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Re: dinosaur copulation?



From: Derek Tearne <derek@nezsdc.fujitsu.co.nz>
 > > 
 > > >swf@elsegundoca.ncr.com wrote:
 > > And yet, somehow, humans have _evolved_ so that their individual lifetimes
 > > _do_ exceed their reproductive lifetime, ...  
 > 
 > Humans have not _evolved_ into this situation.  It has been brought about 
 > by both an extended family where the old are cared for and by advances 
 > in nutrition and public health.  The natural life span of a human in 
 > either a jungle or other similar environment is around 40 to 50 years, 
 > which corresponds remarkably closely to the reproductive life span
 > of females.  

Be careful here - is this the raw average life expectancy?

If so, your conclusion is incorrect.  An age-corrected life
expectancy is more appropriate for determining prevalence of
older ages.  In most societies I know of (other than modern
industrial ones), mortality shows a strong bimodal shape, with
a high child mortality, and then reduced mortality until old age.
In this pattern, once someone has survived childhood, survival
to old age (that is > 65) is common, even normal.

The high mortality in the younger age brackets pulls down
the average a great deal.
 > 
 > Life expectancies of modern peoples ion third world countries fit 
 > very closely to this age breakdown.
 > 
 > Country       Male             Female
 > Algeria        56               58
 > Burkino Faso   41               44
 > Mali           41               43
 > Nigeria        41               44
 > Senegal        41               44

Yep, this looks like raw life expectancies, not age-corrected
ones.

This means that even in these societies ages in excess of 65
are *not* unusual.

In fact, as far as we can tell from anthropology and archaeology,
what you call the extended family (including clan and tribal
moieties) is the norm in almost all human societies, and has been
so at least as long as modern humans have been around (and probably
rather longer).

Thus, this extended life span *has* evolved in humans, and is
a legacy of our primate ancestry.

If this were not so, then what we call old age would set in
at menopause, not 20 years later as it actually does.
 > 
 > I would expect that preagricultural societies would have life expectancies in
 > the late thirties.
 >
 > 
 > The historical evidence suggests that women essentially continued having 
 > babies until they died and that other members of the group then looked 
 > after them.

Exactly - this is the basic evolutionary advantage to extended
maximum life spans.  It means a larger pool of potential care-
givers if a woman dies in child birth.  [Note, again, that it
is mostly death during child birth that kills adult women in
pre-industrial societies prior to old age - this means that those
who manage to survive past this are excellent candidates for
caring for their sisters' and nieces' orphans].

swf@elsegundoca.ncr.com         sarima@netcom.com

The peace of God be with you.