A study led by Prof Kristen Hawkes at the University of Utah provides new mathematical support for the ‘grandmother hypothesis,’ a theory that humans evolved longer adult lifespans than apes because grandmothers helped feed their grandchildren.

Prof Hawkes with colleagues proposed the hypothesis in 1997. It stemmed from observations by the scientists in the 80s when they lived with Hadza hunter-gatherer people in Tanzania and watched older women spend their days collecting tubers and other foods for their grandchildren. Except for humans, all other primates and mammals collect their own food after weaning.

“Grandmothering was the initial step toward making us who we are,” said Prof Hawkes, who co-authored the study published in the Proceedings of the Royal Society B.

The new computer 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 study shows 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.

The competing ‘hunting hypothesis’ holds that as resources dried up for human ancestors in Africa, hunting became better than foraging for finding food, and that led to natural selection for bigger brains capable of learning better hunting methods and clever use of hunting weapons. Women formed ‘pair bonds’ with men who brought home meat.

Many anthropologists argue that increasing brain size in our ape-like ancestors was the major factor in humans developing lifespans different from apes. But the new computer simulation ignored brain size, hunting and pair bonding, and showed that even a weak grandmother effect can make the simulated creatures evolve from chimp-like longevity to human longevity.

“The shift to longer adult lifespan caused by grandmothering is what underlies subsequent important changes in human evolution, including increasing brain size,” Prof Hawkes said.

“If you are a chimpanzee, gorilla or orangutan baby, your mom is thinking about nothing but you. But if you are a human baby, your mom has other kids she is worrying about, and that means now there is selection on you – which was not on any other apes – to much more actively engage her: Mom! Pay attention to me!”

“Grandmothering gave us the kind of upbringing that made us more dependent on each other socially and prone to engage each other’s attention,” she said.

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Bibliographic information: Peter S. Kim et al. Increased longevity evolves from grandmothering. Proc. R. Soc. B, published online before print October 24, 2012; doi: 10.1098/rspb.2012.1751