Studies of hunter-gatherers in Tanzania shed light on why humans, unlike other primates, undergo menopause.

Language, tool making and higher-order cognition all distinguish humans from their primate ancestors, but it’s possible that none of these would have evolved if not for the development of another uniquely human trait: menopause.

According to Laurence Shaw, medical director at the London Bridge Fertility, Gynecology and Genetics Center, that’s because menopause begat grandmothers, and grandmothers mean daycare and extra nutrition for demanding human babies.

Shaw researches the Hadza, a tribe of hunter-gatherers in Tanzania originally studied by University of Utah anthropology professor Kristen Hawkes. Unlike women in the developed world, Hadza women don’t live 30 or 40 years past the age of menopause. However, they do experience the decline in fertility that occurs prior to actual menopause, beginning around age 35.

This decline, Shaw argues, is the real evolutionary advantage of menopause. Women who are no longer able to have children have more time to babysit and forage for food. Both of these functions are important because at this point in her life, a Hadza woman’s daughter—who is likely around 17 years old and at the peak of her fertility—is busy giving birth to more children.

“[The grandmother] is investing in the second generation of her own line,” Shaw said. “None of the other primates—and certainly none of the great apes—show this consistency.”

All other primates instead undergo what is known as somatic wastage: They are capable of reproducing until they die.

While helpful for hunter-gatherers, this decline in fertility is a hindrance in modern life, Shaw says. Women now often delay childbearing until they are less fertile, and more women live past menopause, becoming more susceptible to health problems like osteoporosis, bowel cancer and Alzheimer’s disease.

Paradoxically, Shaw’s finding might actually prove beneficial to those wanting to have kids at an older age. Scientists already know that the process of fertility decline is controlled by genes that promote apoptosis, or programmed cell death, which causes a reduction in the number of eggs in a woman’s ovaries. Knowing this, scientists might be able to use gene therapy to allow women to conceive naturally later in life, Shaw said.
The concept of an early peak in fertility also puts the occurrence of teenage pregnancies in a new context.

“There’s a lot of stigma attached to being a teenage mother,” said Sarah Elton, an anatomy lecturer at the Hull York Medical School. But from an evolutionary point of view, it makes more sense for women to reproduce when they are the most fertile and have enough energy to care for a newborn.

“The challenge for us in our society—for healthcare provisions and social guidance—is to understand where [menopause] came from,” Shaw said. “The original work that was presented in anthropology is excellent, but a doctor hasn’t had his turn at having an interpretation of that, and that’s what I’m really doing.”

Originally published July 23, 2006

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