"We went there not to study grandmothers, but to learn how do you pull it off, live off wild foods," Hawkes said. "The amazing productivity of these old ladies emerged out of the data."
The postmenopausal women were actively gathering food and caring for children. Hawkes wondered if "grandmothering" could explain why humans often live for decades after their offspring grow up, while chimpanzees, orangutans and gorillas become frail after menopause and rarely reach the age of 50.
"That's biggest difference between us and the other apes," Hawkes said.
While humans evolved in East Africa, according to her now-famous hypothesis, grandmothers began playing a more active role in caring for children. This allowed mothers to wean babies fast and have another child sooner, thereby increasing human fertility. It also lent a selective advantage to genetic traits that allowed women to live beyond their child-bearing years.
The theory sounds great on paper, but there was not much statistical string to tie it together, critics pointed out. Maybe human women were living longer for reasons that had nothing to do with child care, and they took it up to stay busy. Critics also argued that the surviving postmenopausal women may actually have hurt their grandchildren's chances by consuming resources that would have fed them.
This week, Hawkes answered with mathematical data published in the British journal Proceedings of the Royal Society B. The study was conducted with mathematical biologist Peter Kim, a former U. postdoctoral researcher now on the University of Sydney math faculty, and James Coxworth, a U. doctoral student in anthropology.
The team devised a computer simulation to investigate how a chimpanzee's life span would evolve in the presence of grandmothering.
The simulation allowed for a weak grandmother effect, according to Hawkes. Only females over the age of 45 could care for children and they could care for only one at a time, while the child had to be at least 2 years old and was not necessarily the caregiver's grandchild.
Even with restricting grandmothers' roles, the simulated results indicated that in just 60,000 years or 2,400 generations, which is not that long on an evolutionary scale this creature's life span would reach human-like dimensions. It grew from 25 years beyond adulthood to 49 years.
"The longer the life span the more years there were to grandmother, the greater advantage to mothers who were longer lived. Longevity was favoring itself," Hawkes said. What drove ancestral humans to look after grandchildren, Hawkes believes, was East Africa's changing landscape, which was becoming less forested as the climate dried out and opened up grasslands.
"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."
The primate species that stayed near food sources that newly weaned offspring could gather and eat "are our great ape cousins," Hawkes said. "The ones that began to exploit resources little kids couldn't handle [such as wild game and hard-shelled nuts, as opposed to fruits], opened this window for grandmothering and eventually evolved into humans."
The National Science Foundation and the Australian Research Council funded the study.