Evolutionary biologists have theorized for some time now that grandmothers are responsible for our relatively long lifespans. The idea is that, by virtue of caregiving grandmothers, daughters were free to have more offspring. Consequently, selectional pressures favored proto-humans who had long lives — a trait that got passed down from generation to generation. Now, all this sounds great on paper, but it has completely lacked proof. Until now.
Postmenopausal longevity has baffled biologists for decades. Theoretically, it doesn’t make a whole lot of evolutionary sense for individuals to live beyond their child-rearing years. There must be some explanation, therefore, for why humans live way past their reproductive expiry date. This is why the so-called “grandmother hypothesis” has been a particularly exciting area of inquiry. It essentially suggests that grandmas “subsidize” their daughters’ fertility when they take care of their own grandchildren.
To prove this — or at the very least show that the theory is mathematically sound — University of Utah anthropologist Kristen Hawkes turned to the power of computer simulations. She began with the assumption that early humans had the same lifespan as chimpanzees (about 30-40 years), but then added such variables as the age at which a grandmother could start to take care of a grandchild, the resultant reproduction rate, mutation rates, and so on. After pressing the go button and allowing the process to unfold for generation upon generations, Hawkes’s simulation showed that the simulated proto-humans went from having a chimp-like lifespan to a human-like lifespan in less than 60,000 years. Specifically, her models revealed that the presence of caregiving grandmothers endowed her simulants with an additional 49 years of life after adulthood was reached (which contrasts to the 25 years found in chimps). And in fact, depending on the variables, it could have taken as little as 24,000 years.
In other words, Hawkes’s model showed that the presence of grandmothers doubled human adult-lifespan over a relatively short period of evolutionary time (essentially, a few thousand generations). Via io9