The building blocks of life may have rained down from Earth's early atmosphere, according to a recent study that challenges our understanding of life's origins. Researchers at the University of Colorado Boulder have discovered that billions of years ago, the young Earth's atmosphere could have been a factory for sulfur-bearing organic molecules, which are essential for biology. This finding contradicts the long-held belief that such sulfur biomolecules appeared only after life was already established.
Sulfur, alongside carbon, hydrogen, nitrogen, and phosphorus, is one of the essential elements for life. It is found in amino acids like cysteine and cofactors that drive metabolism. While early Earth's atmosphere certainly contained sulfur, scientists previously thought it couldn't generate organic sulfur compounds on its own. Instead, they believed that molecules like those found in amino acids were produced exclusively by living systems.
However, the James Webb Space Telescope's detection of dimethyl sulfide (DMS) on exoplanet K2-18b, a molecule produced by marine algae on modern Earth, challenged this view. In collaboration with Ellie Browne, Nate Reed demonstrated in the lab that DMS can form abiotically from common atmospheric gases under light, suggesting that some 'life-like' sulfur molecules can arise without biology.
In their new study, the team simulated a prebiotic atmosphere by shining light through a gas mixture of methane, carbon dioxide, hydrogen sulfide, and nitrogen, believed to be common before life evolved. Using ultra-sensitive mass spectrometry, they 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.
The team scaled their lab yields to planetary size and concluded that an early Earth atmosphere could have synthesized enough cysteine to provision roughly one octillion cells. While this is less than today's biological census, it still represents a staggering prebiotic stockpile.
The authors argue that these molecules likely rained out onto land and ocean surfaces, seeding the very environments where life is thought to have taken root. This challenges the notion of specialized hotspots, such as volcanic terrains and hydrothermal vents, as the only places where life could have originated.
Instead, the study suggests that some of these more complex molecules were already widespread under non-specialized conditions, which might have made it easier for life to get going. This finding has implications for the search for life signals, as certain 'life signals' may be more ambiguous than hoped, requiring careful consideration of planetary composition, radiation environment, and atmospheric mixing ratios.
The study, published in the journal Proceedings of the National Academy of Sciences, highlights the active role of Earth's early atmosphere in delivering the building blocks of life to the surface long before cells assembled. It invites further exploration of the complex interplay between the atmosphere, chemistry, and the origins of life on our planet.
