|MadSci Network: Earth Sciences|
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.
Try the links in the MadSci Library for more information on Earth Sciences.