In modern society, grandmothers are often called upon to babysit. But a few million years ago, when primate grandmothers first started doing that, they apparently had a major impact on human evolution. Scientists believe it’s a big reason why we live much longer than other primates. It’s called the “grandmother hypothesis.”

University of Utah Anthropology Professor Kristen Hawkes says humans are distinct among primates when it comes to longevity.

“One of the things that’s really different about us humans, compared to our closest living relatives, the other great apes, is that we have these really long lifespans. We reach adulthood later and then we have much longer adult lives. And an especially important thing about that is that women usually live through the childbearing years and are healthy and productive well beyond,” she said.

Other primates are not as lucky.

“In other great apes, females, if they make it to adulthood, they usually die in their childbearing years and they get to be old, frail and gray and less able to do all the things that we associate with getting old. Well, of course, it happens to all of us, but it happens slower and later to us compared to the other great apes,” she said.

Hawkes said climate change may have set things in motion by affecting food supplies. Savannahs started replacing forests in Africa.

“One of the things it did was restrict the availability of the kinds of things that little kids, little apes, can feed themselves on. So that meant that ancestral moms had two choices. They could either follow the retreating forests, or if they stayed in those environments, then they just would have to feed their kids themselves. The kids couldn’t do it,” she said.

So, if mothers decided to feed their offspring themselves they would not be able to give birth as often. They’d just be too busy finding food. Here’s where granny primate steps in to help.

She said, “It would also mean that older females, whose fertility was coming to an end, could now make a big difference in their fitness by helping their daughters feed those grandchildren. And that would mean that moms could wean earlier.”

The act of early babysitting had long-range effects.

“That whole array of changes could account for why we have longer adult lifespans. We age more slowly. We mature later. Our kids are actually dependent longer, but we wean them earlier than the other apes do. And that hypothesis has been on the table for a while,” said Hawkes.

The caring for their daughters’ offspring may have triggered genetic changes that allowed older females to live longer. Those changes were eventually passed down. Computer simulations show that chimps, who reached adulthood at age 13, lived another 15 or 16 years. But humans in developed countries, who reached adulthood at 19, generally lived another 60 years or more.

Hawks and her colleagues believe the lengthening of the lifespan happened pretty quickly in scientific terms – between 24,000 and 60,000 years.

“This combination of grandmothering and increased longevity go together. When there’s grandmothering that makes more grandmothers. And it makes longevity increase from an apelike range into a humanlike range,” she said.

The “grandmother hypothesis” had its roots in research done in the 1980s. Hawkes and anthropologist James O’Connell lived among the hunter-gatherer Hadza people in Tanzania. Older women in that community spent their day gathering food for their grandchildren.

Hawkes says grandmothering made us more socially dependent on each other and “prone to engage each other’s attention.”