Co-author Susan Alberts of Duke University and the National Evolutionary Synthesis Center, said that unlike other primates, women tend to have a long post-reproductive life.
She said that even before modern medicine, many women lived for 30 to 35 years after their last child was born.
In the study, Alberts and colleagues compared mortality and fertility data for seven species of wild primates to similar data for the !Kung people of Southern Africa, a human population of hunter-gatherers with limited access to modern medicine or birth control.
The nonhuman primate data were based on long-term observations of 700 adult females, including capuchins in Costa Rica, muriqui monkeys in Brazil, baboons and blue monkeys in Kenya, chimpanzees in Tanzania, gorillas in Rwanda and sifakas in Madagascar.
For each species, the researchers estimated the pace of reproductive decline - measured as the probability, at each age, that a female's childbirth will be her last - and compared it with the rate of decline in overall health, measured as the odds of dying with each passing birthday.
Alberts said that in this way they were able to compare the rate of aging in the reproductive system with the rate of aging in the rest of the body.
The results suggest that in nonhuman primates, reproductive decline is surpassed by declines in survival, so that very few females run out of reproductive steam before they die.
But in human females the reproductive system shuts down much more rapidly than the rest of the body.
Alberts said that 50 percent of women experience menopause by the age of 50, and fertility starts to decline about two decades before that.
In both humans and chimpanzees, for example, female fertility starts to decline in the late 30s and early 40s.
Alberts said that 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.
If evolution has given us longer lifespans than our primate cousins, why hasn't female reproduction kept pace? And in a world where individuals with more offspring tend to win the evolutionary contest, why shut down reproduction with decades of survival still ahead?
It may be that older females who forego future breeding to invest in the survival of their existing children and grandchildren gain a greater evolutionary edge than those who continue to reproduce. Once a baby chimp is weaned it can forage for itself, whereas human infants are nutritionally dependent long after they leave the breast.
Alberts explained that human kids can benefit greatly from having mothers and grandmothers who are still alive and not tied up with helpless infants.
Another possibility is that mammalian eggs simply have a limited shelf life.
The study has been published in the Proceedings of the National Academy of Sciences.
--ANI (Posted on 30-07-2013)