Re: What was the natural radioactivity in prehistoric time

Interesting that you should ask this question - I spent about 7 years 
during my master's and doctoral research trying to find an answer.  Let 
me see if I can summarize it in less than the 500 pages I originally 
wrote!

There are four major sources of natural radiation - radiation from rocks 
and soils, cosmic radiation, radiation from isotopes that are part of our 
bodies, and radon.  There's also ultraviolet radiation, although this is 
somewhat different from the others.  It's important to realize that these 
have all been present for as long as life has been on earth.  

To start with the geological sources of radiation, the major sources of 
radiation are uranium, thorium, and a radioactive isotope of potassium.  
All of these were formed in a supernova explosion about 5 or 6 billion 
years ago.  When the earth first formed, they were evenly mixed in all of 
the rocks but, over time, they have become increasingly concentrated in 
the crust of the earth because of their geochemical properties.  At the 
same time, they have all be radioactively decaying, each with a 
characteristic half-life.  The net result is that, for about the first 
two billion years of the earth's history, radiation levels from these 
geologic emitters remained roughly constant at levels about 2-3 times 
what we see today.  After that, they gradually dropped to current 
radiation levels due to radioactive decay.

Radon comes from the decay of uranium in the rocks and soil, so radon 
concentrations will directly track with uranium concentrations.  However, 
radon emits alpha radiation, which will only penetrate a few microns in 
water.  This means that, until life developed gills, we had no radiation 
dose from radon.  So, in effect, dose from radon suddenly shot up from 
virtually nothing to nearly current levels, and it's remained fairly 
constant for the last few hundred million years.  The reason it's been 
fairly constant is because the half-life of its parent nuclide (U-238) is 
nearly 4.4 billion years - far too long to see any significant changes in 
the time since complex life first formed.

Cosmic radiation comes from the sun as well as from events outside the 
solar system.  Solar radiation does not penetrate into the atmosphere 
very well, so most of our cosmic radiation comes from outside the solar 
system.  As solar activity increases, our radiation dose from the sun 
increases a little bit, but the increased solar activity helps to exclude 
galactic cosmic rays from the inner solar system.  When the solar system 
was young, the sun spun more rapidly than it does today, so the solar 
magnetic field was more intense.  This caused a much stronger solar wind, 
which caused a much higher radiation dose from solar cosmic rays, and a 
reduced dose from galactic cosmic radiation.  The net result is that 
cosmic radiation dose today is higher than it was in the distant past 
because the galactic cosmic rays are much more damaging than solar cosmic 
rays.

UV radiation comes from the sun.  Through time, the sun has become hotter 
and has emitted higher levels of UV radiation.  However, the early earth 
had no ozone in the atmosphere, so this UV reached the earth's surface 
without any attenuation.  When the ozone layer formed, the amount of UV 
reaching the earth's surface dropped dramatically to today's levels.  All 
in all, sea-level UV levels have dropped by a factor of 400 since the 
earth first formed.  When you consider that DNA absorbs UV very 
efficiencly in the same wavelengths that ozone absorbs, you can calculate 
that the DNA-weighted UV irradiance (which accounts for this) has dropped 
by a factor of about 1000 since the earth first formed.  However, this 
would only affect living organisms that live in the "photic zone" - that 
are exposed to sunlight on a regular basis.  

The last source of natural radiation is from radioactive potassium in our 
own bodies.  Today, this gives us about 10% of our total radition dose.  
Although the potassium concentrations in living organisms have likely not 
changed much over time, the amount of radioactive potassium-40 has 
changed by a large amount.  Actually, since life first evolved, the 
radiation dose from internal emitters has dropped by a factor of 8.

When you put all of this together, what we find is that overall, natural 
radiation exposure was about 6-7 times higher in the distant past 
compared to today, not counting UV.  Adding in UV, we find that the 
radiation dose has dropped even more radically.

So what does all this mean?

First, our cells all have DNA repair mechanisms that probably appeared 
early in the history of life.  This means that we are probably able to 
deal with higher levels of radiation exposure than exist today.  So maybe 
we can be exposed to higher levels of radiation without having any ill 
effects.  This is an area requiring more research.

Unfortunately, there is not much published on any of this except in the 
scientific literature, so I really can't suggest any web sites or books 
for the general public.