Billions of years ago, Earth's sky was doing chemistry that scientists thought only living things could do. A new study in the Proceedings of the National Academy of Sciences shows that the young planet's atmosphere was generating sulfur-based molecules—the kind that show up in every living cell today, from bacteria to humans.
Sulfur is everywhere in life. It's woven into amino acids, which are the building blocks of proteins. For decades, scientists assumed these sulfur molecules only appeared after life was already here, produced by organisms themselves. Early attempts to recreate ancient Earth conditions in the lab kept failing to generate meaningful amounts of sulfur biomolecules on their own.
The Sky as a Chemical Factory
Researchers at CU Boulder challenged this assumption by running new experiments. They mixed methane, carbon dioxide, hydrogen sulfide, and nitrogen—gases that existed in Earth's early atmosphere—and exposed them to light, mimicking what the young planet's sky would have done. Using sensitive mass spectrometers, they found something striking: the mixture produced a range of sulfur biomolecules, including cysteine and taurine (both amino acids) and coenzyme M, a molecule crucial for metabolism.
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Start Your News DetoxThis wasn't a fluke. The team ran the numbers on what an entire ancient atmosphere could have produced. Their calculations suggested early Earth's sky generated enough cysteine to support roughly one octillion cells—an enormous amount that dwarfs the roughly one nonillion cells living on Earth today.
The insight shifts how we think about life's origins. These sulfur molecules would have fallen back to Earth through rainfall, essentially seeding the surface with complex chemistry. Life probably still needed specialized conditions—think hydrothermal vents or volcanic zones—to actually begin. But having these more sophisticated building blocks already present, created for free by the atmosphere, would have made the leap to living systems easier.
It's a reminder that the early Earth wasn't a blank slate waiting for life to invent everything from scratch. The planet's own chemistry was already doing much of the heavy lifting.
