Natural selection favors certain characteristics that help promote reproductive success according to the canon in evolutionary biology. This usually means that the force of selection has the ability to remove harmful changes that occur during early life and throughout the reproductive years. The story says that selection becomes blind to what happens to our bodies when fertility ceases. Our cells are more at risk after menopause. The majority of animals will die shortly after fertility ends.
Humans and some species of whales are in a unique club of animals that live long after their reproductive lives are over. How can we live a long time in the shadows?
Michael Gurven is an anthropology professor at UC Santa Barbara. Chimpanzees, our closest primate brethren, have a link between fertility and lifespan that is very pronounced. Women can live for a long time after having children. We have a post-reproductive life stage and we don't just gain a few extra years.
In a paper published in the Proceedings of the National Academy of Sciences, senior author Gurven, with former UCSB graduate student and population ecologist Raziel Davison, challenge the longstanding view that the force of natural selection in humans must decline to zero once reproduction is complete.
Recent improvements in health and medicine are not the sole reason for a long lifespan. Gurven said that long life is part of who we are as humans.
There is a secret to our success. Our grandpas.
Gurven said that ideas about the potential value of older adults have been floating around for a long time. The paper asks what the force of selection might be once you consider the contributions of older adults.
One of the leading ideas for human longevity is called the Grandmother Hypothesis, which states that maternal grandmothers can increase their fitness by helping improve the survival of their granddaughters, which in turn will enable their daughters to have more children. The grandmother's genetic material is passed down through fitness effects.
That is not reproduction, but it is an indirect reproduction. The ability to pool resources, and not just rely on your own efforts, is a game-changer for highly social animals.
In their paper, the researchers show that resource sharing between old and young has a major role to play in the force of selection. Sharing of food is an example of non-industrial societies.
According to Gurven, it takes up to two decades before people produce more food than they consume. A lot of food needs to be shared to get kids to the point where they can fend for themselves. Adults fill most of the need with their ability to get more food than they need for themselves, a strategy that has lasted pre-industrial societies for a long time.
The survival and fertility of close kin and other group members are improved by the large surplus that adults produce. The indirect fitness value of adults is higher than that of reproductive-aged adults. Using demographic and economic data from multiple hunter-gatherers and horticulturists, we found that the surplus provided by older adults generated positive selection for their survival. It's worth up to a few extra kids because of the extra fitness in late adulthood.
Gurven claims that elders are valuable but only up to a point. Some grandmas are not worth their weight. Hunter-gatherers and farmers end up taking in more resources than they give by the late 70s. The circle of close kin who will benefit from their help is small because most of their grandkids will be dependents by the mid-seventies.
Food isn't the only thing. Children are taught and socialized in a variety of ways. While they don't contribute as much to the food surplus, older adults have a lifetime of skills they can use to ease the burden on parents, as well as knowledge and training that they can pass on to their children.
"If you take into account that elders are also actively involved in helping others, then it adds even more fitness value to their activity and to them being alive." Elder usefulness helps ensure that they receive from the surpluses, protections and care from their group. Interdependence runs from old to young and young to old.
There could be some kickback if you're part of my social world. To the extent that we are interdependent, I am vested in your interest. I want you to be skilled so that you can help me down the road.
Our skills- intensive strategies and long-term investments in the health of the group preceded and evolved with our shift to our particular situation, according to Gurven and Davison.
Chimpanzees are able to find their own food by age 5. They have less skill and produce less surplus. The authors show that if a chimp-like ancestor shared their food more widely, they could still generate enough fitness contributions to increase the force of selection.
Human longevity is a story about cooperation, according to Gurven. Chimpanzee grandmothers are rarely seen doing anything for their children.
The implication that we owe it to elders everywhere is an important reminder, even though the authors say their work is more about how long life came to first exist in the Homo line.
"Despite elders being more numerous today than ever before, there's still much ageism and underappreciation of older adults." Many shrugged their shoulders when it became clear that COVID was most deadly for elderly people.
He said that much of the value of our elders goes undiscovered. It's time to think seriously about how to connect the generations.
There is a story
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