Grandparents, hug your grandchildren. They just may be the reason you’re here.
Research on the evolutionary roots of human aging patterns suggests that raising human young is so tough it takes both parents and grandparents to do it.
That explains the reason humans continue to live – and work – far beyond their prime reproductive years. It also explains why children’s survival rates increase as they get older and as more resources are invested in them.
The research, which builds on existing ideas of the importance of grandmothers in raising children in primitive societies, was aired Friday (Nov. 14) during the second in a series of lectures on population studies.
The series is sponsored by the School of Public Health’s Deans Office, the Provost’s Office, and the Center for Population and Development Studies.
The lecture, “Rethinking the Evolutionary Theory of Aging: Transfers, not Births, Shape Senescence in Social Species,” was delivered by Ron Lee, a professor at the University of California, Berkeley. Lee published his work in the Aug. 5 issue of The Proceedings of the National Academy of Sciences.
Lee told the audience gathered in William James Hall’s basement auditorium that he became interested in the question of why we age as we do by accident. He was preparing for a class on aging, and after reviewing research on the biology of aging, was dissatisfied with the research’s conclusions.
Fertility and aging:
Prominent theories link human life span to fertility. And indeed, people do decline in health after fertility ends, roughly in their 40s.
But fertility alone doesn’t explain why we don’t die more rapidly after we’re done having kids, Lee said.
Lee used the Pacific salmon as an extreme example of a life cycle tied tightly to fertility. The Pacific salmon returns to the streams where it was born, breeds, and then dies.
Fertility also doesn’t explain why infants die at a higher rate than older children. If fertility were the sole driving force in human mortality, changes in the death rate wouldn’t be seen until fertility kicks in during the teenage years, when humans can start having children.
For answers, Lee turned to modern hunter-gatherer bands, including the Ache in South America’s Amazon River basin, whose lifestyle most closely resembles the hunter-gatherer lifestyle that ancient humans practiced.
Lee began studying the transfer of resources from generation to generation, measuring calories transferred to and from individuals. He found that adults in these hunter-gatherer societies produce more than they consume well into old age, until they’re near death.
Children, on the other hand, consume more than they produce until they reach 20 or 21. While the fact that children can’t take care of themselves is not surprising, Lee said the amount of resources that children consume is.
Raising a child to adulthood in these hunter-gatherer bands takes three to five times the resources it takes in agricultural communities. That’s roughly the equivalent of 10 years’ worth of adult consumption.
“Children are not carrying their own weight until age 20 or 21 in these groups,” Lee said. “I was surprised at how costly that was.”
The average age of consumption for these bands is 10 to 15 years younger than the average age of production. Lee said the evidence from these transfers of resources explains human life history far better than fertility alone.
“Investments in children are very large and they continue after cessation of fertility,” Lee said. “Children are very costly…. This explains postreproductive survival because older men and women are continuing to contribute.”
Similarities to modern-day U.S.
Comparing his data on hunter-gatherers to modern U.S. populations, Lee found very similar patterns. The major difference, he said, is that in the modern United States, production from the elderly falls off steeply. Those over age 60 again become net consumers of resources rather than remaining the producers their hunter-gatherer counterparts are.
The emerging picture, Lee said, implies that there may be no natural limit on human life spans. Natural selection continues to work at older ages, though weakly, by improving the survival of grandchildren who are helped by both their parents and grandparents.
Lee described a recent meeting between scientists and the Social Security Administration on future population growth and aging. Though some scientists told administrators they thought humans today are nearing the limits of their natural life spans, Lee said he wasn’t so sure. This mechanism of intergenerational transfer may provide an evolutionary rationale for life spans to continue to lengthen.
“That may be good for us older people and for everyone who’s going to be older, but it does present some problems for Social Security,” Lee said.