Earth’s ancient sky may have played a larger role in the beginnings of life than scientists once believed.
According to a study published Dec. 1 in the Proceedings of the National Academy of Sciences, researchers from CU Boulder and their collaborators report that billions of years ago, the young planet’s atmosphere may have been generating sulfur-based molecules that are known today as important components for life.
This discovery challenges the long-standing idea that these sulfur molecules formed only after life had already taken hold on Earth.
“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 research while working in the Department of Chemistry and the Cooperative Institute for Research in Environmental Sciences (CIRES) at CU Boulder.
Sulfur’s Importance and Why the Findings Matter
Sulfur, much like carbon, is a vital element found in every form of life, from bacteria to humans. It appears in certain amino acids, which serve as the basic building blocks of proteins.
Although sulfur was present in the early atmosphere, most scientists believed that organic sulfur molecules, such as amino acids, arose only after living organisms were already present and producing them.
Earlier attempts to simulate early Earth conditions often failed to generate meaningful amounts of sulfur biomolecules before life existed. When these molecules did appear, they formed only under unusual or highly specific conditions that were unlikely to have been common across the planet.
Because of this background, the scientific community reacted strongly when the James Webb Space Telescope detected dimethyl sulfide, a sulfur compound produced by marine algae on present-day Earth, in the atmosphere of an exoplanet called K2-18b. Many considered it a possible sign of life.
New Experiments Reveal Atmospheric Chemistry at Work
However, previous work by Reed and senior author Ellie Browne, a chemistry professor and CIRES fellow, showed that dimethyl sulfide could form naturally in the lab using only light and simple atmospheric gases. This indicated that the molecule might appear even on worlds without life.
In their latest experiment, Browne, Reed, and their team tested what Earth’s early sky might have been capable of producing. They illuminated a mixture of methane, carbon dioxide, hydrogen sulfide, and nitrogen to recreate atmospheric conditions from before life emerged.
Working with sulfur is challenging, Browne noted. The element sticks to laboratory equipment, and in the atmosphere, sulfur-based molecules are present at extremely low levels compared to CO2 and nitrogen. “You have to have equipment that can measure incredibly tiny quantities of the products,” she said.
Using a very sensitive mass spectrometer to identify and measure chemical compounds, the researchers discovered that their early Earth simulation produced a wide range of sulfur biomolecules. These included the amino acids cysteine and taurine, along with coenzyme M, which plays a key role in metabolism.
A Sky Capable of Supporting a Growing Ecosystem
The team then estimated how much cysteine an entire ancient atmosphere might generate. Their calculations suggested that early Earth’s sky could have produced enough cysteine to support about one octillion (one followed by 27 zeros) cells. By comparison, modern Earth contains roughly 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 researchers propose that these atmospheric biomolecules may have fallen to the surface through rainfall, potentially delivering the chemistry needed to help life begin.
“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.”
