Yet 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, you know, 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.”

Professor Hawkes says climate change may have affected food supplies. She says grasslands started taking the place of 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. You know, the kids couldn’t do it.”

So, if mothers decided to feed their children themselves, they would not be able to give birth as often. They would just be too busy finding food. That is where the grandmother can help.

“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 then that moms could wean earlier.”

Professor Hawkes says the act of early babysitting had long-range effects.