UC Santa Barbara researchers report:
According to long-standing canon in evolutionary biology, natural selection is cruelly selfish, favoring traits that help promote reproductive success. This usually means that the so-called “force” of selection is well equipped to remove harmful mutations that appear during early life and throughout the reproductive years. However, by the age fertility ceases, the story goes that selection becomes blind to what happens to our bodies. After the age of menopause, our cells are more vulnerable to harmful mutations. In the vast majority of animals, this usually means that death follows shortly after fertility ends.
Which puts humans (and some species of whale) in a unique club: animals that continue to live long after their reproductive lives end. How is it that we can live decades in selection’s shadow?
“From the perspective of natural selection, long post-menopausal life is a puzzle,” said UC Santa Barbara anthropology professor Michael Gurven. In most animals, including chimpanzees—our closest primate brethren—this link between fertility and longevity is very pronounced, where survival drops in sync with the ability to reproduce. Meanwhile in humans, women can live for decades after their ability to have children ends. “We don’t just gain a few extra years—we have a true post-reproductive life stage,” Gurven said.
In a paper published in the Proceedings of the National Academy of Sciences, senior author Gurven, with former UCSB postdoctoral fellow and population ecologist Raziel Davison, challenge the longstanding view that the force of natural selection in humans must decline to zero once reproduction is complete.
They assert that a long post-reproductive lifespan is not just due to recent advancements in health and medicine. “The potential for long life is part of who we are as humans, an evolved feature of the life course,” Gurven said.
The secret to our success? Our grandparents.
“Ideas about the potential value of older adults have been floating around for awhile,” Gurven said. “Our paper formalizes those ideas, and asks what the force of selection might be once you take into account the contributions of older adults.”
For example, one of the leading ideas for human longevity is called the Grandmother Hypothesis—the idea that, through their efforts, maternal grandmothers can increase their fitness by helping improve the survival of their grandchildren, thereby enabling their daughters to have more children. Such fitness effects help ensure that the grandmother’s DNA is passed down.
“And so that’s not reproduction, but it’s sort of an indirect reproduction. The ability to pool resources, and not just rely on your own efforts, is a game changer for highly social animals like humans,” Davison said.
In their paper, the researchers take the kernel of that idea—intergenerational transfers, or resource sharing between old and young—and show that it, too, has played a fundamental role in the force of selection at different ages. Food sharing in non-industrial societies is perhaps the most obvious example.
“We show that elders are valuable, but only up to a point,” contends Gurven. “Not all grandmothers are worth their weight. By about their mid-seventies, hunter-gatherers and farmers end up soaking up more resources than they provide. Plus, by their mid-seventies, most of their grandkids won’t be dependents anymore, and so the circle of close kin who stand to benefit from their help is small.”
But food isn’t everything. Beyond getting fed, children are also taught and socialized, trained in relevant skills and worldviews. This is where older adults can make their biggest contributions: While they don’t contribute as much to the food surplus, they have the accumulation of a lifetime of skills they can deploy to ease the burden of childcare on parents, as well as knowledge and training that they can pass on to their grandchildren.
In contrast, chimpanzees—who represent our best guess as to what humans’ last common ancestor may have been like—are able to forage for themselves by age 5. However, their foraging activities require less skill, and they produce minimal surplus. Even so, the authors show that if a chimpanzee-like ancestor would share their food more widely, they could still generate enough indirect fitness contributions to increase the force of selection in later adulthood.
“What this suggests is that human longevity is really a story about cooperation,” said Gurven. “Chimpanzee grandmothers are rarely observed doing anything for their grandkids.”
Full article available at Phys.Org.
Suggesting that grandmothers have worth only in helping raise grandchildren, to the end that this results in their DNA being passed down, reveals how the evolutionary paradigm has deadened its adherents to higher values that humans have held throughout history.