why do we die?

Ron Grunwald grun at acpub.duke.edu
Fri Oct 20 21:04:10 EST 1995


In article <DGrFr4.8D3 at uns.bris.ac.uk>, tp5145 at irix.bris.ac.uk (TJD.
Prior) wrote:

> As a physics student I was thinking and the question came to me Why do we
> die, Why cant we just regenerate our selves forever. As nobody could give
> me a satisfactory answer am asking you. If you could answer this simple!!
> question in basic terms I would I would be most greatfull.

No simple answer here. However, common notions run to the problem of how
to deal with the accumulation of errors in a replicating system.
Maintenance of the information content of the genome, ie regeneration, 
requires energy. Perfect and perpetual regeneration would require an
infinite amount of energy, or more specifically that an organism invest
all acccumulated energy into repair of DNA errors. 

That would be no fun, beside being highly unlikely (the second law of
thermodynamics rears its ugly head!). Instead we live with the
accumulation of occasional errors, most of which are well tolerated. But
eventually even an unlikley combination of errors in a single cell will
occur that will be disastrous for the whole lot, as in the case of cancer.


Natural selection favors genomes which persist, but then how can a genome
persist without 'regeneration'? Selective advantage is obtained by a
genome which can replicate multiple copies of itself as independent
organisms. Selection against those offspring with excess errors acts as an
expensive, but ultimately succesful repair system. The production of
independent individuals also offers the advantage of recombination repair
of DNA, a process which requires sex between individuals (or something
very much like it). 

So if the genome is best perpetuated by reproduction rather than
'regeneration',  when and why should the parental organism die? Although
it may be true that the accumlation of DNA errors causes inevitable
senescence, this statement doesn't address the question of when we die.
The life span of organisms is highly variable and indeed some are quite
long (trees, for example). The levels of DNA repair machinery varies
between orgainsms and, in principle, an increased investment into DNA
repair would allow you to live longer.  

But at what cost? The costs of a long life span have to considered both in
terms of the impact on the individual's reproductive potential
(reproduction competes with repair for resources) and also on the success
of the offspring (the parent at some point competes with the offspring).
There is a large literature on this topic concerning life history
evolution that struggles with the particulars, including the problem of
'optimal' lifespan,  for each organism. I don't know very much about the
topic though. 

As for this human organism, I just want to live long enough to meet my
grandchildren and be an annoyance to my children. After that they can take
over from there. 

Hope this helps

-- 
Ron Grunwald                             



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