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).

And in terms of the variables, Hawkes erred on the side of caution by making the effect as "weak" as possible. For instance, she assumed that a female couldn't be a grandmother until the age of 45 or after 75, that she couldn't care for the child until it was two years-old, and that she could only care for one child at a time (but it did not have to be her own grandchild). The simulation also assumed that newborns had a 5% chance of a gene mutation that could lead to either a shorter or longer lifespan — a figure that was derived from earlier research.


Moreover, the model started with the assumption that only 1% of females were able to live to a grandmothering stage. But as the simulation showed, it only took 24,000 to 60,000 years for the bots to achieve a lifespan comparable to human hunter-gatherers (in which about 43% of adult women are grandmothers).

It's worth noting that, for the sake of scientific thoroughness, Hawkes also took male lifespans into account. Specifically, she had to make some assumptions about the costs imposed on males due to increased longevity. Males had to put more energy and metabolism into maintaining their now longer-lived bodies, so Hawkes suspects they put less effort into competing with other males over females during young adulthood. To this end, she tested three different degrees to which males were competitive during the reproductive phase.


Hawkes's study also excluded assumptions about brain size, learning, or pair bonds — showing that these variables were not required to produce the life-extending effect.

You can read the entire study at Proceedings of the Royal Society B.

Top image somchai rakin/shutterstock. Inset image: Lee J. Siegel, University of Utah.