Earth’s earliest atmosphere may have done more than surround the planet — it may have helped make life possible. A new study published in Proceedings of the National Academy of Sciences suggests that billions of years ago, the prebiotic sky could generate sulfur-based biomolecules essential to life, challenging the long-standing view that these compounds emerged only after living systems took hold.
Rather than being confined to rare, extreme environments, some of life’s chemical building blocks may have been widespread under ordinary atmospheric conditions.
“Life probably required some very specialized conditions to get started, like near volcanoes or hydrothermal vents with complex chemistry,” said Ellie Browne, the study’s senior author, in a press release. “We used to think life had to start completely from scratch, but 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.”
Why Sulfur Is Central to the Origin of Life
Sulfur sits at the heart of modern biology. It stabilizes proteins, helps enzymes do their work, and plays a central role in metabolism. Yet in most origin-of-life models, sulfur chemistry enters the story relatively late — after living systems are already up and running.
That assumption also shaped how sulfur is interpreted beyond Earth. When the James Webb Space Telescope detected dimethyl sulfide in the atmosphere of K2-18b, the molecule drew attention because it’s closely tied to marine life on our planet. But recent lab work shows that the same compound can form without life at all — raising the possibility that sulfur chemistry was already active on Earth before biology began.
Simulating Earth’s Early Atmosphere
To test whether sulfur biomolecules could form before life existed, researchers recreated a version of Earth’s early atmosphere using a mixture of methane, carbon dioxide, nitrogen, and hydrogen sulfide — gases thought to have been common before oxygen accumulated in the air. The mixture was then exposed to ultraviolet light to simulate solar radiation under prebiotic conditions.
Tracking sulfur chemistry under those conditions is notoriously difficult. These compounds form in extremely small amounts and tend to cling to laboratory surfaces before they can be measured.
“You have to have equipment that can measure incredibly tiny quantities of the products,” Browne said in the press release.
Using sensitive mass spectrometry, the simulated atmosphere produced a range of sulfur-bearing biomolecules. These included the amino acids cysteine and taurine, along with coenzyme M, a compound critical to metabolism in some living systems today.
When the results were scaled up to the size of Earth’s full atmosphere, the early sky could have generated enough cysteine to support on the order of one octillion cells. While that is far fewer than the estimated number of cells on the planet today, it represents a substantial chemical reservoir for a world that had not yet crossed the line into biology.
“While it’s not as many as what’s present now, that was still a lot of cysteine in an environment without life,” Reed added. “It might be enough for a budding global ecosystem, where life is just getting started.”
Rather than remaining suspended in the air, the newly formed molecules would likely have returned to the surface with rain, delivering sulfur-rich material into oceans, lakes, and shallow waters where further chemical evolution could unfold.
What Earth’s Early Atmosphere Means for Life on Other Planets
If Earth’s atmosphere can build these molecules without life, similar chemistry could also operate on other worlds — complicating how scientists interpret chemical signs beyond our planet.
“Our study could help us understand the evolution of life at its earliest stages,” Reed said.
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- This article references information from a new study that was published in the Proceedings of the National Academy of Sciences: An Archean atmosphere rich in sulfur biomolecules
