Life’s building blocks may have rained down from Earth’s early atmosphere

According to a recent study, Earth’s atmosphere might have been more important for the origin of life story than we gave it credit for. 

Researchers at the University of Colorado Boulder (CU Boulder) found that billions of years ago the young sky could have been a factory for sulfur-bearing organic molecules that biology depends on.

The result challenges the long-standing assumption that such sulfur biomolecules appeared only after life was already underway. 

“Our study could help us understand the evolution of life at its earliest stages,” said first author Nate Reed, a postdoctoral fellow at NASA who conducted the work at CU Boulder.

Why sulfur matters to life

Sulfur sits alongside carbon, hydrogen, nitrogen, oxygen, and phosphorus on life’s shortlist of essential elements. It’s embedded in amino acids like cysteine and in cofactors that drive metabolism. 

While early Earth certainly contained sulfur in its atmosphere, scientists long believed it couldn’t generate organic sulfur compounds on its own.

Instead, they thought molecules like those found in amino acids were produced only by living systems.

That view got shakier after the James Webb Space Telescope (JWST) reported dimethyl sulfide (DMS) on exoplanet K2-18b. This molecule is made by marine algae on modern Earth and often touted as a potential biosignature. 

In collaboration with Ellie Browne, Reed had already shown in the lab that DMS can form abiotically from common atmospheric gases under light, hinting that some “life-like” sulfur molecules can arise without biology.

Simulating prebiotic atmospheres

In the new work, the team simulated a prebiotic atmosphere by shining light through a gas mixture of methane, carbon dioxide, hydrogen sulfide, and nitrogen. These gases are believed to be common before life evolved. 

Working with sulfur isn’t simple since this gas clings to equipment and typically exists at trace levels compared with CO2 and nitrogen.

“You have to have equipment that can measure incredibly tiny quantities of the products,” said Browne.

Using ultra-sensitive mass spectrometry, the group detected a suite of sulfur-rich biomolecules forming directly in the “air,” including the amino acids cysteine and taurine, plus coenzyme M, a key player in metabolic chemistry.

Biomolecules that rained down

The team scaled their lab yields to planetary size. Their conclusion: an early Earth atmosphere could have synthesized enough cysteine to provision roughly one octillion (10²⁷) cells.

This is short of today’s biological census (about 10³⁰ cells), but still a staggering prebiotic stockpile. 

“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,” Reed said. 

Those molecules wouldn’t have stayed aloft forever. The authors argue they likely rained out onto land and ocean surfaces, seeding the very environments where life is thought to have taken root.

Complex molecules that were widespread

Origin of life scenarios often point to specialized hotspots, such as volcanic terrains and hydrothermal vents, where energy and chemistry combine to spark complexity. Browne doesn’t dismiss those niches. Yet, she reframes the starting conditions. 

“Life probably required some very specialized conditions to get started, like near volcanoes or hydrothermal vents with complex chemistry,” she said. 

“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.”

The search for life signals

If an atmosphere like early Earth’s can generate DMS and other sulfur organics abiotically, then certain “life signals” may be more ambiguous than hoped. 

This matters both for deciphering our own past and for exoplanet scouting. The takeaway isn’t that sulfur biosignatures are useless, but rather that context matters.

Planetary composition, radiation environment, and atmospheric mixing ratios will determine whether sulfur organics point to life, to photochemistry, or to a blend of both.

By showing that sunlight plus a plausible early atmosphere can knit together multiple sulfur biomolecules at once, the CU Boulder team widens the set of prebiotic sources feeding life’s first chemistry sets. 

The findings suggest that Earth’s skies weren’t just a passive backdrop. They were an active conveyor, delivering ready-made pieces of the metabolic toolkit to the surface long before cells assembled.

The study is published in the journal Proceedings of the National Academy of Sciences.

Image Credit: ESA/Hubble, M. Kornmesser

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–