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Menopause evolution's way to end moms-in-law reign

5 April, 2008

A new study has found that menopause, which marks the end of menstruation, has an “evolutionary” significance.

According to two British scientists, menopause is “the evolutionary way of solving the age-old tensions between women and their dominant mothers-in-law.” In their study, published in the Proceedings of the National Academy of Sciences, Dr Michael Cant of Exeter University and Dr Rufus Johnstone of Cambridge University, have put forward the new theory of how evolution shaped the fertility of women. They also believe that the new finding could give clues to the genetic basis of premature menopause and other diseases of low fertility.

Most women lose the ability to conceive at about age 50. It has puzzled scientists why, after carrying out their most important evolutionary function, they not only go on to live much longer but outlive men, too.

The most popular theory, according to an article in Telegraph, is that “menopause is nature’s way of saying it is time to become a grandmother, so women can look after grandchildren in their fifties and sixties, giving their hard pressed daughters a break from motherhood to allow them to become pregnant again.”

In tight-knit ancient communities, there was often food to feed only one woman’s offspring, and the daughters-in-law won the fight for resources.

Dr Michael Cant and Dr Rufus Johnstone offer a new answer to this problem, based on the realization that our ancestors lived in small groups and members of those groups had to compete for food and other resources, which is critical for raising the young.

The Telegraph quoted Dr Michael Cant as saying, "The timing of reproductive cessation in humans is best understood as an adaptation to minimize reproductive competition between generations of females in the same family unit. Simply put, it is in the interest of mothers – and their genes – to stop having children when younger family members start to reproduce.”

"Humans,” according to Dr Cant, “are unique among primates because there is almost no overlap of reproductive generations. In natural fertility populations, women on average have their first baby at 19 years, and their last baby at 38 years; in other words, women stop breeding when the next generation starts to breed.”

The two British researchers argue that there is a “mathematical logic” to the menopause. Dr Cant says: “The point of life is to pass genes down the generations.

Looked at this way, it is in the interest to mothers in law to give their daughters-in-law a clear berth, particularly in simpler societies where extended families lived in one place. An older female does best to avoid competing because she is related to her daughter-in-law’s offspring (and therefore share’s an interest in her reproductive success), but not vice versa.”

“Despite vast differences in wealth, resources and access to medicine,” the researchers wrote in their study, “women in all societies experience menopause. This suggests that the human fertility schedule is hard-wired into our genetic makeup as a consequence of our evolutionary history, prior to more recent cultural and technological advances.”

The study, they added, also helps to explain why in some societies, particularly in Africa and Asia, women are required by social law to stop having children when their first grandchild is born.




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