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The grandmother hypothesis says that when grandmothers help feed their grandchildren after weaning, their daughters can produce more children at shorter intervals; the children become younger at weaning but older when they first can feed themselves and when they reach adulthood; and women end up with postmenopausal lifespans just like ours.
By allowing their daughters to have more children, a few ancestral females who lived long enough to become grandmothers passed their longevity genes to more descendants, who had longer adult lifespans as a result.
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.
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.
"Grandmothering was the initial step toward making us who we are," says Kristen Hawkes, a distinguished professor of anthropology at the University of Utah and senior author of the new study published Oct.
That, says Hawkes, gave 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." University of Utah.
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.
Based on earlier research, the simulation assumed that any newborn had a 5 percent chance of a gene mutation that could lead to either a shorter or a longer lifespan.
The difference in how fast the doubling occurred depends on different assumptions about how much a longer lifespan costs males: Living longer means males must put more energy and metabolism into maintaining their bodies longer, so they put less vigor into competing with other males over females during young adulthood.
The simulation tested three different degrees to which males are competitive in reproducing. The competing "hunting hypothesis" holds that as resources dried up for human ancestors in Africa, hunting became better than foraging for finding food, and that led to natural selection for bigger brains capable of learning better hunting methods and clever use of hunting weapons.