Human Longevity: How Your Grandparents Are the Secret to Your Long Life

According to a long-standing canon in evolutionary biology, natural selection is ruthlessly self-serving, favoring traits that improve the possibility of fertile reproduction. This frequently suggests that the so-called "force" of selection is capable of removing dangerous mutations that arise throughout childhood and the reproductive years.

However, it is believed that by the time fertility starts to diminish, selection stops caring about our physical health. After menopause, our cells are more vulnerable to harmful mutations. This frequently indicates that mortality commonly follows the cessation of fertility in the vast majority of animals.

This puts humans (as well as several whale species) in an exclusive club: beings that continue to exist long after the end of their reproductive lives. How can we tolerate decades of living in the shadow of selection?

According to Michael Gurven, an anthropology professor at the University of California Santa Barbara, "extended post-menopausal life is a conundrum from the standpoint of natural selection."

In most species, including chimpanzees, our closest living relatives among the primates, this link between fertility and lifespan is extremely obvious, with the chance of surviving declining in direct proportion to the capability to breed. In contrast, women may survive for decades in humans despite losing the ability to procreate.

We have a real post-reproductive life stage, not simply a few more years, Gurven claimed.

In a study published in the Proceedings of the National Academy of Sciences, Gurven, a senior author, and Raziel Davison, a population ecologist and former postdoctoral researcher at UCSB, disprove the notion that the power of natural selection in humans must completely disappear after sexual reproduction.

A lengthy post-reproductive lifespan, according to them, is not merely a result of recent improvements in health and medicine. According to Gurven, "the capacity for long life is a component of who we are as humans, an evolved aspect of the life process."

The key to our accomplishments? Grandparents of ours.

Ideas regarding the potential worth of senior citizens have been around for some time, according to Gurven. Our research formalizes such concepts and investigates the force of selection after accounting for the contributions of senior citizens.

The Grandmother Hypothesis, for instance, proposes that maternal grandparents might enhance their fitness by contributing to the survival of their grandchildren, allowing their daughters to have more children. It is one of the most popular theories explaining human longevity. Such fitness consequences aid in preserving the grandmother's DNA.

That is sort of an indirect reproduction, not reproduction, so to speak. For highly sociable creatures like humans, the capacity to pool resources and not simply rely on your own efforts is a game changer, according to Davison.

The researchers explain in their work that intergenerational transfers, or resource sharing between old and young, have also played a vital part in the force of selection at various ages by taking the core of that theory. The most obvious example in non-industrial society is probably sharing food.

Gurven, who has researched the economy and demographics of the Tsimané and other indigenous communities in South America, stated that it can take up to two decades from birth until people generate more food than they consume. For children to reach the stage where they can support themselves and function as productive group members, a lot of food must be obtained and distributed. Adults fulfill the majority of this requirement by being able to purchase more food than they actually require. This provisioning approach has long supported pre-industrial communities and also persists in modern societies.

The considerable excess that adults create, according to Davison's model, "helps promote the survival and fertility of near kin, as well as of other group members who dependably share their food, too." "It turns out that the indirect fitness value of adults is likewise highest among those of reproductive age when seen through the prism of food production and its repercussions. However, we discover that the surplus generated by older individuals also causes positive selection for their survival using demographic and economic data from several hunter-gatherers and horticulturists. We estimate that all of this additional fitness in later life is worth up to a few more children!

Gurven claims, "We show that elders are useful, but only to a point. "All grandmothers are not created equal. Hunter-gatherers and farmers eventually use up more resources than they produce by the middle of their seventies. Additionally, by the time they are in their mid-seventies, the majority of their grandchildren won't be dependents any longer, so there are few close relatives who may need their assistance.

Food, however, isn't everything. Children receive nutrition, but they also receive education, socialization, and training in pertinent skills and worldviews. The greatest contributions may be made by older people in this situation because, despite their smaller contributions to the food surplus, they have a lifetime's worth of skills that they can use to help parents with childcare and knowledge and training that they can impart to their grandkids.

The seniors' participation in actively assisting others with foraging, according to Gurven, "adds even greater fitness value to their activities and to their being alive." Elders not only give to the group, but their usefulness also ensures that they benefit from the surpluses, safeguards, and care provided by their community. In other words, there is reliance both from the young to the old.

"There could be some payback if you're part of my social environment," Davison said. So, insofar as we are interdependent, I have an interest in you that goes beyond basic kinship. I want you to be as proficient as you can be since some of your output could be useful to me later.

According to Gurven and Davison, our skills-intensive strategies and long-term investments in the health of the group preceded and evolved with our shift to our particular human life history, with its extended childhood and unusually long post-reproductive stage. Instead of our long lifespans creating opportunities that led to a human-like foraging economy and social behavior, the opposite is more likely.

In contrast, chimpanzees, who are the closest living relatives of humans, are capable of foraging on their own by the age of five. However, they don't create much excess and their foraging activities don't demand much expertise. The authors still demonstrate that a chimpanzee-like ancestor may still produce sufficient indirect fitness contributions to heighten selection pressure in later adulthood if they shared their food more freely.

This shows that collaboration is truly the key to human lifespan, said Gurven. Chimpanzee grandmothers are rarely seen taking care of their grandchildren, according to research.

The suggestion that we owe it to seniors worldwide is a crucial reminder going forward, even if the authors claim that the focus of their work is primarily on how the ability for long life initially emerged in the Homo lineage.

Despite the fact that there are more older people than ever before, ageism and underappreciation of older folks remain widespread, according to Gurven. "Many people shrugged their shoulders about the importance of lockdown or other serious measures because COVID looked to be most lethal just for older persons.

He said, "Much of the enormous worth of our elders goes untapped." It's time to really consider how to bridge the generations and make use of some of their knowledge and experience.

By UNIVERSITY OF CALIFORNIA - SANTA BARBARA