RE: Radiation Evolutionary Effects

["Karam, Andrew" <Andrew_Karam@URMC.Rochester.edu>]

Dear Kent (and others),

A few thoughts occur to me regarding your question.  I don't know if they
directly address it, but I hope they'll help.

First is that you are probably correct in that any effects that wait until
after we reproduce (plus enough time to raise kids to fend for themselves)
probably have little to no evolutionary impact.  One caveat would be that,
in species in which one or both sexes have a prolonged reproductive life,
surviving through that entire span would give the chance of more offspring.
So, in human terms, we can start having kids in our teens, but if we
continue having children until we're in our 40s, we'll not only be very
tired, but will have more chances to pass our genes along, too.  So there is
one evolutionary advantage to delaying the onset of cancer somewhat.  Otto
Raabe has done some work showing that, as dose decreases, time until
carcinogenesis increases.  This gives an "effective threshold" because
postponing the onset of carcinogenesis until after you die of something else
means there will be no cancer death.  In other words, we only die once!

Second, it would seem to me that, at low dose rates, the mean time between
DNA damaging events is probably longer than the mean time to repair damaged
DNA.  That suggests that, provided the flux is low enough, damage should be
repaired pretty consistently and we should suffer no ill effects.  From this
perspective, our ability to survive low levels of exposure should depend in
part on the rapidity with which our repair mechanisms work, and this should
have some relationship to the radiation environment in which they evolved.

Third, it is unlikely that our radiation environment has changed markedly
over the duration of the human species or, for that matter, over the
duration of mammalian life (about 100 million years or so).  This is because
the major factors affecting background beta-gamma radiation levels have
half-lives in the billions of years and will not have decayed markedly in
that time.  

Fourth, remember that background radiation presently accounts for only about
5% or less of all background mutations - the rest come from a number of
other sources.  These include reactive oxygen metabolites, free oxygen in
our cells, and random errors that just crop up.  For an interesting
treatment of this last, you should consider reading "What is Life?" by
Schroedinger.

Finally, DNA damage repair is necessary for life as we know it because you
have to be able to pass on genetic information in a reasonably consistent
form from one generation to the next.  The damage repair must be accurate
and it must work quickly enough to contend with background plus normal
variations.  For life to exist at all, then, it almost certainly evolved
damage repair early and has probably continued refining these mechanisms
ever since.  A little bit of mutation is good - it's called evolution.  A
lot of mutation is bad - in us it's called birth defects or cancer.  What
would be interesting would be to see if there are any differences in the
repair mechanisms between, say, a Galapagos tortoise (or any other very
long-lived species) and those of insects, mice, or other short-lived
species.

Andy

Andrew Karam, CHP              (716) 275-1473 (voice)
Radiation Safety Officer          (716) 275-3781 (office)
University of Rochester           (716) 256-0365 (fax)
601 Elmwood Ave. Box HPH   Rochester, NY  14642

Andrew_Karam@URMC.Rochester.edu
http://Intranet.urmc.rochester.edu/RadiationSafety

The brain is a wonderful organ.  It starts working the moment you get up
in the morning and does not stop until you get into the office.  
Robert Frost
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