According to a long-standing canon in evolutionary biology, natural selection is cruelly selfish, promoting traits that promote reproductive success. This usually means that the so-called “power” of selection is well equipped to remove deleterious mutations that occur during early life and throughout the reproductive years. But by the age that fertility stops, the story goes that selection becomes blind to what happens to our bodies. After menopause, our cells are more vulnerable to harmful mutations. In the vast majority of animals, this usually means death follows shortly after fertility has ended.
That puts humans (and some species of whales) in a unique club: animals that continue to live long after their reproductive lives have ended. How come we can live in the shadow of selection for decades?
“From the perspective of natural selection, longevity after menopause is a puzzle,” said Michael Gurven, a professor of anthropology at UC Santa Barbara. In most animals, including chimpanzees – our closest primate brethren – this link between fertility and longevity is very pronounced, where survival is synchronized with the ability to reproduce. Meanwhile, in humans, women can live for decades after their ability to have children ends. “We don’t just get a few extra years — we have a real post-reproductive life stage,” Gurven said.
In a paper published in the Proceedings of the National Academy of Sciencessenior author Gurven, with former UCSB postdoctoral fellow and population ecologist Raziel Davison, challenge the long-held view that the power of natural selection in humans should drop to zero once reproduction is complete.
They argue that a long post-reproductive lifespan isn’t just due to recent advances in health and medicine. “The potential for longevity is part of who we are as humans, an evolved feature of the life course,” Gurven said.
The secret of our success? Our grandparents.
“Ideas about the potential value of older adults been floating around for a while,” Gurven said. “Our paper formalizes those ideas and asks what the power of selection might be when 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 maternal grandmothers through their efforts can increase their fitness by helping to improve the survival of their grandchildren, allowing their daughters to have more children. Such fitness effects ensure that the grandmother’s DNA is passed on.
“And so that’s not reproduction, but a kind of indirect reproduction. The ability to pool resources, and not just rely on your own efforts, is a game changer for very social animals like people,” Davison said.
In their paper, the researchers take the gist of that idea — intergenerational transfers, or the sharing of resources between old and young — and show that it has also played a fundamental role in the power of selection at different ages. Sharing food in non-industrial societies is perhaps the most obvious example.
“It takes up to two decades from birth for people to produce more food than they consume,” said Gurven, who has studied the economics and demographics of the Tsimane and other indigenous groups in South America. A lot of food must be purchased and shared in order for children to be able to fend for themselves and be productive group members. Adults fulfill most of this need with their ability to obtain more food than they need, a supply strategy that has perpetuated pre-industrial societies for centuries and is also spilling over into industrialized societies.
“In our model, the large surplus that adults produce helps improve survival and fertility in close relatives, as well as other group members who share their food reliably,” Davison said. “Viewed through the lens of food production and its effects, it appears that the indirect fitness value of adults is also highest in adults of reproductive age. But using demographic and economic data from multiple hunter-gatherers and horticulturists, we find that the excess of older adults also generates positive selection for their survival. We’re calculating all this extra fitness in late adulthood to be worth a few extra kids!”
“We show that elders are valuable, but only up to a point,” says Gurven. “Not all grannies are worth their weight. They are around 70 hunter-gatherers and farmers end up taking in more resources than they supply. In addition, most of their grandchildren will no longer be dependent in their mid-seventies, so the circle of close relatives who would benefit from their help is small.”
But food isn’t everything. In addition to being fed, children are also taught and socialized, trained in relevant skills and worldviews. This is where older adults can make their biggest contribution: While they don’t contribute as much to the food surplus, they have the accumulation of lifelong skills they can use to ease the burden of childcare for parents, as well as knowledge and education they can. pass it on to their grandchildren.
“Once you take into account that elders are also actively involved in helping others forage, it adds even more fitness value to their activity and to life,” Gurven said. “Not only do the elderly contribute to the group, but their usefulness helps ensure that they also receive from their group’s surpluses, protection and care. In other words, interdependence comes from two sides, from old to young and from young to old.”
“If you’re part of my social world, there might be some kickbacks,” Davison explained. “So to the extent that we depend on each other, I’m grounded in your interest, more than just a simple relationship. I’m interested in making you as capable as possible, because some of your productivity can get me started.”
Gurven and Davison found that rather than our longevity providing opportunities that led to a human-like foraging economy and social behavior, the reverse is more likely: our skill-intensive strategies and long-term investments in group health predated and evolved with our shift to our specific human life history, with its extended childhood and unusually long post-reproductive stage.
In contrast, chimpanzees — representing our best guess as to what humans’ last common ancestor looked like — are able to forage for themselves by the age of 5 years. However, their foraging activities require less skill and produce minimal surplus. Still, the authors show that if a chimpanzee-like ancestor shared their food more widely, they could still generate enough indirect fitness contributions to increase selection power at later adulthood.
“What this suggests is that human longevity is really a story of cooperation,” Gurven said. “Chimpanzee grandmothers are rarely seen doing anything for their grandchildren.”
While the authors say their work is more about how longevity first emerged in the Gay lineage, the implication that we owe it to the elderly everywhere is an important reminder of the future.
“Despite the fact that the elderly are much more numerous today than ever before, there is still a lot of ageism and undervaluation of older adults,” Gurven said. “When COVID seemed deadliest only to older adults, many shrugged the urgency of lockdown or other important precautions.
“Much of the tremendous value of our elders remains untapped,” he added. “It’s time to think seriously about how we can reconnect the generations and harness some of that older wisdom and expertise.”
Raziel Davison et al, The Importance of Elders: Expanding Hamilton’s Selective Power with Intergenerational Transmissions, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2200073119
University of California – Santa Barbara
Quote: Researchers claim that human longevity is in part due to contributions from the elderly (2022, July 7) retrieved July 7, 2022 from https://phys.org/news/2022-07-human-lifespan-due -contributions-elsewhere .html
This document is copyrighted. Other than fair dealing for personal study or research, nothing may be reproduced without written permission. The content is provided for informational purposes only.