Thymine may have acted as DNA’s early sunscreen

Early life on Earth faced intense ultraviolet radiation from the sun because the planet had no ozone layer. In that harsh environment the first genetic molecules had to survive repeated UV hits, which can break the chemical bonds that hold DNA together. A chemical building block of DNA, thymine, is more prone to UV damage than uracil, the similar base found in RNA, yet over time thymine became the preferred base in DNA.

The experimental test

Researchers at Texas A&M University tested how thymine and uracil react to UV light that mimics early Earth conditions. They used biophotonics tools such as absorption, fluorescence and Raman spectroscopy to track how the molecules absorb, emit and scatter light during UV exposure. The experiments showed that thymine tends to convert UV damage into a form that is easier to reverse.

This ability suggests thymine could have acted as a molecular sunscreen, absorbing harmful UV energy and keeping the damage confined to the genetic bases rather than spreading to more vulnerable parts of the molecule. The result would allow early genetic material to repair itself more efficiently, giving a survival advantage to organisms that used thymine instead of uracil. The study published in the Proceedings of the National Academy of Sciences explains why thymine replaced uracil as life’s genetic building block.

The team plans to extend their work to investigate whether these chemical building blocks formed on Earth or were delivered by meteorites, and to explore how early genetic molecules assembled. The same spectral signatures they measured could also help future research in faster, more sensitive pathogen detection and in monitoring the effectiveness of UV disinfection in water treatment. Funding for the project came from the Air Force Office of Scientific Research and Texas A&M Engineering Experiment Station.