Earth may once have looked a lot like Saturn’s moon Titan, with a methane-rich haze in the atmosphere of the planet. Now, a new study has investigated how long we had this haze for.

According to the study, led by the University of Maryland (UMD) and published in Proceedings of the National Academy of Sciences, more than 2.4 billion years ago Earth had its methane-rich haze for a million years. Previous studies have shown evidence for this haze, but now we have a clearer picture of what happened.

Using detailed chemical records and atmospheric models, the team reconstructed our atmospheric chemistry immediately before the Great Oxidation Event (or Great Oxygenation Event). In particular, they looked at anomalous patterns of sulfur isotopes at the time, which are useful in recreating ancient atmospheric conditions.

This methane would have come from the only life on Earth at the time, bacteria, which filled the air with a haze similar to what we currently observe on Saturn’s moon Titan. There, the atmosphere is so thick that we cannot even see the surface from orbit.

Persisting for just a million years, it was enough to drive away the large amount of hydrogen in our atmosphere, letting oxygen flood it. During the Great Oxidation Event, the concentration of oxygen in Earth’s atmosphere increased about 10,000 times. This ultimately allowed life on our planet to thrive, notably multicellular life.

“The transformation of Earth’s air from a toxic mix to a more welcoming, oxygen-rich atmosphere happened in a geological instant,” said James Farquhar, a professor of geology at UMD and a co-author of the study, in a statement. “With this study, we finally have the first complete picture of how methane haze made this happen.”