Searching For The Evolutionary Origins Of Menopause
Susan Bowen for redOrbit.com – Your Universe Online
Here’s another difference to add to the list of things that make humans different from other primates: Human women tend to live for many years after they can no longer bear children. In a study published in the Proceedings of the National Academy of Sciences, a team of scientists compared mortality and fertility data for seven species of wild primates to the data for a modern hunter-gatherer human population.
The !Kung people of Southern Africa were chosen because they have little access to modern medicine or birth control. According to a Duke University report, “this is the first study to compare humans with multiple primate species living in the wild.”
The non-human primate data were based on the long-term observation of 700 adult females. Among the primates studies were capuchins in Costa Rica, muriqui monkeys in Brazil, baboons and blue monkeys in Kenya, chimpanzees in Tanzania, gorillas in Rwanda and sifakas in Madagascar.
“Unlike other primates, women tend to have a long post-reproductive life,” noted co-author Susan Alberts of Duke and the National Evolutionary Synthesis Center.
The study compared the pace of reproductive decline with the rate of the decline in overall health for each species. Reproductive decline was defined as the probability at each age that a female would bear her last child. Health decline was defined as the odds of dying with each passing year. “This way we were able to compare the rate of aging in the reproductive system with the rate of aging in the rest of the body,” Alberts said.
The results showed non-human primates tend to die while still able to reproduce. In human women, however, the reproductive system shuts down much earlier than the rest of the body. “Half of women experience menopause by the age of 50, and fertility starts to decline about two decades before that,” Alberts said.
Another critical difference is that humans live longer. For example, in both humans and chimpanzees, fertility can start to decline by the late 30s. “[But] even in human populations with little access to modern medicine, like the !Kung [hunter-gatherers in this study], most women survive for decades after their last child is born. Nonhuman primates rarely do that,” explained Alberts.
The precise reason why human women continue to live long beyond their reproductive years remains unclear. One theory that has gained traction in recent years is an evolutionary advantage was conferred on women who could invest their golden years in ensuring the survival of the next generation of relatives rather than continuing to give birth. Unlike other primates, human children remain more or less helpless and unable to forage for their own food long after they stop breast feeding. “[Human children] can benefit greatly from having mothers and grandmothers who are still alive and not tied up with helpless infants,” Alberts said.
Supporting this theory was a 2003 study which found among the primitive Hadza people of Tanzania, grandmothers supply enough food surplus and babysitting services to allow their daughters to successfully raise a larger number of children.
Another possible explanation is human women simply outlive their egg supply. Female primates are born with all of the eggs they are ever going to have, and as they age the quality of these eggs decline as the DNA stored inside progressively degenerates. More study needs to be done on this so-called “shelf life” theory. Killer whales seem to follow a pattern similar to humans, while elephant females sometimes give birth into their 50s or 60s, which surpasses the usual lifespan for mammalian eggs.