On ancient Earth, sulfur biomolecules could have formed without life

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