A Discussion of the Grandmother Hypothesis
We humans are longer-lived than our near primate cousins, and this is a comparatively recent development in evolutionary time. It is thought that this longevity arises from our greater intelligence and culture. When grandparents can contribute to the reproductive success of grandchildren, there is a selection pressure favoring mechanisms that allow for individual survival to older ages than would otherwise be the case. Chimpanzees do not have sufficient intelligence and culture for grandparents to greatly influence the success of their grandchildren, natural selection does not operate as strongly on the shape of later life, and thus chimpanzees are not as long-lived as humans.
According to long-standing canon in evolutionary biology, natural selection is cruelly selfish, favoring traits that help promote reproductive success. 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 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? 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.
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. In a new paper, researchers take the kernel of that idea - intergenerational transfers, or resource sharing between old and young - and show that it 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.
"In our model, the large surplus that adults produce helps improve the survival and fertility of close kin, and of other group members who reliably share their food, too. Viewed through the lens of food production and its effects, it turns out that the indirect fitness value of adults is also highest among reproductive-aged adults. But using demographic and economic data from multiple hunter-gatherers and horticulturalists, we find that the surplus provided by older adults also generates positive selection for their survival. Once you take into account that elders are also actively involved in helping others forage, then it adds even more fitness value to their activity and to them being alive."
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. Chimpanzee grandmothers are rarely observed doing anything for their grandkids."
Link: https://www.news.ucsb.edu/2022/020677/importance-elders