Earth’s Ancient Sky May Have Supplied Ingredients for Life Before It Began

Earth’s first atmosphere may have done more than surround the planet: it may have helped make life possible. A new study published in Proceedings of the National Academy of Sciences suggests that billions of years ago, the prebiotic sky could have generated sulfur-based biomolecules essential for life, challenging the long-held view that these compounds only emerged after living systems took hold.

Rather than being confined to rare and extreme environments, some chemical elements of life may have been widespread in ordinary atmospheric conditions.

“Life likely required very specialized conditions to get started, such as near volcanoes or hydrothermal vents with complex chemistry,” Ellie Browne, the study’s lead author, said in a press release. “We used to think that life had to start from scratch, but our results suggest that some of these more complex molecules were already widespread in non-specialized conditions, which might have made it a little easier for life to get started.”

Why sulfur is central to the origin of life

Sulfur is at the heart of modern biology. It stabilizes proteins, helps enzymes do their job, and plays a central role in metabolism. Yet in most models of the origin of life, sulfur chemistry comes into play relatively late, once living systems are already operational.

This hypothesis has also shaped how sulfur is interpreted beyond Earth. When the James Webb Space Telescope detected dimethyl sulfide in the atmosphere of K2-18b, the molecule attracted attention because it is closely related to marine life on our planet. But recent laboratory work shows that the same compound can form without any life, raising the possibility that sulfur chemistry was already active on Earth before biology began.

Simulate the Earth’s first atmosphere

To test whether sulfur biomolecules could form before life existed, the researchers recreated a version of Earth’s early atmosphere using a mixture of methane, carbon dioxide, nitrogen and hydrogen sulfide – gases that would have been common before oxygen accumulated in the air. The mixture was then exposed to ultraviolet light to simulate solar radiation under prebiotic conditions.

Tracking sulfur chemistry under these conditions is notoriously difficult. These compounds form in extremely small amounts and tend to stick to laboratory surfaces before they can be measured.

“You need to have equipment that can measure incredibly tiny quantities of product,” Browne said in the press release.

Using sensitive mass spectrometry, the simulated atmosphere produced a range of sulfur biomolecules. These included the amino acids cysteine ​​and taurine, as well as coenzyme M, a compound essential to the metabolism of some living systems today.

When the results were extended to the size of Earth’s atmosphere, the early sky could have generated enough cysteine ​​to support the order of an octillion cells. Although this figure is much lower than the estimated number of cells on the planet today, it represents an important chemical reservoir for a world that has not yet crossed the frontier of biology.

“Although it’s not as much as what’s present now, it was still a lot of cysteine ​​in a lifeless environment,” Reed added. “This could be enough for a nascent global ecosystem, where life is just beginning.”

Rather than remaining suspended in the air, the newly formed molecules would likely have returned to the surface with rain, dumping sulfur-rich material into oceans, lakes and shallow waters where further chemical evolution could take place.

What Earth’s Early Atmosphere Means for Life on Other Planets

If Earth’s atmosphere can build these lifeless molecules, similar chemistry could also work on other worlds, complicating how scientists interpret chemical signs beyond our planet.

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

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