geolgy

Study pinpoints timing of oxygen’s first appearance in Earth’s atmosphere

Study pinpoints timing of oxygen’s first appearance in Earth’s atmosphere

Today, 21 percent of the air we breathe is made up of molecular oxygen. But this gas was not always in such ample, life-sustaining supply, and in fact was largely absent from the atmosphere for the first 2 billion years of Earth’s history. When, then, did oxygen first accumulate in the atmosphere?