Re: News: Menopause: blame it on evolution.


Matthew wrote:
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.

The chickens may not be relevant. I'm kinda whimsical and sometimes I
put stuff in just for fun, or without checking carefully. But
apparently chicken reproduction (in the sense of making eggs) is
directly affected by the presence or non-presence of eggs already laid
to be attended on - sat on for warmth and defence. Human reproduction
isn't, except of course that pregnancy interrupts the menstrual cycle.

On the other hand... well, I'm not sure if there are "few" or "many"
women choosing to use science to have children after their natural
menopause time. If it makes it into the newspapers, then I guess it's
unusual. This isn't to say "right" or "wrong", either.

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?

Well, even in primitive present-day societies, /basic/ care and stone-
age technology allow most adults to survive most of the time, compared
to hunter-gatherer existence with predators picking us off.

I would expect you 100-year-old woman to be protected or even carried
by younger members of the tribe, incidentally. As it is, it often
happens that some old person stops a bank robbery or a mugging, or
something.

.

Relevant Pages

  • Re: News: Menopause: blame it on evolution.
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  • 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? ...
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  • Re: News: Menopause: blame it on evolution.
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    (talk.origins)
  • Re: News: Menopause: blame it on evolution.
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