The helpfulness of grandmothers is the reason humans don't die in their thirties the way chimpanzees do, new research shows - and may even explain our bigger brains.
Computer simulations show that with only a little grandmotherly care, animals with chimpanzee lifespans evolve to reach a human level in less than 60,000 years.
"Grandmothering was the initial step toward making us who we are," says Kristen Hawkes of the University of Utah.
The simulated creatures were given an initial lifespan of 25 years from reaching adulthood. But after 24,000 to 60,000 years of grandmothers caring for grandchildren, the creatures reaching adulthood lived another 49 years, just like human hunter-gatherers.
The long-standing 'grandmother hypothesis' says that when grandmothers help feed their grandchildren after weaning, their daughters can produce more children at shorter intervals. Thus, a few ancestral females who lived long enough to become grandmothers passed their longevity genes to more descendants.
Bigger brains have also been suggested as an explanation for human longevity. Crucially, though, the model made no assumption about brain size, showing that the grandmother effect was enough all on its own. Indeed, says Hawkes, it may itself explain increasing brain size.
"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 says, giving 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."
That's a lot to thank your grandma for.