Natural background radiation is an important source of the spontaneous mutations that fuel evolution.

Hermann J. Muller showed in 1927 that X-rays could increase inherited mutations in fruit flies. Once radiation was known to mutate DNA, the weak radiation constantly arriving from cosmic rays, rocks, building materials, and radioisotopes inside organisms became a plausible natural source of evolutionary variation. In 1956, the U.S. National Academy of Sciences’ BEAR Genetics Committee concluded that background radiation supplied enough mutations for evolutionary purposes.

Radiation does cause mutations, but later work showed a problem of scale. Modelling for an E. coli experiment estimated about 6 × 10^-5 radiation-track interactions per cell per day at the surface laboratory, roughly one interaction for every 17,000 cells. The expected point-mutation rate was about 100 times higher. Replication errors, spontaneous DNA chemistry, reactive molecules, and imperfect repair generate mutations much more frequently.

Researchers then tested the evolutionary prediction directly. They grew 24 E. coli populations for 500 generations at a surface laboratory and at the Modane Underground Laboratory beneath 1,700 m of rock. The absorbed dose rate fell from 214 to 26 nGy per hour, a 6.8-fold reduction. Fitness increased in both environments, with no statistically significant evidence that the low-radiation populations adapted more slowly.

Very low radiation can still affect cell physiology. Experiments with other microbes have found changes in growth, stress responses, and gene expression when background radiation is strongly reduced. The narrowed conclusion concerns mutation supply: ordinary background radiation can damage DNA, but it appears too sparse to provide an important share of the spontaneous mutations that drive evolution.