BERLIN - It is a scientific theory of longevity that has the whiff of homemade pie: people live longer thanks to the grandmothers who help raise them.
Though first put forth more than a half-century ago, the “grandmother hypothesis” has gotten renewed attention with a recent study published in the Proceedings of the Royal Society B: Biological Sciences by University of Utah anthropologist anthropologist Kristen Hawkes.
While other mammals like chimpanzees chase females who are no longer fertile from their midst, Hawkes notes that human women are among the very few mammals who live for decades after menopause.
In 1957, American evolutionary biologist George Christopher Williams came up with the the "grandmother hypothesis" based on the fact that a woman could have more children at shorter intervals when her mother was available to care for her grandchildren. In effect, the “grandmothers enhanced their daughters’ fertility by taking care of grandchildren,” explains Hawkes.
Her study used a computer simulation of evolution to support the grandmother hypothesis. And with grandmothers on hand to look after them until they can feed themselves, children’s survival rates are higher. Hawkes says this important role explains why women live so long.
Not everyone agrees. James Vaupel, director of the Max Planck Institute for Demographic Research in Rostock, Germany, says that studies regularly appear that appear to show such effects. But "grandmothers have a very high mortality rate, and historically not many children had a grandmother to care for them,” he points out.
Vaupel also dampens the fun of another study that appears to support the grandmother hypothesis. A month ago, Emma Foster of the University of Exeter published a study of orcas in Science. The killer whales live in social groups and are one of the very few mammals where females, like humans, undergo menopause. Orcas can live to be 90 years of age.
Orca females mainly stay with their sons and the sons’ offspring. Both sons and their offspring frequently died if the mother or grandmother did.
Is this proof that grandmothers play a role in extending our longevity? "Not at all," says Vaupel. "That study doesn’t take into account that for orcas generally there are good and bad years, depending on environmental circumstances. In bad years, more orcas in general die, so that means more orca grandmothers, mothers, sons, grandsons die too."
"Grandmothers are not the reason humans live longer," Vaupel states. Together with his colleague Oskar Burger he favors another explanation. They published a study in a recent Proceedings of the National Academy of Sciences showing that natural evolution (which the grandmother hypothesis is based on) is not what makes longer human life possible – cultural factors are.
"We have a better grip on environmental dangers, and better nutrition, medicine and care as we age," Vaupel says. In countries like Germany, over the past four generations, this means that a 72-year-old man today has the same mortality rate as a 30-year-old man 200,000 years ago.
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