Under the kind of harsh ultraviolet light that bathed the young Earth, uracil, the genetic letter RNA uses, broke down about seven times faster than thymine, the letter DNA uses in its place. The result comes from a laboratory study by chemists at the University of Wisconsin-Madison, published in the Proceedings of the National Academy of Sciences.

The gap was even wider when the molecules were attached to their sugars. As nucleosides, the building blocks used in genetic chains, uridine degraded about 67 times faster than thymidine, according to a summary of the work by Earth.com.

That offers a concrete clue to one of biology's long-running puzzles. Many scientists think early life relied on RNA before DNA took over long-term storage of genes, and the authors suggest strong sunlight may have favored thymine during that shift. But the same study also found that ultraviolet light alone cannot explain why life settled on its five-letter genetic alphabet.

Testing 24 Molecules Under an Ozone-Free Sky

Before oxygen built up in the atmosphere during the Great Oxidation Event, Earth had no ozone layer to block ultraviolet light. Any molecules sitting in shallow water would have been exposed to radiation strong enough to break chemical bonds.

The idea that sunlight acted as a filter is not new. Earlier research found that life's bases shed absorbed ultraviolet energy within trillionths of a second, faster than many lookalike molecules, which led some scientists to propose that ultraviolet light destroyed less stable candidates and left life's letters behind. Ring molecules like these have been found in meteorites and in samples returned from the asteroid Ryugu, and the young Sun, though only about 70 percent as bright as today, likely emitted similar levels of the relevant ultraviolet light.

The research team, led by first author Marcos Jusino-Maldonado in the lab of chemist and astronomer Zoe Todd, dissolved 24 molecules in water. Nine were part of life's genetic alphabet, including four bases and five nucleosides, and 15 were lookalike molecules that life does not use. Guanine was left out because it did not dissolve well enough.

The samples were exposed to 254-nanometer ultraviolet light from mercury lamps after most oxygen had been removed. The scientists then tracked how quickly each molecule disappeared, repeating each test at least three times.

Across all 24 molecules, the fastest-degrading one broke down about 650 times faster than the slowest. Adenosine, one of life's own molecules, was the most stable of all. Uridine, another of life's molecules, broke down fastest.

Life's Letters Were Not the Toughest

That spread is the study's main message — and it cuts against a tidy story. Life's molecules appeared across the full range of stability rather than clustering at the durable end, and several lookalikes outlasted them. Melamine, a ring molecule life does not use, was the second most stable compound tested.

The authors made their conclusion clear in the paper's title, "Continuous UV irradiation played an inconclusive role in selecting the canonical genetic alphabet." In other words, surviving sunlight was probably one pressure among several, not the deciding factor.

Other factors the team lists include how well a molecule supports RNA copying, how abundant it could have been on early Earth, and chance. The single-ring molecules known as pyrimidines fared poorly overall, with 10 of the 12 tested ranking among the 12 least stable.

The thymine result stands out because it lines up with a known transition. RNA uses uracil and DNA uses thymine, and if early genetic systems faced constant ultraviolet exposure, a more durable letter could have offered an advantage as longer-lived genetic storage emerged.

A Second Study Reaches a Different Emphasis

The question is drawing attention from more than one lab. A separate PNAS paper published in August by researchers at Texas A&M University, led by Peter Rentzepis, compared thymine and uracil under 265-nanometer ultraviolet light and reached a more nuanced picture.

That team found thymine more photoreactive overall, but it formed the most permanent type of damage more slowly than uracil, steering harm toward a repairable form instead. "These findings support the concept that nucleobases evolved as 'molecular sunscreens,'" the Texas A&M authors wrote, arguing that the molecules may have been selected to control damage rather than avoid it.

The two studies measured different things under different wavelengths — breakdown rates in one, damage pathways in the other — so they do not directly contradict each other. Both, however, point to ultraviolet stress as a plausible pressure on early genetic chemistry while stopping short of a final answer.

Limits Readers Should Know

The Wisconsin results come from molecules in water under lamps, not from real planetary conditions. The lamps emitted a single wavelength, while the young Sun emitted ultraviolet across roughly 200 to 300 nanometers, so molecules that absorb at other wavelengths could behave differently in real sunlight.

The experiments also left out natural shielding that early molecules might have had, such as deep water, other dissolved chemicals, and tiny pores in rock. Some origin-of-life scenarios, including those near deep-sea vents, do not involve sunlight at all. Some of the ultraviolet damage was reversible when samples were warmed in the dark, so the measured rates may overstate real-world losses.

The study is laboratory chemistry, peer-reviewed and published online Sept. 28 ahead of the journal's Oct. 13 issue. It does not show how life began, and it does not change any public health guidance about ultraviolet exposure today.

The team plans to repeat the experiments across a broader range of ultraviolet wavelengths. For now, the reason life chose its particular five letters remains open, but the thymine finding gives researchers a measurable starting point for testing how a harsher sky may have shaped the chemistry that came before cells.

What Readers Want to Know

What did the study find about thymine and uracil?

Under 254-nanometer ultraviolet light, uracil broke down about 7 times faster than thymine, and uridine broke down about 67 times faster than thymidine.

Does this explain why DNA uses thymine?

It offers a possible clue. The authors suggest ultraviolet light may have favored thymine during a shift from RNA toward DNA, but they call UV's overall role in selecting life's alphabet inconclusive.

Why was ultraviolet light stronger on early Earth?

Before oxygen built up in the atmosphere, there was no ozone layer to block ultraviolet radiation from the Sun.

Were life's genetic molecules the most UV-resistant?

No. Life's molecules spanned the full range of stability, and several molecules life does not use were more durable.

Who did the research?

Chemists at the University of Wisconsin-Madison, led by Marcos Jusino-Maldonado in Zoe Todd's lab, published in PNAS.

What are the study's main limits?

It used a single wavelength in laboratory water samples and did not account for natural shielding or origin-of-life settings without sunlight.