Study suggests ancient skies rained down ingredients for life on Earth

Earth’s earliest atmosphere may have done more than shield the planet from the Sun. New research suggests the sky itself helped supply early life with key sulfur-based molecules, long thought to appear only after biology took hold.

A study published in the Proceedings of the National Academy of Sciences reports that Earth’s ancient air likely produced important sulfur compounds before life existed. Scientists at the University of Colorado Boulder and their partners recreated conditions from billions of years ago and found that light and common gases could make molecules tied to today’s living cells.

“Our study could help us understand the evolution of life at its earliest stages,” said Nate Reed, the study’s first author and a postdoctoral fellow at NASA. He completed the research while working at CU Boulder’s Department of Chemistry and the Cooperative Institute for Research in Environmental Sciences.

Sulfur sits beside carbon as a basic building block of life. You find it in amino acids, the pieces that form proteins, and in chemicals that help cells manage energy. For decades, many researchers believed sulfur-based biomolecules emerged later, crafted by life rather than supplied to it.

Earlier experiments struggled to make sulfur compounds without special setups that did not resemble real conditions on young Earth. As a result, the origin of life and the later role of sulfur seemed loosely connected. The new work links them.

The team tested a simple idea. If early air held methane, carbon dioxide, hydrogen sulfide and nitrogen, and if sunlight struck that mix, could useful chemistry follow? To find out, they shone light on those gases in a lab chamber designed to mimic long-ago conditions.

Working with sulfur poses a challenge. It sticks to equipment, and its airborne levels are often tiny compared with nitrogen and carbon dioxide. “You have to have equipment that can measure incredibly tiny quantities of the products,” said Ellie Browne, a chemistry professor at CU Boulder and the project’s senior researcher.

The group used a sensitive mass spectrometer, a tool that can spot minute chemical traces. What they found surprised them. The air mixture produced a range of sulfur-bearing compounds linked to modern biology.

Among the products were cysteine and taurine, two amino acids, and coenzyme M, a molecule tied to metabolism. The team also found hints of methionine and homocysteine, both important to life.

Finding the molecules was only part of the question. The next challenge was scale. Could an entire atmosphere produce enough material to matter?

The researchers used their data to estimate how much cysteine the ancient sky could have added to Earth’s surface. The answer was striking. Their model suggests the air could have supplied cysteine sufficient for about one octillion cells. That is a one followed by 27 zeros.

Today, scientists estimate Earth hosts about one nonillion cells, or a one with 30 zeros. The ancient figure is smaller, but still enormous for a world without life.

“While it’s not as many as what’s present now, that was still a lot of cysteine in an environment without life,” Reed said. “It might be enough for a budding global ecosystem, where life is just getting started.”

The picture that emerges is vivid. The sky could have acted like a vast factory, crafting molecules high above and sending them down in rain. Those deliveries may have stocked early oceans and shores with materials that young life could use.

“Life probably required some very specialized conditions to get started, like near volcanoes or hydrothermal vents with complex chemistry,” Browne 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 study also speaks to the search for life beyond Earth. In 2023, the James Webb Space Telescope detected dimethyl sulfide on a distant planet called K2-18b. On Earth, that gas comes from marine algae. The discovery raised talk of biology.

Yet Reed and Browne earlier showed dimethyl sulfide can form in the lab with only light and common gases. That means such signals may not always point to life. The new study strengthens that caution.

If a planet’s sky can make sulfur compounds on its own, scientists must weigh chemistry as well as biology when reading alien air. The work does not rule out life elsewhere, but it adds context.

It also reshapes how you think about Earth’s past. Instead of a lifeless planet waiting for a spark, the picture now includes a planet already rich in useful tools. The atmosphere may have given early biology a head start.

The results shift how scientists design experiments about life’s origins. Researchers can now test models that include sulfur from the start, which may speed progress in understanding how the first cells formed.

The work also guides future missions that study exoplanet atmospheres. Scientists will refine how they interpret sulfur gases and avoid false alarms.

Over time, this knowledge can improve the search for life and deepen understanding of Earth’s early history, which informs geology, chemistry and biology alike.

Research findings are available online in the journal PNAS.

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