Earth’s atmosphere might have contributed to the origin of life more than previously thought.

In a study published Dec. 1 in the "Proceedings of the National Academy of Sciences," CU Boulder researchers and collaborators reveal that billions of years ago, the planet’s early sky might have been producing sulfur-containing molecules that were essential ingredients for life.

The finding challenges a long-held theory that these sulfur molecules emerged only after life had already formed.

“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 as a postdoctoral researcher in the Department of Chemistry and the Cooperative Institute for Research in Environmental Sciences (CIRES) at CU Boulder.

Nate Reed and Ellie Browne (Credit: Patrick Campbell/CU Boulder)

Just like carbon, sulfur is an essential element found in all life forms, from single-cell bacteria to humans. It is part of some amino acids, which are the building blocks of protein.

While the young Earth’s atmosphere contained sulfur elements, scientists had long thought that organic sulfur compounds, or biomolecules like amino acids, emerged later as a product of the living system.

In previous simulations of early Earth, scientists either failed to detect meaningful amounts of sulfur biomolecules before life existed, or created the molecules only under specialized conditions that were unlikely to be widespread on this planet.

As a result, when the James Webb Space Telescope detected dimethyl sulfide, an organic sulfur compound produced by marine algae on Earth, on another planet called K2-18b, many thought it was a possible sign of life on other planets.

But in previous work, Reed and the study’s senior author, Ellie Browne, a chemistry professor and a CIRES fellow, successfully created dimethyl sulfide in their lab using only light and common atmospheric gases. This suggested that this molecule could arise in places void of life.

This time, Browne, Reed and their team set off to see what early Earth’s sky could have contributed. They shone light on a gas mixture containing methane, carbon dioxide, hydrogen sulfide and nitrogen to simulate Earth’s atmosphere before life emerged.

Sulfur is a difficult element to work with in the lab, according to Browne. It tends to stick to all equipment, and in the atmosphere, sulfur molecules tend to exist at very low concentrations compared to CO2 and nitrogen. “You have to have equipment that can measure incredibly tiny quantities of the products,” she added.

Ellie Browne (Credit: Patrick Campbell/CU Boulder)

Using a highly sensitive mass spectrometry instrument that can identify and measure different chemical compounds, Browne’s team found that the early Earth simulation produced a whole suite of sulfur biomolecules, including the amino acids cysteine and taurine, as well as coenzyme M, a compound critical for metabolism.

When the team scaled their lab results to calculate how much cysteine an entire atmosphere could produce, they found that the early Earth’s sky might have brought cysteine to supply about one octillion—one followed by 27 zeros—cells. Currently, Earth boasts about one nonillion—one followed by 30 zeros—cells.  

“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.

The team said these biomolecules formed in Earth’s atmosphere might have fallen onto the ground or oceans with rain, helping to get life started.

“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.”