From: Damien Broderick (damien@ariel.ucs.unimelb.edu.au)
Date: Wed Oct 01 1997 - 16:57:50 MDT
At 11:58 AM 9/30/97 -0400, Darrell wrote stuff.
I read your post with initial disappointment that turned around as I went
on. I wish you'd sorted out the logic before posting. You start by making
the oft-heard but absurd claim
> Unfortunately, the selection pressure towards indefinate longevity is
>very weak. [...] once an organism has finished breeding and rearing
>children there is no particular evolutionary advantage in living much
>longer
We've been down this track before. You're assuming part of what needs to
be demonstrated - that older organisms automatically cease breeding
reliably. That's contingently true of complex critters, as it happens, but
what we need to ask is why it should be so. (In other words, it is
logically possible that an endlessly healthy, self-repairing and fertile
couple could production-line their joint genes until the end of time, with
a superior inclusive fitness function for their very own selfish genes than
their watered-down cousin gene pool. Both Darwin and Mendel would rub
their hands with glee without a single censorious word of reproof.)
> 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.
Closer to making sense. This again, however, tends to assume what needs to
be shown. What are the characteristics that keep you fighting fit as a
rug-rat, a child, an adolescent and a young parent but go on to sap your
reproductive health in later years? Having a good immune system, once it's
been primed, should give you a snappy start in life, but surely should also
*continue* as a benefit. (On the other hand, we set out with a limited if
Vast immunity repertoire, and the little bastards who breed in us keep
changing their formula, so the simple passage of time might be enough to
make oldsters vulnerable to Widowmaker '99 - but that doesn't explain these
ugly crowsfeet and those hot flushes.) Certain specific trade-offs have
been suggested by experts such as George Williams, but the devil is in the
details. Hand-waving can support either tendency, I think.
Note that this hangs on the assumption that some metabolic mechanisms
especially beneficial to the infant, say, will turn and bite later on. I
dunno - maybe if you're a chubby cholesterol-rich kid you do better than
the lean and hungry look, and then the arterial accumulation knocks you off
after 50 fertile years. But we know there are snazzy mechanisms for
healing sick DNA and other damage - a different point, granted - because
our gonadal DNA is 3 billion years old and not a day the worse for it (same
homeobox sequences, often enough - highly conservative). Aside from those
pesky risks to purity of essence, to wit--
>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.
Well... okay, that makes sense - but is exposure to free radicals and
cosmic rays and really bad television programs for 40 years so much more
damaging, even lethal, than for 20 years? My hunch is that the machinery
for keeping gonadal DNA clean and shiny is specialised and too expensive to
share around generally, and that some suboptimal babies are not due to
damaged ova but to clapped out Mommies and their poorly maintained
life-support system innards. True, Down syndrome is a specific
embryological glitch, an extra somatic chromosome acquired during meiosis,
but might Down births be due not so much to an excess of these foetuses
from older mothers and maybe fathers as to a breakdown in the standard
flushout mechanism for teratological deviations? (Any experts out there?)
> To get superlongevity we'll have to bootstrap and cast away our
>genetic history.
Surely. It will presumably require - at least in the short term - the
physical introduction of cleaners and sweepers at the nano scale, getting
their purity guarantees and power supply from outside the body, from
culturgens rather than directly from genes. Works with memes, sort of.
Damien Broderick
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