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The evolution of aging

Doug Hosack Doug! at nih.gov
Mon Jan 25 18:21:13 EST 1999

ufotruth at ix.netcom.com wrote in message
<36a77253.131151593 at nntp.ix.netcom.com>...
>Perhaps also it is a mechanism to make sure that evolution of a
>species continues. If individual organisms lived too long, and
>consumed too many important resources, they could slow down the
>evolutionary development of their species.
>If our ancestors a hundred thousand years ago had an extend lifespan
>and lived for long periods of time (until they were killed by disease,
>animal attack, or acident) then I doubt we humans would be living in
>houses today. Probably because evolution would have slowed down

Assuming that all other factors like "age of reproduction" and "reproductive
success" remain equal, long life would not slow down evolution.  You seem to
be forwarding the theory that evolution selected for aging as "good for the
species."  I recently had a discussion with Jennifer Petersen, who holds the
same belief, and I thought you might like to read my reply to her:

At first glance, it makes sense that evolution would promote "turnover" of
individuals for genetic diversity.  But take a moment to imagine a single
group of primitive humans in which some individuals age and others do not
(all other factors being equal.)  Both subgroups would have a few
individuals with extraordinary fitness.  However, the aging "fittest" people
would eventually die, and the non-aging "fittest" people would continue to
survive (and pass on their genes) as long as they were the most fit.
Furthermore, the non-aging population would have just as much diversity in
their offspring as the aging population, and would therefore be just as
adaptable.  In this scenario, evolution would soon select AGAINST aging.

So why didn't evolution select against aging?  Well, it did (does) up to a
point . . .
That point is the time at which the average individual in its natural
environment normally dies due to predation or disease or whatever.  Let's
refer to this as the "environmentally-determined lifespan."  Individuals who
age before the environmentally-determined lifespan obviously have less
children than those who don't.  But those who age AFTER the
environmentally-determined lifespan have just as many children in a lifetime
as those who don't age at all.  Therefore evolution selects against aging
until that point at which most individuals in a given environment would get
killed anyway.  After this point, there is no advantage either way.  This
explains why "hard to kill" animals like tortoises(hard shell),
birds(flight), humans(intelligence) and elephants(sheer size) live so much
longer than similar animals.  The classic example is mice which live 2 years
vs. bats which live 40 years.  Both have a similar size, morphology and
metabolic rate.  Hell, bats are basically winged mice . . . bats are just
harder to kill since they are not confined to the ground.

So aging certainly isn't a process imposed by evolution as something that is
"good for the species."  It's more like entropy that occurs after evolution
has lost the power to demand optimal health.  People have argued with me

 "immortality would cause a population to grow to the limit of its
resources, which would then not allow children to successfully compete with
adults, even if the children are more evolved.  This would lead to
evolutionary stagnation, so evolution doesn't permit this."

Unfortunately, evolution has no mechanism for stating rules like
"immortality is not allowed."  Random mutations and recombinations of traits
arise, and evolution is simply the non-judgmental course of events that
ensues.  The truth is that evolution simply hasn't selected FOR immortality.
In other words, non-aging individuals very well MIGHT have existed through
random chance, but rather than out-competing mortals (as outlined in my
second paragraph), they simply got killed by a saber-tooth tiger along with
everyone else . . .

With this in mind, you can see that systems that put a limit on lifespan,
such as telomere-shortening, do not exist for the PURPOSE of ensuring a
limited lifespan or "turnover of individuals."  They exist because they
offer an advantage early in life, but have the unfortunate SIDE effect of
limiting lifespan (which evolution has no power to further lengthen.)  The
current thinking about telomere-shortening is the "defense against cancer"
theory.  But even if this is not the actual reason, you can be sure that
evolution has selected for telomere-shortening for some good reason.


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