Are grandmas what makes us human?

We know of just three species — humans, killer whales and short-finned pilot whales — whose females can live long beyond their reproductive years. In the 1980s, an American anthropologist named Kristen Hawkes and two colleagues came up with a theory. They had gone to northern Tanzania to study the foraging habits of the Hadza, the last known hunter-gatherers in Africa. While there, the scholars were struck by how strong the tribe's old women were and how, rather than live off the fruits of others' labor, they worked hard digging up the tribe's main starch staple, a deeply-buried tuber. "Their acquisition rates were similar to the rates of younger women," Hawkes told me, "but these old ladies were spending even more time" than their daughters gathering food, leaving camp earlier, coming back later, and bringing back more than they needed. The anthropologists also noticed that many children with grandmothers or great-aunts had faster growth rates than their counterparts. Two decades later, the grandmother hypothesis has gone from oddball conjecture to one of the dominant theories of why we live so long, breed so fast, and are so smart. The extra calories and care supplied by women in their long post-fertile period subsidized the long pre-fertile period that is childhood. And that's what made us fully human. Photo by Diego López Román via Flickr.