The following is a discussion of evolution. Any anthropomorphic speech
should be taken metaphorically.
Unfortunately, the selection pressure towards indefinate longevity is
very weak. Natural selection favors selection of replicators that
replicate. That is, once an organism has finished breeding and rearing
children there is no particular evolutionary advantage in living much
longer, and one could argue even a disadvantage, as that individual would
take up resources that could be used by the younger, breeding generation.
Natural selection not only favors reproduction, but also favors the
preservation of siblings and cousins, as they also share some of the
individual's genes. Primates (including early humans)typically live in
packs in which many if not all of the members are interrelated. If long
living genes resulted in resources being taken away from nephews and
cousins, there would actually be a selection pressure towards shorter life
spans. In this situation evolution would have no incentive to make
lifespans a great deal longer than reproductive age. Rather, evolution
would guide lifespans into a certain range, long enough to reproduce and
raise children, but not so long that there are too many old barren mouths
to feed. A rough estimate of this timespan in humans would be 60 years,
assuming
that the children are protected, taught, cared for a much lengthier period
than in most animals, and this seems to be born out by what actually
happens in humans. It is generally considered that sometime in the early
to mid
60s is the time when people stop doing much productive work and start
resting on their laurels and Social Security benefits until they die.
Also keep in mind that in the past many more people did not survive to
full maturity. Any genetic trait that exchanges robustness in youth for
more problems in old age would be favored. Cell repairs fall into this
category. Typically, aging damage is not a significant threat to survival
until well after reproductive years. Maintaining the body in peak
condition throughout breeding age would require spending energy that might
be better used otherwise. This may be an explanation of why people pass
out of reproductive age. The first place that cell damage will cause
serious problems is when damaged sperm or eggs pass their inheritence to
an embryo. This is why woman over 40 having children have a much higher
risk of having babies with Down's Syndrome. A Down's baby is
reproductively speaking an enormous waste of energy, especially when it
comes to caring for it after it is born. Maintaining the body in peak
condition is too likely to lead to starvation, so there is a gradual
tapering off of fertility until the end result is a barren organism with
little in the way of evolutionary pressures. One sidenote is to observe
that men are fertile quite a bit after woman. This may be that men
produce sperm constantly, while all eggs are produce prior to birth. This
means the eggs have much more time to suffer cellular damage, and further
fertility because evolutionarily untenable sometime in the 40s.
To get superlongevity we'll have to bootstrap and cast away our
genetic history. We can engineer our genes, and now have the capability
to create guided evolution that favors maximum lifespans of the
individual. Here is an interesting question to ponder: If we genetically
engineer a race of apes to have lifespans in excess of 200 years, and then
drop them on a nice planet somewhere, would natural selection gradually
undo what humans had done and reduce lifespan to a much shorter length?
It may be that genetic traits that we would consider to be a good idea
need the constant maintainence of civilization to maintain, and would not
last indefinately in the constantly resource scarce environment of pure
competition.
For more detailed explanation of natural selection's pressure on the aging
process read Richard Dawkings 'River Out of Eden'
Darrell Parfitt