Ancient Earth’s Sky May Have Rained Life’s Building Blocks

Billions of years ago, Earth’s atmosphere might’ve been churning out the sulfur-based molecules essential for life, then delivering them to the surface in rainfall. A team at CU Boulder used lab simulations of the planet’s early sky to produce cysteine, taurine, and other sulfur biomolecules, challenging the assumption that life had to “invent” these compounds through evolution. The results, published December 1 in the Proceedings of the National Academy of Sciences, suggest the young planet’s atmosphere could’ve supplied enough cysteine alone to support roughly one octillion cells.

That’s a staggering number, though still a thousand times less than what exists on Earth today. But in an environment without life, it represents a generous head start.

Rethinking Sulfur’s Role in Life’s Origins

Sulfur sits alongside carbon as a fundamental building block, woven into amino acids and proteins across every domain of life. Scientists had long believed that organic sulfur compounds emerged only after living systems evolved to produce them. Earlier attempts to simulate prebiotic Earth either failed to generate meaningful quantities of sulfur biomolecules or required such specialized conditions that they seemed irrelevant to the planet’s broader chemistry.

The assumption grew so entrenched that when the James Webb Space Telescope detected dimethyl sulfide on exoplanet K2-18b, many researchers flagged it as a potential biosignature. On Earth, marine algae produce that compound. But Nate Reed and Ellie Browne had already demonstrated in prior work that dimethyl sulfide could form without biology, using only light and common atmospheric gases.

Reed, now a postdoctoral fellow at NASA who conducted the research at CU Boulder’s Department of Chemistry and the Cooperative Institute for Research in Environmental Sciences, wanted to push further. Working with Browne, a chemistry professor and CIRES fellow, the team constructed a laboratory atmosphere mimicking Earth before life emerged: methane, carbon dioxide, hydrogen sulfide, nitrogen, and ultraviolet light.

“Our study could help us understand the evolution of life at its earliest stages,” Reed said.

Sulfur compounds proved exceptionally difficult to work with. The element clings to lab equipment, and atmospheric concentrations of sulfur molecules run far below those of carbon dioxide or nitrogen. Browne’s team needed mass spectrometry instruments sensitive enough to detect trace quantities, equipment capable of identifying and measuring compounds at vanishingly low levels.

An Atmospheric Factory for Life’s Ingredients

What they found surprised them. The simulated early Earth atmosphere produced an entire suite of sulfur biomolecules: cysteine and taurine, both amino acids, plus coenzyme M, a compound critical for cellular metabolism. The reactions occurred without specialized conditions, no volcanic vents or hydrothermal systems required.

When the researchers scaled their lab results to planetary dimensions, the numbers became striking. Earth’s early atmosphere might’ve generated enough cysteine to supply approximately one octillion cells. For context, modern Earth contains roughly one nonillion cells, a thousand times more. But there’s something quietly radical about imagining that much biochemical raw material floating down from the sky onto a lifeless planet.

The implications extend beyond Earth. If atmospheric chemistry alone can generate sulfur biomolecules under relatively common conditions, then their presence elsewhere in the universe might not signal biology at all. The finding complicates how scientists interpret potential biosignatures on distant worlds.

It also reframes the narrative of life’s emergence. Rather than evolving in isolated pockets near geothermal energy sources and slowly assembling every necessary molecule from scratch, early life might’ve had access to a planet-wide supply of complex organic compounds. The atmosphere was seeding the surface, raining down ingredients that accumulated in oceans and tide pools.

“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,” Browne said.

Whether that made the crucial difference, whether it tipped the balance from chemistry to biology, remains unknown. But the work narrows the gap between prebiotic chemistry and the universal dependence of life on sulfur. That disconnect always seemed arbitrary, a puzzle piece that didn’t quite fit. Now there’s a mechanism that bridges the divide, even if the full picture stays obscured.

Proceedings of the National Academy of Sciences study