Number 461 (Story #1), December 10, 1999 by Phillip F. Schewe and Ben Stein NATURALLY OCCURRING RADIATION LEVELS ARE MUCH LOWER TODAY on Earth than when life first appeared, a new analysis has shown (Andrew Karam,716-275-1473, Andrew_Karam@URMC.Rochester.edu), suggesting that all living organisms--which have mutation-repair mechanisms very similar to those first developed by primordial life forms--were once equipped to handle larger doses of background nuclear radiation than modern life forms. Presently, humans receive a dose of about 360 millirems per year of radiation from natural sources, plus typically about 63 mrem/yr from anthropogenic sources. Perhaps surprisingly, a major source (about 40 mrem/yr) of naturally occurring radiation is inside our bodies--in the form of potassium, a nutrient essential for many things such as generating signals between cells. All natural sources of potassium contain some radioactive potassium-40 (K-40). But life first began about 4 billion years ago--about 3 K-40 half-lives ago--meaning that the radiation dose from potassium today is about one-eighth of what it was 4 billion years ago. Geologic sources of radiation (about 28 mrem/yr) include uranium, thorium, and potassium present in rocks and minerals in the earth's crust. Studying published data of 1100 rocks, and assuming that the continental crust had formed early (a scenario favored by the rock record), the researchers estimated that radiation from these sources is now about one-half of what it was 4 billion years ago, because many of these radioisotopes decayed in the intervening time. Not considered in the present study were cosmic sources (about 27 mrem/yr) and radon (typically about 200 mrem/yr); the authors are making these the subject of ongoing research. (Karam and Leslie, Health Physics, December 1999.)