According to 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 “force” of selection is well equipped to remove harmful mutations that appear during early life and throughout the reproductive years. 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 harmful 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?
“From the perspective of natural selection, long post-menopausal life is a puzzle,” said UC Santa Barbara anthropology professor Michael Gurven. 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. “We don’t just gain a few extra years — we have a true post-reproductive life stage,” Gurven said.
In a paper published in the Proceedings of the National Academy of Sciences, senior author Gurven, with former UCSB postdoctoral fellow 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.
They assert that a long post-reproductive lifespan is not just due to recent advancements in health and medicine. “The potential for long life is part of who we are as humans, an evolved feature of the life course,” Gurven said.
The secret to our success? Our grandparents.
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