Re: News: Menopause: blame it on evolution.

Robert Carnegie wrote:
Matthew wrote:
Ye Old One wrote:
Menopause: blame it on evolution
Thursday, 20 September 2007
by Carolyn Barry
Cosmos Online

http://www.cosmosmagazine.com/node/1593

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.

Funny that the high rate of birth defects for women over 35ish isn't
mentioned. Defective / dead offspring tend to be contra-selective.

Isn't that covered by "the quality of eggs decreases"? Although that
seems to put the cart before the horse. How are they rating egg
quality at different ages, other than by successful reproduction?

I think basically the question is "Why do individuals of a species
sustained by reproduction and replacement of those individuals,
survive the end of their reproductive ability?" After all, they
compete for resources with reproducers. Well, in many species, non-
reproducers don't survive. Some are eaten by their mate or by their
offspring. Or the biology runs down and switches off along with the
reproductive capacity. In other species, elders and other non-
reproducers can defend and nurture the current and future reproducers.

To recap what seems to have been demonstrated in this statistical/
theoretical exercise, in a species like ours where reproduction is
also risky (I think I've seen put at two per cent the rate of placenta
previa, in which without intervention the birth process will burst the
placenta and simply bleed mother and offspring to death very quickly),
and in which reproduction at an older age is riskier - what seems to
have been demonstrated is that to have dead-when-infertile or longer-
fertile females leads to a lower future projected population than the
way that things actually are. (Since "longer fertile" includes
"probably dies in childbirth".)

And we were just talking about chickens... when you take their eggs
away, they lay replacement eggs. A major industry is founded on
this. If they keep 'em, they hold off on making more until hatching
time. In other words, their reproductive engine pauses making eggs
while there are laid eggs to sit on.


I don't get the part about the chickens - how is this relevant to menopause? Are you suggesting that women delay menopause if they haven't had any children? Are you suggesting that a 59 year old woman would choose to have a child simply because menopause got delayed until 60? Are you suggesting that more children will be born if we delay menopause until 55 or so? This line of thought seems naive to me, but maybe I'm being USA-centric? Or maybe I'm just dense and missed your point.


I think the notion of "gradual" is severely overstated. To see if memory served me, I googled "birth defects age" and just grabbed the first one:
http://www.babyhopes.com/articles/birthdefects.html

===========
Risk of Birth Defects
The risk of having a baby with chromosomal disorders increase as a woman grows older. The most common of these disorders is Down syndrome, a combination of mental retardation and physical abnormalities caused by the presence of an extra chromosome. At age 25, a woman has about a 1-in-1,250 chance of having a baby with Down syndrome; at age 30, a 1-in-1,000 chance; at age 35, a 1-in-400 chance; at age 40, a 1-in-100 chance; and at 45, a 1-in-30 chance.
===========

At: http://www.musckids.com/health_library/hrpregnant/over30.htm
There is a small chart that shows what I believe is a decidedly non-linear risk of birth defects VS. age. Especially when you consider that women can become fertile from say 9-13 years old in the US (damn those cow hormones).

At: http://www.wcox.com.au/glossary.htm
You can scroll down to the chart "MATERNAL AGE" which shows what I believe is also a non-gradual progression (a very nice year-by-year chart BTW).

I'm not going to vouch for any of the sources listed above, but all of it reinforces the research I did for a high school paper back in the early seventies (yeah I know, what the hell did I know back then?). The point is that there is a sweet spot for reproduction between 19 and 30 where the (many sundry) risks of childbirth are low and outside that range it gets non-linearly more dangerous - especially at the older end of that range.


All that aside, I may not understand exactly what the article was proposing, but the assertion of natural selection on today's humans is absurd because the weak humans aren't killed off and the "best of the best" rarely ever have time to raise a dozen or two kids. I suspect that religion and poverty are stronger selectors for modern procreation. Where I live, the onset of menopause has virtually no bearing on the upper limit of having children. The vast majority of women have either had kids and moved on, or they actively prevent pregnancy due to the risks of older pregnancy.

The assertion that humans stop making eggs much earlier prior-to-death than other animals rings a bit hollow to me. I think it's being overstated. Humans live roughly twice as long as they should - due to human meddling. If women died at 60ish and stopped producing eggs at 50ish, that seems pretty close to death relatively speaking.

What other animal could survive it's natural environment if you medically doubled it's lifespan? Have you seen how fast a 100 year old woman can run? Could she avoid being eaten by predator? It just seems silly to make that comparison with other animals in their natural environments. Does your 13 year old dog have puppies? Do other animals stop making eggs *later* than 50/60ths (over 83%) of their lifespan? How much later? I honestly don't know, but it doesn't seem so unusual if humans didn't live so much longer than they should.

Other facets of the study are troubling as well. Let's assume that evolution of menopause didn't just occur in the last few centuries. Let's assume that the majority of it's positive selection probably goes back thousands of years. In a modern setting (the setting of this survey) the selective forces are radically different than in the past. The past is what did the selecting, not the recent present.

It seems to me in a planet of modern hospitals and 6 billion-ish people, the selection forces acting on say a two year old child are virtually nothing like a thousand years ago, let alone thirty thousand years ago.

Today, in the USA (for the sake of argument), the largest killer of children ages 5-17 is probably automobiles and children under 5 is household accidents or drowning etc.. If you live elsewhere, malaria kills 3000 people a day, mostly children under 5 years old.

I'm having trouble figuring out how grandma has a *significant* effect on selection in either of those situations. In my own case, my child's private school was far more diligent than my mother ever was at supervising my son. Even *I* was a lot better at it than mom ever was. I don't mean to imply mom was a bad mother, but the information available to concerned parents is far better than "Dr. Spock". IMO, grandma doesn't seem to possess any special skills not otherwise available in modern society to a modern working couple. I understand this is USA-centric, YMMV.

In the case of say ten or twenty thousand years ago, I imagine that all the mothers would tend to watch over all the kids, they didn't go off to grandma's house and drop the kids off while they went to gather food. I guess I'm just having trouble with the assertion of "the kind old grandma" providing a selective advantage for the survival rate of the kids. During the time that menopause was supposed to evolve, humans were in relatively small groups where there were plenty of people handy to look after kids. In modern contexts, grandma is increasingly useful especially since she is still rather healthy and mobile thanks to modern medicine. The problem is, modern society completely confounds any natural selection that might otherwise control the breeding / survival rates of humans.

From a slightly different angle, if menopause selects for longevity of the species, then why don't women wait until age 19 before producing eggs? Having a child at 10-16 is fraught with life-threatening problems as much as having a child over 40. Personally, I think menopause is more of an artifact of other selective forces concerning all the myriad things that occur from conception to birth. I won't argue that menopause can have some handy side-effects for modern man and quite possibly those side-effects might be beneficial in a VERY narrow and limited scope, but it seems implausible that menopause "planned" for grandma to babysit.

It seems to me that a girl under 12 that died while unable to give birth to a full sized baby should have selected for delayed egg release in so much as menopause would contribute to the babysitter pool. And yet, it didn't.

I realize that all this is arguing from incredulity, and that survey sounds legit, but it flys in the face of so much of what I *do* know, that it leaves me a bit perplexed. What am I not seeing?

.

Relevant Pages

  • Re: News: Menopause: blame it on evolution.
    ... Because women go through menopause and can no longer have their own ... eggs only during a short window of time. ... help take care of her daughter's offspring, ... other than by successful reproduction? ...
    (talk.origins)
  • Re: News: Menopause: blame it on evolution.
    ... Menopause, the natural cessation of female reproductive functioning, ... eggs only during a short window of time. ... when puberty and menstruation kick in at around age 13. ... other than by successful reproduction? ...
    (talk.origins)
  • Study finds menopause symptomps can be predicted
    ... And while doctors can't actually count the number of eggs in an ovary, ... menopause will set in and how many fertile years a woman has left. ... At about age 37, a woman has about 25,000 eggs left, and at menopause ... Assisted reproductive technology could not be relied upon to ...
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  • Study finds menopause symptomps can be predicted
    ... And while doctors can't actually count the number of eggs in an ovary, ... menopause will set in and how many fertile years a woman has left. ... At about age 37, a woman has about 25,000 eggs left, and at menopause ... Assisted reproductive technology could not be relied upon to ...
    (alt.med.fibromyalgia)
  • Study finds menopause symptomps can be predicted
    ... And while doctors can't actually count the number of eggs in an ovary, ... menopause will set in and how many fertile years a woman has left. ... At about age 37, a woman has about 25,000 eggs left, and at menopause ... Assisted reproductive technology could not be relied upon to ...
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