According to the long canon of evolutionary biology, natural selection is brutally selfish, favoring traits that promote reproductive success. This usually means that the so-called “force” of selection is well equipped to remove harmful mutations that appear early in life and during the reproductive years. However, by age fertility ceases, the story goes that selection becomes blind to what is happening to our bodies. After menopause, our cells become more vulnerable to harmful mutations. In the vast majority of animals, this usually means that death occurs shortly after the end of fertility.
Which puts humans (and some species of whales) in a unique club: animals that continue to live long after their reproductive lives are over. How is it that we can live for decades in the shadow of selection?
“From a natural selection point of view, longevity after menopause is a mystery,” said UC Santa Barbara anthropology professor Michael Gurven. In most animals, including chimpanzees – our closest primate brothers – this link between fertility and longevity is very pronounced, where survival falls in sync with reproductive capacity. Meanwhile, in humans, women can live for decades after they are no longer able to have children. “We’re not just getting a few extra years — we have a true post-reproductive life stage,” Gurven said.
In an article published in Proceedings of the National Academy of Sciencessenior author Gurven, with former UCSB doctoral student and population ecologist Raziel Davison, challenge the long-held view that the strength of natural selection in humans should decline to zero after reproduction is complete.
They argue that the long post-reproductive lifespan is not only due to recent advances in health and medicine. “The potential for longevity is part of who we are as humans, an evolutionary feature of the life course,” Gurven said.
The secret to our success? Our grandparents.
“Ideas about the potential value of the elderly have been floating around for some time,” Gurven said. “Our paper formalizes these ideas and asks what the strength of selection might be when the contribution of older adults is taken into account.”
For example, one of the leading ideas about human longevity is called the grandmother hypothesis, the idea that maternal grandmothers can improve their fitness through their efforts, helping to improve the survival of their grandchildren, thereby allowing their daughters to have more children. Such fitness effects help ensure that grandmother’s DNA is inherited.
“So it’s not a reproduction, but a kind of indirect reproduction. The ability to pool resources rather than relying solely on one’s own efforts is a game-changer for highly social animals like humans,” said Davison.
In their paper, the researchers take the heart of this idea — intergenerational transmission, or the sharing of resources between old and young — and show that it also played a fundamental role in the strength of selection at different ages. Food exchange in non-industrial societies is perhaps the most obvious example.
“It takes up to two decades from birth before people produce more food than they consume,” said Gurven, who has studied the economics and demography of the Tsimane and other indigenous groups in South America. Getting children to the point where they can take care of themselves and be productive members of the group requires buying and sharing a lot of food. Adults meet most of this need by their ability to forage more food than they need for themselves, a provisioning strategy that sustained pre-industrial societies for centuries and carries over into industrialized societies as well.
“In our model, the large surplus produced by adults helps to improve the survival and fecundity of close relatives and other group members who also share their food reliably,” Davison said. “Viewed through the lens of food production and its effects, it appears that the indirect fitness of adults is also highest among adults of reproductive age. But using the demographic and economic data of many hunter-gatherers and horticulturalists, we find that the surplus provided by older adults also creates positive selection for their survival. We think all that extra fitness in late adulthood is worth a few extra kids!”
“We’re showing that elders are valuable, but only up to a point,” Gurven says. “Not all grandmothers are worth their weight. By about the mid-seventies, hunter-gatherers and farmers end up absorbing more resources than they provide. In addition, by the mid-seventies, most of their grandchildren will no longer be dependents, so the circle of close relatives who can use their help is small.’
But food is not everything. In addition to feeding, children are also taught and socialized, taught appropriate skills and worldviews. This is where older people can make their greatest contribution: although they do not contribute much to the food surplus, they have lifelong skills that they can apply to ease the burden of childcare on parents, and the knowledge and training, which they can pass on to their grandchildren.
“When you consider that elders are also actively involved in helping others forage for food, it adds even more value to their fitness and to the fact that they are alive,” Gurven said. “Elders not only contribute to the group, but their usefulness helps ensure that they also receive surplus, protection and care from their group. In other words, interdependence goes both ways, from old to young and from young to old.’
“If you’re part of my social world, there can be pushback,” Davison explained. “So to the extent that we are interdependent, I am committed to your interests beyond mere kinship. I am interested in you being as skilled as possible because some of your performance may help me down the road.”
Gurven and Davison found that our long lifespans did not open up the opportunities that led to human economies and social behavior, but rather the opposite—our skill-intensive strategies and long-term investments in group health preceded and evolved alongside our transition to our uniquely human life history. with her prolonged childhood and unusually long post-reproductive stage.
In contrast, chimpanzees, which represent our best guess at what the last common human ancestor was 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 were to share its food more widely, it could still generate enough of an indirect fitness contribution to increase the strength of selection in later adulthood.
“This shows that human longevity is really a story of cooperation,” Gurven said. “It’s rare to see chimpanzee grandmothers doing anything for their grandchildren.”
Although the authors say that their work is more about how the ability to live for the first time existed in Homo genealogy, the hint that we owe these elders around the world, is an important reminder of the future.
“Even though there are many more seniors today than at any time in the past, there is still significant ageism and underestimation of older people,” Gurven said. “When COVID seemed most deadly only to the elderly, many shrugged off the urgency of lockdowns or other serious precautions.
“Much of the enormous value of our old people remains untapped,” he added. “It is time to think seriously about how to reconnect the generations and use some of this wisdom and experience of the elders.”