resource by taking care of their grandchildren.
There are many controversies about what happened in human evolution. But no one debates that there were two dramatic changes in what biologists call our "life-history": Besides living much longer than our primate relatives, our babies depend on adults for much longer.
Young chimps gather as much food as they eat by the time they are 7 or so. But even in forager societies, human children pull their weight only when they are teenagers.
Why would our babies be helpless for so long?
That long immaturity helps make us smart: It gives us a long protected time to grow large brains and to use those brains to learn about the world we live in. Human beings can learn to adapt to an exceptionally wide variety of environments, and those skills of learning and culture develop in the early years of life.
But that immaturity has a cost. It means that biological mothers can't keep babies going all by themselves: They need help. In forager societies, grandmothers provide a substantial amount of child care as well as nutrition. Barry Hewlett at Washington State University and his colleagues found, much to their surprise, that grandmothers even shared breast-feeding with mothers. Some grandmoms just served as big pacifiers, but some, even after menopause, could "relactate," actually producing milk. (For my part I think I'll stick to the 21st-century version of helping to feed my 5-month-old granddaughter with electric pumps, freezers and bottles.)
Hawkes's "grandmother hypothesis" proposes that grandmotherhood developed in tandem with our long childhood. In fact, she argues, the evolution of grandmothers was exactly what allowed our long childhood - and the learning and culture that go with it - to emerge. In mathematical models, you can see what happens if, at first, just a few women live past menopause and use that time to support their grandchildren (who, of course, share their genes). The "grandmother trait" can rapidly take hold and spread. And the more that grandmothers contribute, the longer the period of immaturity can be.
So on Father's Day this Sunday, as we celebrate dads across the nation over Bloody Marys and eggs Benedict, let's raise an additional toast to the gray-haired grandmoms behind the scenes.
Alison Gopnik is a psychology professor in California. Adapted from the Wall Street Journal by permission of the author.