"So moms had two choices," Hawkes said. "They could either follow the retreating forests, where foods were available that weaned infants could collect, or continue to feed the kids after the kids are weaned. That is a problem for mothers because it means you can't have the next kid while you are occupied with this one."
Other women were able to step in and help, showing a benefit to grandmothering, also allowing the population to continue to grow and flourish.
"My grandparents died before I was born, so it took me becoming a mom to know about the influence of the backup team known as grandparents and power aunts," said Cori Connors, a Farmington resident, mother, grandmother and musician.
Connors said her mother would pay the kids to learn and recite poetry, which encouraged them to become poets, songwriters, musicians and patrons of the arts "because she was astute enough to bribe them."
She said she misses her mother's influence.
The computer simulations involved in the new University of Utah study help to support the grandmother hypothesis, measuring a change in adult longevity over time.
Chimps that reach adulthood (age 13) live an average of another 15 or 16 years, without help from a grandmothering figure. People in developed nations who reach adulthood (about age 19) live an average of another 60 years or so, to the late 70s or early 80s.
The simulation begins with only 1 percent of women living to grandmother age and able to care for grandchildren, but over 24,000 to 60,000 simulated years, about 43 percent of adult women are grandmothers, according to the study.
From adulthood, additional years of life doubled from 25 years to 49 years from the beginning to the end of the simulated length of time.
"Grandmothering gave us the kind of upbringing that made us more dependent on each other socially and prone to engage each other's attention," Hawkes said.
The idea 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," she said.
"It's the best job in the whole world," Nevins said. "My mom used to say, 'I love you kids, but, oh my goodness, the love I have for my grandkids is unexplainable,' and now I know how she feels."
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