The Grandmother Hypothesis says our mothers' mothers are why we live so long.
In the 1980s, Kristen Hawkes and James O'Connell spent time with Hadza hunter-gatherers. They noticed that the older women in the society spent their days collecting tubers and other food for their grandchildren. That was the proverbial fallen apple that sparked Hawkes' interest in the Grandmother Theory, which says that humans evolved to live so long because grandmothers were around to help take care of the young'uns.
The Grandmother Hypothesis goes further than to establish the importance of grandmas. In our early years as a species, the theory goes, older women helped gather food for their offsprings' offspring. In so doing, they were freeing up their daughters to have more children, more quickly. So the most evolutionarily fit grandmothers have the most grandchildren, to whom they pass on their longevity-promoting genes.
The theory's been surprisingly controversial, given its "aww" factor: who wouldn't want another reason to celebrate grandmas? Hawkes, who has been exploring this theory for years, developed a new mathematical model in response to a 2010 study that appeared to disprove it. Beginning with a population that lived for about as long as apes typically do, she modeled evolution over time and showed that "even weak grandmothering drives the evolution of longevity from an ape-like value into the human range." All other female primates fail to live much past their childbearing years. But she showed that after less than 60,000 years, and with "only a little bit of grandmothering," the human lifespan doubled.
Hawkes then takes this even further, suggesting that when certain apes started pursuing more complex resources -- for example, by developing tools for hunting -- grandmothers came about in order to ensure that small children weren't left behind. With the kids provided for, natural selection was free to favor those with larger brains, thus paving the way for those apes to evolve into humans. And grandmothers' style of upbringing, with its emphasis on social dependence, 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." Grandmothers, Hawkes says, are what make us human.
In the interest of being as cautious as possible, the model doesn't account for brain size or the role of men, and doesn't even assume that women would only assist their own grandchildren. Even so, it shows how eventually grandmothers grew from representing one percent of female caregivers to 43 percent -- thus achieving "grandmothering equilibrium."
Although we now credit public health measures for the fact that our lifespans continue to increase, grandmotherly involvement remains the norm in plenty of modern societies. Think of it as a reminder that they may have been the ones to give us the initial boost.
Incidentally, the Grandmother Effect also appears to occur in whales.