Are grandmothers an evolutionary necessity? The contributions of older women to society have long been debated by anthropologists. In the animal world, females often don’t live much past their reproductive years. But in our world, women live into their 80s and beyond — a fact that may be explained, in part, by evolutionary forces.

“It’s the norm in human population that women are vigorous and productive long past their fertility,’’ noted Kristen Hawkes, an anthropologist at the University of Utah. She spoke yesterday at the North American Menopause Society meeting in Dallas.

Today many women feel marginalized once they reach menopause. But research suggests that far from being a burden to societies, grandmothers have played an important role in the evolution of human longevity. Studies of modern hunter-gatherers in Tanzania, Venezuela and Eastern Paraguay — societies that offer insights into how humans evolved — consistently show that Grandma is doing much of the work.

Researchers have even measured the muscle strength of men and women in these communities and weighed the baskets and bundles carted around by them. Often, the scientists find, women in their 60s are as strong as women in their 20s. “It’s the women over 40 who are carrying the heavy loads,’’ said Dr. Hawkes.

The research is the basis for the grandmother hypothesis that may help explain why menopause occurs. The basic idea is that an end to a woman’s reproductive years allows her to channel her energy and resources into caring for her children and grandchildren, thereby providing her descendants with a survival advantage.

Until recently, many researchers argued that menopause isn’t natural and that modern medicines simply have increased life expectancy well beyond what nature intended. But while it’s true that the average life expectancy for women was just 40 years only a century ago, recent studies have found the number was skewed by high infant mortality rates at the time. Plenty of women were living well past age 40, Dr. Hawkes said. Even the Bible recognized that women can live well beyond their fertile years, NAMS executive director Dr. Wulf Utian noted.

In hunter-gatherer cultures today, said Dr. Hawkes, “women are strong and economically productive into their 60s….Women are not being helped along by others. The flow of help is going into the other direction.”

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