On an early Earth exposed to intense ultraviolet radiation, the evolution of DNA may have depended on an unexpected molecular advantage.

Early life on Earth faced intense ultraviolet (UV) radiation from the sun without the protection of an ozone layer. Yet DNA came to rely on thymine, a chemical building block that is more prone to UV damage than uracil, its counterpart in single-stranded RNA.

A study at Texas A&M University suggests that thymine may have helped early genetic molecules survive by directing that damage into a form that was easier to reverse. The findings, published in Proceedings of the National Academy of Sciences (PNAS), offer a possible explanation for why thymine replaced uracil as genetic systems evolved toward DNA.