Earth’s transition to permanently hosting an oxygenated atmosphere was a halting process that took 100 million years longer than previously believed, according to a new study.

When Earth first formed 4.5 billion years ago, the atmosphere contained almost no oxygen. But 2.43 billion years ago, something happened: Oxygen levels started rising, then falling, accompanied by massive changes in climate, including several glaciations that may have covered the entire globe in ice.