When it comes to the origins of life on Earth, sulfur-containing biomolecules have long presented scientists with a chicken-and-egg-type conundrum. Life today needs cysteine, taurine, and other organosulfur molecules to survive, but biological activity is largely considered the most prevalent source of such compounds. So what came first, abiotically formed organosulfur molecules or life itself?
“There’s been this general thought that life evolved initially without those molecules because we had no good way of making them under the generic conditions that would be prevalent on the early Earth,” says Ellie Browne, an associate professor of chemistry at the University of Colorado Boulder. But last year her group showed that dimethyl sulfide—a compound seemingly produced exclusively by life and widely considered a potential biosignature—was generated by ultraviolet (UV) light shining on simple gas mixtures. The finding made Browne wonder what other organosulfur molecules might have formed in Earth’s atmosphere when life was first evolving, 2.5–4 billion years ago.
Led by postdoctoral researcher Nate Reed, Browne and an interdisciplinary team of scientists have now demonstrated that a slew of organosulfur molecules form when simple gas mixtures formulated to mimic the Archean Earth’s atmosphere are exposed to UV light (Proc. Natl. Acad. Sci. U.S.A. 2025, DOI: 10.1073/pnas.2516779122). Many are biologically relevant.
“We didn’t have oxygen or ozone during this time period,” Browne explains, so the gas mixtures contained nitrogen, methane, carbon dioxide, and hydrogen sulfide. After preparing gas mixtures, the researchers photolyzed the gases by exposing them to UV light from a deuterium lamp for a few minutes. And when the scientists caught and analyzed the resulting aerosols with two mass spectrometry techniques, they discovered small amounts of cysteine, homocysteine, methionine, coenzyme M, cysteine sulfinic acid, taurine, methyl sulfonic acid, and methyl sulfate.
Line structures of cysteine (left) and taurine (right)
“The reaction cell that we do [the experiments] in is the size of a small water bottle, so there’s not a lot of stuff that we make,” Browne says. “But when you scale up a small water bottle to the size of the whole atmosphere, then you can start making a lot of these things.”
In fact, the researchers estimate that 105–1010 moles of cysteine could have deposited onto early Earth every year as the haze of aerosols naturally fell from the sky. “It ends up being really competitive with other estimates of how some more-complex molecules could have been delivered to the surface,” Browne says.
The findings excite Sarah E. Moran, a researcher at the Space Telescope Science Institute who was not involved in the study. “In planetary science and exoplanets and early-Earth studies, very, very few sulfur experiments have been done,” she says. For a long time, researchers largely ignored sulfur when designing chemical models to explore the possible atmospheric chemistry of the Archean Earth, Moran explains. “This Colorado lab is showing, very powerfully, you can’t ignore it.”
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