RE: Ancient radiation levels were higher than today's levels

["Bradshaw, Keith" <Keith.Bradshaw@nnc.co.uk>]

	Dear RADSAFERS

	Implicit in these arguments about background radiation levels and
the speed of evolution seems to be that background radiation is a
significant cause of mutations compared to other mechanisms.  I read
recently that every cell in our bodies averages 10000 DNA lesions per day
(can anyone confirm/refute this?), and the cause was put down to free
radicals as a result of food oxidation.

	So my question is, what proportion of DNA lesions are caused by
background radiation?  You can probably see my argument.  If radiation is
responsible for 100%, then a doubling of the radiation dose might give a
doubling of the evolution rate, but if it's responsible for only 0.1%, then
doubling it might only give a 0.2% increase in the rate, which is surely
indetectable.

	BTW, I doubt that the opponents of the LNT model on this list would
like doubling the background to significantly increase the evolution rate,
because that would imply that low level radiation is a major mutagen today.

	Regards

	keith.bradshaw@nnc.co.uk

	>What we're looking into now is occasional events ("periodic"
implies a
> regularity that is not present) with the capability of raising radiation
> levels to much higher levels than normal.  A number of people have made
> rather extravagant claims about supernovae and gamma ray bursts causing
> periodic mass extinctions, but we tend to discount those.  However, there
> is
> little doubt that a nearby supernova or a gamma ray burst anywhere in the
> galaxy can raise background levels by a few hundred mrad (or a few mGy)
> briefly and that this periodicity should be on the order of every few
> million years.  In addition, nearby sun-like stars have been observed to
> experience periodic "superflares" that emit a LOT of energy over a few
> days,
> but the average time between such events is not yet well constrained.
> Putting these together, it seems possible that, on an evolutionarily
> significant time scale, rad levels may have been raised enough so that the
> ability to address higher levels of radiation damage might not have been
> lost. 


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