“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.”

Caring for their daughters’ children may have caused genetic changes that, in turn, caused older females to live longer. Those changes were eventually passed down from one generation to the next.

Computer models show that chimpanzees who reached adulthood at age 13 lived another 15 or 16 years. But humans in developed countries generally lived another 60 years or more.

Professor Hawkes believes the lengthening of lifespan happened 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.”The “grandmother hypothesis” comes from research done in the 1980s. Professor 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.

Professor Hawkes says grandmothering made humans more socially dependent on each other and, in her words, “prone to engage each other’s attention.”