If you are in a special relationship with another person, thank grandma — not just yours, but all grandmothers since humans evolved.
University of Utah anthropologist Kristen Hawkes is known for the “grandmother hypothesis”, which credits prehistoric grandmothering for our long human lifespan. Now, Hawkes has used computer simulations to link grandmothering and longevity to a surplus of older fertile men and, in turn, to the male tendency to guard a female mate from the competition and form a “pair bond” with her instead of mating with numerous partners. “It looks like grandmothering was crucial to the development of pair bonds in humans”, says Hawkes, senior author of the new study published online in the September 7 edition of the journal Proceedings of the National Academy of Sciences.
Pairs bond thanks to them
“Pair bonds are universal in human societies and distinguish us from our closest living relatives,” Hawkes and colleagues write in the study. “Our hypothesis is that human pair bonds evolved with increasing payoffs for mate guarding, which resulted from the evolution of our grandmothering life history.”
That conclusion contradicts the traditional view that pair bonding “resulted from male hunters feeding females and their offspring in exchange for paternity of those kids so the males have descendants and pass on genes,” Hawkes says. The grandma hypothesis holds that “the key to why moms can have next babies sooner is not because of dad bringing home the bacon but because of grandma helping feed the weaned children. That favoured increased longevity as longer-lived grandmothers helped more.”
The new study focuses on the resulting excess of older males competing for mates, a likely source of men’s preference for young women. “This is different than what you see in chimpanzees, where males prefer older females,” says Hawkes, a distinguished professor of anthropology and National Academy of Sciences member. As human longevity increased, there were “lots more old guys, so you have an increasing number of males in the paternity competition, and the only way you can become a father is with a fertile female, which means younger females. So males who had preference for younger females were more likely to leave descendants.”
Grandmother hypothesis
Hawkes performed the study with first author James Coxworth, a postdoctoral fellow at the University of Utah; mathematical biologist Peter Kim, a former University of Utah postdoctoral researcher now at the University of Sydney; and computer specialist John McQueen, also at University of Sydney. The grandmother hypothesis says that in the earlier days, few females lived past their fertile years. But changing environments led to the use of food like buried tubers that weaned children couldn’t dig themselves. So older females helped feed the kids, allowing their daughters to have the next baby sooner.
By allowing their daughters to have more kids, grandmothers' longevity genes became increasingly common in the population and human lifespan increased. A 2012 computer simulation study by Hawkes and colleagues supported the hypothesis, finding that without grandmothering, simulated lifespans reach equilibrium when they match those of great apes, but with grandmothering, the computed lifespans get longer like those of humans, often into the 70s or 80s.
