Computer Simulation Shows Grandmas Made Humans Live Longer

Computer simulations provide new mathematical support for the “grandmother hypothesis” – a famous theory that humans evolved longer adult lifespans than apes because grandmothers helped feed their grandchildren.

“Grandmothering was the initial step toward making us who we are,” says Kristen Hawkes, a distinguished professor of anthropology at the University of Utah and senior author of the new study published Oct. 24 by the British journal Proceedings of the Royal Society B.

The simulations indicate that with only a little bit of grandmothering – and without any assumptions about human brain size – animals with chimpanzee lifespans evolve in less than 60,000 years so they have a human lifespan. Female chimps rarely live past child-bearing years, usually into their 30s and sometimes their 40s. Human females often live decades past their child-bearing years.

The findings showed that from the time adulthood is reached, the simulated creatures lived another 25 years like chimps, yet after 24,000 to 60,000 years of grandmothers caring for grandchildren, the creatures who reached adulthood lived another 49 years – as do human hunter-gatherers.

The grandmother hypothesis says that when grandmothers help feed their grandchildren after weaning, their daughters can produce more children at shorter intervals; the children become younger at weaning but older when they first can feed themselves and when they reach adulthood; and women end up with postmenopausal lifespans just like ours.

By allowing their daughters to have more children, a few ancestral females who lived long enough to become grandmothers passed their longevity genes to more descendants, who had longer adult lifespans as a result.