SALT LAKE CITY, Oct. 24, 2012 – Computer simulations provide new mathematical support for the "grandmother hypothesis" – a famous theory that humans evolved longer adult lifespans than apes because grandmothers helped feed their grandchildren.
The simulations indicate that with only a little bit of grandmothering – and without any assumptions about human brain size – animals with chimpanzee lifespans evolve in less than 60,000 years so they have a human lifespan.
The findings showed that from the time adulthood is reached, the simulated creatures lived another 25 years like chimps, yet after 24,000 to 60,000 years of grandmothers caring for grandchildren, the creatures who reached adulthood lived another 49 years – as do human hunter-gatherers.
But it is possible that the benefits grandmothers provide to their grandchildren might be the result of long postmenopausal lifespans that evolved for other reasons, so the new study set out to determine if grandmothering alone could result in the evolution of ape-like life histories into long postmenopausal lifespans seen in humans.
The researchers were conservative, making the grandmother effect "weak" by assuming that a woman couldn't be a grandmother until age 45 or after age 75, that she couldn't care for a child until age 2, and that she could care only for one child and that it could be any child, not just her daughter's child.
The simulation begins with only 1 percent of women living to grandmother age and able to care for grandchildren, but by the end of the 24,000 to 60,000 simulated years, the results are similar to those seen in human hunter-gatherer populations: about 43 percent of adult women are grandmothers.
The new study found that from adulthood, additional years of life doubled from 25 years to 49 years over the simulated 24,000 to 60,000 years.