But in human females the reproductive system shuts down much more rapidly 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.
What distinguishes a human female from her primate cousins is not that the human biological clock ticks faster, but that mortality is so much lower in humans than in other primates, according to work done by University of Utah anthropologist Kristen Hawkes, who was not an author of this study.
This study supports that idea, the researchers say. In both humans and chimpanzees, for example, female fertility starts to decline in the late 30s and early 40s. "[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," Alberts said.
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.
"[Human children] can benefit greatly from having mothers and grandmothers who are still alive and not tied up with helpless infants," Alberts explained.
Another possibility is that mammalian eggs simply have a limited shelf life. According to th
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