Good grandmothers -- those who help to feed and otherwise provide loving care for their grandchildren -- are helping to increase the lifespan of all members of our species, according to new research.
The study, published in the latest Proceedings of the Royal Society B, strengthens what's known as the "grandmother hypothesis." We've been telling you about that for quite a while, such as in a story from earlier this year on humans and killer whales. Both have mothers that often live long beyond menopause.
Now, computer simulations provide new mathematical support for the hypothesis.
Kristen Hawkes, a professor of anthropology at the University of Utah, and her team created computer simulations of animals, some with grandmothers and some without. Even with just a bit of grandmothering, the simulated animals evolved longer lifespans in 60,000 years, which is a drop in the evolutionary time bucket. After reaching adulthood, the animals lived another 49 years, which is what happens in hunter-gatherer human societies.
In the 1980s, Hawkes lived with Tanzania's Hazda hunter-gatherer people. She watched how older women spent their days collecting tubers and other foods for their grandchildren. Except for humans, all other primates and mammals collect their own food after weaning. This extra nutritional boost has really helped humanity.
Grandmothers probably began to provide such care around 2 million years ago, the researchers believe. At that time, the environment where our ancestors lived changed, growing drier, with more open grasslands and fewer forests. That wasn't so good for us, since those forests included trees where human infants could collect and eat fleshy fruits on their own.
"So moms had two choices," Hawkes was quoted as saying in a press release. "They could either follow the retreating forests, where foods were available that weaned infants could collect, or continue to feed the kids after the kids are weaned. That is a problem for mothers because it means you can’t have the next kid while you are occupied with this one."
Females whose childbearing days were ending then chipped in to help dig up things like tubers and crack hard-shelled nuts, tasks that infants cannot handle.
The primates who stayed near food sources that newly weaned offspring could collect "are our great ape cousins," said Hawkes. "The ones that began to exploit resources little kids couldn’t handle opened this window for grandmothering and eventually evolved into humans."
The help these early grandmothers provided was not tied to humans having bigger brains, the computer simulations show. That's important, because it suggests how humans might have evolved big brains in the first place. Good grandmothering came first.
Hawkes and colleagues Peter Kim and James Coxworth believe the shift to longer adult lifespan caused by grandmothering "is what underlies subsequent important changes in human evolution, including increasing brain size," Hawkes said.
"If you are a chimpanzee, gorilla or orangutan baby, your mom is thinking about nothing but you," she continued. "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. That gave rise to "a whole array of social capacities that are then the foundation for the evolution of other distinctly human traits, including pair bonding, bigger brains, learning new skills and our tendency for cooperation."