Re: changing rad levels over time (2) -Reply

[Andrew Karam <karam.1@osu.edu>]

Dear Dr. Frame,

All good points.  Right now, we're just trying to figure out what rad
levels might have been over the course of geologic time.  Interpreting them
and their legacy (if any) is really beyond our current scope of work.  My
understanding is that one of the most prevalent mutagens is oxygen.  Prior
to the existance of free oxygen (about 2 billion years ago) this would have
been much less important.  Also, since the presence of oxygen is one of the
modifying factors for radiation effects, removing oxygen from the
atmosphere and oceans may have further reduced the mutagenic effects of
ionizing radiation.  On the other hand, there is a minimum amount of oxygen
present in water, giving a lower limit for this effect.

According to Prasad's Handbook of Radiobiology, radiation currently
accounts for 1%-6% of all mutations.  All else being equal, in rad levels
ten times those at present would constitute from about 10% to about 38% of
all mutations (6 of 100 mutations today becomes 60 of 160 in the past).  To
the best of my knowledge there is no way to differentiate between genetic
damage due to ionizing radiation and that due to other mechanisms with the
possible exception of double-stranded DNA breaks, which are difficult to
repair, in any event.  With regards to loss of repair mechanisms over time,
I can't really think of a mechanism by which they would be lost, unless
they require energy that a cell could put towards other tasks in a
lower-radiation background.

I appreciate the difficulty of knowing anything about repair mechanisms in
the past.  This is one of the reasons we're not really looking at this now.
 A case may be made, perhaps, that a large range of organisms with the same
repair mechanism today (say, E. coli, humans, frogs, yeast) suggests a
common origin in the distant past, but this is not likely to ever be more
than supposition or speculation.  Although, given a choice between assuming
convergent evolution of similar repair mechanisms and assuming they have
been retained from a common origin, it may be more plausible to assume the
latter (a common origin in the distant past).  This is commonly assumed by
paleontologists with things such as bilateral symmetry, number of digits,
and so forth, over time spans of up to 600 million years.

At present, I think it is interesting to note varying radiation resistance
between different classes of organisms, but I can't go much further than
that.  I feel comfortable with speculating about radiation doses in the
distant past, but I need to learn a lot more biology before I can take that
much further.  This is the reason for my posting you reference.  

More questions than answers, I'm afraid, but it keeps life interesting,
doesn't it?  

Something else that I should have emphasized more in my original posting is
that the work my co-author and I have done has been presented at a few
meetings, but has not yet appeared in a peer-reviewed journal.  The
question I saw posted was one I thought I could help to shed some light on,
but I did not intend to present our information as anything other than
(hopefully) interesting informed opinion.  It is certainly not my intention
to present our results as anything other than preliminary until they have
been through the peer-review process and we have been able to assimilate
the comments and criticisms of our colleagues.

Sincerely,

Andrew Karam
The opinions expressed above are well-reasoned and insightful.  Needless to
say, they are not those of my employer. (with apologies to Michael Feldman)		


Andrew Karam, CHP  (karam.1@osu.edu)
The Ohio State University 
Office of Radiation Safety
1314 Kinnear Road
Columbus, OH  43212			    "The mind is not a vessel to
(614) 292-1284 (phone)		    be filled but a fire to be 
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