Evolutionary oddity: A helping hand from grandmothers may explain why women cease to be fertile so long before the end of their lives.
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SYDNEY: A grandmother's role is more than just spoiling grandkids. Because women go through menopause and can no longer have their own children, they are free to help their grandkids survive, evolutionary biologists say.
Menopause, the natural cessation of female reproductive functioning, is a unique human phenomenon that continues to mystify experts. While males produce sperm continually through their lives, females produce eggs only during a short window of time. They begin releasing eggs when puberty and menstruation kick in at around age 13.
Gradually the number and quality of eggs decreases until menopause hits at around age 50. Other female animals stop producing eggs, but much closer to the end of their lives than us.
Human peculiarity
There are two prevailing theories of why women's reproductive ability stops ahead of time. The first holds that menopause prevents a woman from having children later in her life when she is less able to take care of offspring that need her protection and nurturing for many years. The second proposes that menopause gives a woman freedom to help take care of her daughter's offspring, ensuring her genes survive in the long term.
To determine the evolutionary significance of menopause, Daryl Shanley from the University of Newcastle in England, led a team that developed a computer model using historical demographic data from 5,500 people from two villages in Gambia. The model, detailed this week in the British journal, the Proceedings of the Royal Society B, used the data to project the nature of the population if women reached different ages of menopause.
The team's figures on population growth under various scenarios hinted that a woman rarely undergoes menopause after 50 because she is at greater risk of dying in childbirth and consequently unable to provide care. The study also showed that toddlers' chances of survival are better with a maternal grandmother around. In other words, both the mother and the grandmother's input on child survival are factors in the evolution of menopause.
"Rich insight"
"The Gambian data set and the information it reveals following careful analysis provides a uniquely rich insight into human life history in a natural mortality natural fertility setting," said Shanley. However, "we clearly show that the most important benefit of menopause to a population is by freeing up potential grandmothers to provide care and not by protecting children from the death of their own mothers."
Human babies need much more parental input than almost any other species because they take a long time to mature and become self-sufficient. Because of this dependency, human babies need extra care and the prime candidate for care giving is the grandmother because she has the "greatest assured genetic interest," the authors said.
Previous studies have shown that the age at which women reach menopause has increased with increasing life expectancy. Genetics also plays a role, supporting the notion that natural selection has favoured a push towards later menopause that coincides with the age at which her daughter has children.
"The [authors] are really adding to the plausibility of these theories," commented Robert Attenborough, a biological anthropologist at the Australian National University in Canberra. This study is particularly interesting, he said, because it looks at one of the unique and unusual qualities of being human and asks how it evolved.
Menopause: blame it on evolution
Shakey grounds with regard to theories, I see no conclusive results ever appearing.
Are human life expectancies in the distant past beyond 50?
Is menopause strictly a reproductive change?
Could it also occur in the male body?