On Fri, 21 Apr 1995, Steve Chambers wrote:
>> In the hope of stimulating further debate, I'm going to play devil's
> advocate. The following might be seen as evidence against such a
> multiple process model of aging:
>> 1) It seems that homo sapiens' maximum lifespan may be double that of
> his most immediate ancestor. Some might argue (notably Cutler) that
> 150,000 years is such a short period of evolutionary history that
> advantagous mutations could only have influenced a FEW aging processes.
What data is there to support a doubled lifespan for modern humans vs
their immediate ancestors? You could point to a near doubling of average
lifespan over the last 1000 years and it would not mean any genetic
change, but rather, reflect better public health measures, better medical
treatment, better nutrition, etc.
Most animals in the wild experience shorter lifespans than their
captive counterparts (MOST, not ALL). It is not due to any genetic
shift, but is simply a matter of protection from predation, steady diet,
and veterinary care. Essentially, the processes that increase their
lifespan are the same that can be pointed to to have increased ours. In
any case, there is NO evolutionarily adaptive advantage for humans to
experience selection for even longer lifespans. Selection always acts,
in one way or another, on reproductive success, and here is is not just
quanitity, but simply reproducing in the first place. Since evolution is
tied inexorably to reproduction, it is undesireable to set up selection
FOR increased lifespan IF it means even higher reproduction rates.
>> 2) Calorie Restriction has a significant life-extending influence, and it
> influences MANY age-related changes. The effect seems to be universal.
> Some might offer this as evidence that there are only a FEW (and maybe
> only one) underlying aging processes.
Not necessarily. I would put forward the rather simple explanation of
this effect as being due to a reduction in the rate of metabolism.
Metabolism itself produces many harmful side reactions/molecular species
that our bodies have to deal with (radicals and other byproducts).
Decreasing caloric intake _significantly_ (as is required for these
studies to bear fruit) would necessitate a significant drop in metabolism
rate. The fact that the animals still age would indicate that that
process is still programed. Keep in mind that there ARE changes that are
detrimental to the aged: various hormones are no longer trancribed,
others are reduced, and other genes are permanently shut down. You
cannot simply espouse that methods be found to start them all up again
for that would cause many more problems. Many of the hormones and gene
products are necessary for growth and development. To increase or
restart their production would be detrimental (many of these same gene
products, if given to an adult, leads to various complications and side
effects). In any case, do you want to be in a perpetual state of
puberty?
There is no easy answer and the ageing process is inexorably tied to
development and growth. Development and growth are controlled by a very
complex constellation of genes and regulators. Screw with one and all
balances are thrown out.
>> 3) Recent evidence (eg. Carey et al, 1992; Curtsinger et al, 1992)
> suggests that mortality rates may decline in late life of many species.
> Some might suggest that this argues against there being MANY aging
> processes.
Hardly. Those who make it to old age and are still functional might well
simply carry a slower developmental clock. Every one of us is different
in our rate of development, afterall (recall the fast-developing girls in
grade school and junior high? Same overall genetic structure and
biochemistry, different rates of initiation). Such people who reach
significant old age STILL age and die. Their bones ARE brittle and of
lessor mass (largely due to lack or reduction in various hormones
mentioned previously), their muscle mass IS lower (same reason), their
reaction time is slower, collagen is in rough shape from years of normal
wear and tear...and it isn't replaced (again, developmental program
shutting off certain genes at certain developmental levels). Many of
these effects CAN be slowed or partially reversed by exercise of mind AND
body, but they cannot be completely reversed. Many genes are off or
down-regulated and will remain that way regardless of physical or mental
activity.
Patrick
