Not to sound crass—but why are so many postmenopausal women still alive? Most female mammals procreate until they die, but human women tend to live well past their reproductive years. Biologists Virpi Lummaa of the University of Sheffield in England and Mirkka Lahdenperä of the University of Turku in Finland find evidence that the added longevity may have evolved to help grandmothers protect their genetic legacy.
The researchers pored over records of roughly 2,800 Canadian and Finnish women who lived between 1700 and 1900. The data show that for every 10 years a woman lived beyond age 50, she gained two grandchildren, on average. Lummaa sees this pattern as a reflection of the long period of nurturing that human offspring require. Children who lose their mothers at an early age are evidently less likely to survive to adulthood and start healthy families of their own. Recent studies of African tribes hint that nonreproductive grandmothers aid the expansion of the family by providing labor, emotional support, and life experience to their kin. “Old women can increase the reproductive success of their adult children in many ways if they survive long enough,” Lummaa says.