In article <2ufi84$433 at nntp2.Stanford.EDU> galahad at leland.Stanford.EDU (Scott Compton) writes:
|| This statement sounds flawed to me if we are looking at evolution
| of traits. Life expectancies are most likely on the increase within
| the past few hundred years. The average lifespan of a women living
| a million years ago might be *earlier* than menopause. Furthermore,
| how do we really know that the time in which menopause starts has
| stayed a constant (in the midlife range). It would seem more likely
| that menopause has variability since some women start in their 30's
| and some women start in the 50's or even later. Such a variance might
| suggest that menopause could shift over time. If the life expectancy
| is about 50 and that is when menopause starts on average now, it might
| seem more reasonable to say that with the sudden change of lifespan
| in our recent evolutionary past, the menopause rates are correlated
| to mortality rates if we take the fitness of an organism into
| consideration. I'd like to know if really young women have
| undergone menopause: like a teenager or even earlier. Another
| factor we might consider that is somewhat related is the loss
| of menstural cycles when women exhert themselves too hard (I'm
| thinking of track atheletes). If there is too much stress on
| the woman, the body stops cycling possibly as a response of
| survival to save blood. Could this little fact be indirectly
| coorelated with menopause?
Let me just add a few amateurish, common-sense observations.
1. Our life expectancy has increased gradually in, say, the last
10,000 years since we gave up the nomadic life style. The question is
whether the age of menopause has increased too. I believe the answer
is yes, because the age when menstrual cycles begin has also shifted.
It is around 10 - 14 now. However, our closest cousins, the apes,
reach puberty much faster. If the beginning of menstrual cycles has
adjusted to the changes in our life style, the end might have adjusted
2. Even though our life expectancy has increased gradually for thousands
of years, the most dramatic increase has occurred in the last couple
of centuries, thanks to the medical advances. Physiological changes
cannot occur that fast. I believe that is the reason why most of the
women outlive their fertility.
3. The question posed above, namely whether too much stress on the woman
causes the body to stop cycling, is probably unrelated to the 'natural' end
of menstrual cycles. In fact, it is not only when women exert themselves
too hard that they lose their fertility. Malnutrition leads to the same
result. It was probably Mother Nature's way of preventing unnecessary
births when a tribe went through a hard time.
My conclusion is as follows. If our life style and life expectancy stopped
changing, the age of menopause would probably increase and eventually
become equal to the life span. But most likely they will not stop
changing, so the age of menopause will probably keep lagging.
As for why men retain their ability to have children until they are much
older, I guess for the same reason different organs fail at different times.
For example, most of us lose most of our teeth in our '50s and '60s.
The hearing, on the other hand, start degenerating rapidly in our '70s
and '80s. The vision goes in our '90s (I mean that's when retinal
problems become severe). So, I suppose different organs have different
degrees of reliability.