Proposal: The menopause evolved to ensure that older women were able to help in bringing up their grandchildren. -- New Scientist. December 2008.
Richard Alleyne, Science Correspondent, Daily Telegraph, London, 11 Dec 2008
Humans are the only primate to go through the change of life in which the menstrual cycle ends. It has puzzled scientists why it developed when so many other animals continue breeding into old age. Now a team from Exeter and Cambridge Universities believe they may have come up with the answer. It is all to do with the daughter-in-law.
They suggest that the menopause is the evolutionary result of competition between women and their daughters-in law from thousands of years ago.
Unlike other species the team discovered that in the past it was young females that moved from one social group to another rather than males. Faced with a newcomer, the older women had to choose between having more children or helping to raise their grandchildren. Over the years it was decided it was easier and they had more to gain by helping with the care of the younger generation's children rather than having more of their own.
Dr Michael Cant, of Exeter University, and Dr Rufus Johnstone, of Cambridge University, said evolution shaped the fertility of women. They believe their new finding, borne out by studies of DNA, 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 50, although they not only go on to live much longer but also outlive men. Dr Cant said: "A system in which females become more related to their group as they age has this predicted outcome." In New Scientist he calls the menopause the "ghost of reproductive competitions past".
Dr Cant's work as a zoologist made him mindful that in societies where females cooperate over child-rearing – where food is shared and non-mothers step in to assist in the care of young – one considers the costs and benefits to all the females, because all of them will be competing for the same scarce resources. He said: "Reproductive competition has been overlooked in human studies. It might give us insight into why women stop breeding when they do."
The first clue came when Dr Cant and Dr Johnstone mapped the overlap in reproductive years between human generations. Dr Cant said: "It was such a striking pattern. Although human mothers may survive for the majority of their daughters' lifespan, they will continue to reproduce for at most a small fraction of their daughters' reproductive span."
The researchers were convinced this lack of reproductive overlap in humans evolved to minimise reproductive conflict between the generations. So they began to study patterns of human dispersal, which determines the relationships between competing females within a social unit. In most mammals, young males leave and join another group while young females stay with their mother.
According to Dr. Cant and co-authors, lines of evidence including studies of modern hunter-gatherers suggest that in our ancestors it was overwhelmingly the females who moved.