A new study suggests that billions of years ago, Earth's young atmosphere naturally generated sulfur-based molecules essential for life, challenging previous assumptions. Researchers from the University of Colorado Boulder simulated ancient conditions and found production of amino acids like cysteine and taurine. This discovery implies that the planet may have been pre-stocked with life's building blocks before organisms emerged.
Published on December 1 in the Proceedings of the National Academy of Sciences, the research by scientists from CU Boulder and collaborators reveals that the early Earth's atmosphere could have created sulfur biomolecules without the presence of life. First author Nate Reed, a postdoctoral fellow at NASA who conducted the work at CU Boulder's Department of Chemistry and the Cooperative Institute for Research in Environmental Sciences, stated, "Our study could help us understand the evolution of life at its earliest stages."
Sulfur is crucial for life, appearing in amino acids that form proteins, much like carbon. Traditionally, scientists thought organic sulfur molecules, such as those in amino acids, only formed after living organisms produced them. Past simulations of early Earth conditions rarely yielded significant amounts of these molecules, and when they did, it required rare, specific setups unlikely to occur widely.
This view gained attention when the James Webb Space Telescope detected dimethyl sulfide—a sulfur compound from modern marine algae—in the atmosphere of exoplanet K2-18b, hinting at possible life. However, prior lab work by Reed and senior author Ellie Browne, a chemistry professor and CIRES fellow, demonstrated that dimethyl sulfide can form abiotically using light and basic gases.
In the new experiment, the team exposed a mix of methane, carbon dioxide, hydrogen sulfide, and nitrogen to light, mimicking pre-life atmospheric conditions. Browne noted the challenges: "Working with sulfur is challenging... You have to have equipment that can measure incredibly tiny quantities of the products." Using a sensitive mass spectrometer, they identified cysteine, taurine, and coenzyme M, vital for metabolism.
Estimates indicate the ancient atmosphere could have generated enough cysteine to sustain one octillion cells—far less than today's one nonillion but sufficient for an emerging ecosystem. Reed remarked, "While it's not as many as what's present now, that was still a lot of cysteine in an environment without life. It might be enough for a budding global ecosystem, where life is just getting started."
The team suggests these molecules rained down to the surface, providing chemistry for life's origins. Browne added, "Life probably required some very specialized conditions to get started, like near volcanoes or hydrothermal vents with complex chemistry... our results suggest some of these more complex molecules were already widespread under non-specialized conditions, which might have made it a little easier for life to get going."