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.
Source: The Atlantic. Read full article. (link)