According to a 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 “power” of selection is well equipped to remove harmful mutations that occur during early life and through the reproductive years. But by the age when 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 is terminated.
It puts humans (and some species of whales) in a unique club: animals that continue to live long after their reproductive life has ended. How is it that we can live for decades in the shadow of selection?
“From the perspective of natural selection, long post-menopausal life is a puzzle,” said Michael Gurven, professor of anthropology in Santa Barbara. In most animals, including chimpanzees – our closest primate brethren – this link between fertility and longevity is very clear, where survival decreases in line with the ability to reproduce. Meanwhile, in humans, women can live for decades after their ability to have children ends. “We are not just winning a few extra years – we have a true post-reproductive phase of life,” Gurven said.
In a newspaper published in the Proceedings of the National Academy of Sciencessenior author Gurven, along with former UCSB postdoctoral fellow and population ecologist Raziel Davison, challenges the long-held view that the power of natural selection in humans should drop to zero once reproduction is complete.
They claim that a long post-reproductive life is not just due to recent advances in health and medicine. “The potential for longevity is part of who we are as human beings, a developed feature of the life course,” Gurven said.
The secret of our success? Our grandparents.
“Ideas about the potential value of older adults have been floating around for a while,” Gurven said. “Our paper formalizes those ideas, and asks what the power of selection can be once you consider the contributions of older adults.”
For example, one of the leading ideas for human longevity is called the Grandmother Hypothesis – the idea that grandmothers of mothers can increase their fitness through their efforts by helping to improve the survival of their grandchildren and thus enable their daughters to to have more children. Such fitness effects help ensure that the grandmother’s DNA is transmitted.
“And so it’s not reproduction, but it’s kind 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 people, ”Davison said.
In their paper, the researchers take the core of that idea – intergenerational transfers, or resource sharing between young and old – and show that it also played a fundamental role in the power of selection at different ages. The sharing of food 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,” says Gurven, who studied the economics and demographics of the Tsimané and other Native American groups. Lots of food needs to be purchased and shared to get kids to the point where they can take care of themselves and be productive group members. Adults fill most of this need with their ability to obtain more food than they need for themselves, a supply strategy that has sustained pre-industrial societies for centuries and is also passed on to industrialized societies.
“In our model, the large surplus that adults produce also improves the survival and fertility of relatives, and of other group members who reliably share their food,” Davison said. “Seen through the lens of food production and its effects, it appears that the indirect fitness value of adults is also the highest among adults of reproductive age. But using demographic and economic data from various hunter-gatherers and horticulturists, we find that the “Surplus provided by older adults also generates positive selection for their survival. We calculate all this extra fitness in late adulthood to be worth up to a few extra children!”
“We show that elders are valuable, but only up to a point,” Gurven adds. “Not all grandmothers are worth their weight. By about their mid-seventies, hunter-gatherers and farmers eventually sucked up more resources than they provided. Moreover, most of their grandchildren would no longer be dependent by their mid-seventies, and so the circle of relatives who can benefit from their help is small. “
But food is not everything. In addition to being nurtured, children are also taught and socialized, trained in relevant skills and worldviews. This is where older adults can make their biggest contributions: Although they do not contribute as much to the food surplus, they do build up a lifetime of skills that they can deploy to alleviate the burden of childcare on parents, as well as knowledge and training that they can pass on to their grandchildren.
“If you first consider that older people are also actively involved in helping others eat, then it adds even more fitness value to their activity and that they are alive,” Gurven said. “Not only do elders contribute to the group, but their usefulness helps to ensure that they also receive some of the surpluses, protections and care of their group. In other words, interdependence runs both ways, from old to young, and young to old. “
“If you’re part of my social world, there can be a bit of a setback,” Davison explained. “So to the extent that we are interdependent, I am established in your interest, beyond just simple kinship. I am interested in getting you as skilled as possible because some of your productivity can help me along the way.”
Gurven and Davison found that rather than opening up our longevity opportunities that led to a humane food economy and social behavior, the reverse is more likely – our skill-intensive strategies and long-term investments in the group’s health preceded and developed with our shift to our particular human life history, with its extensive childhood and extraordinarily long post-reproductive stage.
In contrast, chimpanzees – who represent our best guess as to what humans’ last common ancestor may have been – may be able to eat for themselves by the age of five. However, their search activities require less skill, and they produce a minimal surplus. Nevertheless, the authors show that if a chimpanzee-like ancestor shared their food more widely, they could still generate enough indirect fitness contributions to increase the power of selection in later adulthood.
“What it suggests is that human longevity is really a story about collaboration,” Gurven said. “Chimpanzee grandmothers are rarely observed doing anything for their grandchildren.”
Although the authors say that their work is more about how the capacity for longevity first in the Homo descent, the implication that we owe it to elders everywhere, is an important reminder to look ahead.
“Despite older people being much more today than ever before in the past, there is still a lot of age and underestimation of older adults,” Gurven said. “When COVID seemed the deadliest only for older adults, many shrugged their shoulders about the urgency of locking up or other major precautions.
“Much of the great value of our elders is untapped,” he added. “It’s time to think seriously about how to reconnect the generations, and harness some of that older wisdom and expertise.”