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ORIGINS OF HUMAN DEVELOPMENT, 1: THE MENOPAUSE
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.
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.
Replies to This Discussion
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Permalink Reply by Daniel W on -
Before addressing why menopause developed, I have to ask when in human evolution most women survived to menopause? What happens to the hypothesis if most women (and men) did not live into their 40s during past eons of history? What does the evidence say? If most women did not live that long, then the point about roles of menopausal women in society is more difficult to support.
A number of the women who I work with tell me that menopause (at least the hot flashes) developed in order to cut down their heating bills. -
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Permalink Reply by Dawn K on -
About the heating bills ;) If only they didn't come and go the hot flashes might just be useful.
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Permalink Reply by Dawn K on
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It seems like this is a very important question to the hypotheses which otherwise seems to make some sense.
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Permalink Reply by D. E. Hall on -
This is a very interesting line of inquiry. I also asked Daniel's question as I read this, but I do think that there is an answer, we just don't know it yet. Is there any way of knowing if menopause is getting pushed later in life as old age becomes the norm? If that is true, even partially, how then does the 'overlap' or lack thereof change or not (At what point to women start to become fertile and has it changed as menopause has or am I missing something fundamental)? At what point in time are these studies taking place?
Still, my grasp of science is limited compared to that of a great deal of this group. I hope my questions don't sound stupid. -
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Permalink Reply by Daniel W on
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D. E. - your point about change of age at menarche and menopause is valid and insightful. You are not missing something fundamental, you are adding something fundamental. I don't know that there is any way to know when menopause occurred in past history. It would be critical to this hypothesis, to know that. There are very few people who would be expert at this.
We might think about other effects of aging. For example, hair goes grey, men become bald (some of us), skin becomes less supple and wrinkles develop, we get arthritis, we become less strong, and other changes. Are these changes beneficial? Is there a reason to single out menopause?
Wikipedia has some interesting reports on menarche, menopause, and the grandmother hyothesis. Going beyond wikipedia to the primary references would be interesting (although i don't have the time to do that).
I admit to a bias on this - sometimes people come up with a hypothesis to explain something that fits into their own peconceptions, without looking into the underlying principles. In our society, where most of us live far beyond reproductive age, it would be validating to say that there is an evolutionary advantage to living beyond reproductive age. Similar arguments could be made for the question of what is the evolutionary advantage to homosexual behavior (which at least exists in many other species). I don't think that we need to find evolutionary advantage to say that older or otherwise nonreproducing members of our society have vitally important roles. But that is moving to a different discussion.
By the way, your questions don't sound stupid. They are thought provoking and interesting. -
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Permalink Reply by mcs on -
I'm not remotely an expert on the topic, but it piqued my curiosity.
I just came across this article from 2005, which states that gorillas experience menopause, as well. This is seems to be in conflict with the original article which states it's exclusive to humans. I have no idea how reputable this source it, though.
The wikipedia article of the subject also states menopause is not exclusive to humans, but has been observed in other species, and has not been thoroughly researched in other animals. So my confidence in the original article is about out the window. I can't help but doubt that the older female opossum goes through menopause to help bring up another's young.
Just my opinion :) -
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