Importance of Grandparents: Why Do Humans Live Past Their Reproductive Years?

Aging is an inevitable deterioration of an organism's physiological function with increasing age. On the other hand, it also poses an evolutionary paradox in which natural selection designs organisms for optimal survival and reproduction. But why do humans grow old and have long lives past their reproductive years?

According to an article in Nature, scientists and philosophers, including Aristotle, have struggled to answer the said question. There were theories over the years suggesting that grandparents play a key role in longevity. A recently study put the theory to test and see the importance of the elders to the next generation.

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Importance of Grandparents: Why Do Humans Live Past Their Reproductive Years?

Three Evolutionary Explanations for Human Longevity

Evolutionary biology has taught humans that natural selection is cruel and biased, favoring traits that promote reproductive success and removing harmful mutations in early life throughout the reproductive years. However, when fertility and reproductive years are over, natural selection tends to make people more vulnerable to mutations and sickness until there is only death.

But unlike most animals that die after their reproductive years are over, humans and some species of whales continue to live long, putting them in a different club. Michael Gurven, professor of anthropology at the University of California Santa Barbara, said that this perspective of natural selection and long post-menopausal life is a puzzle.

The article in Ecology and Evolution suggests three evolutionary explanations for natural selection for human longevity, particularly female late-life survival. First, genes promote early survival and fertility that promotes long life. Second, men do not have menopause and can reproduce until their 70s, suggesting a selection for late-life survival in men. Lastly, adult offspring benefit from their mothers and survival of children increases in the presence of grandmothers.

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Grandparents are the Secret to Longevity

Researchers assert that the long life after menopause is not due to the advancements in technology in health and medicine. Instead, it is an evolved feature of the life course.

According to the news release, the paper discusses the Grandmother Hypothesis suggesting that the efforts of grandmothers can increase fitness and improve the survival of their grandchildren, letting their daughters have more children. The grandmother's DNA is passed down through indirect reproduction.

The paper also talks about intergenerational transfers or sharing of resources between young and old that has played a crucial role in natural selection at different ages. It has been observed even during the hunter-gatherer era of human civilization. Adults gather food for their young, so they could grow into useful members of the society.

Aside from providing food, grandparents make their biggest contribution by teaching children to socialize and training them with relevant skills and worldviews. They have enough experiences to ease the burden of childcare on parents. They also have knowledge and skills to pass down to their grandchildren.

Gurven concluded that their study shows human longevity as a product of cooperation. Grandmothers of other species are rarely seen taking care of their grandchildren, unlike humans. Therefore, it is time to harness the untapped huge value of elders.

The full results of the study are available in the paper titled "The Importance of Elders: Extending Hamilton's Force of Selection to Include Intergenerational Transfers," published in the Proceedings of the National Academy of Sciences.