If life had not evolved on our planet, would our atmosphere be any different?
Yes. There would be less nitrogen, oxygen, and methane, but more carbon dioxide and carbon monoxide. The chemical composition of the Earth’s atmosphere has changed dramatically over time as a result of living organisms. Today about 21 percent of the air we breathe is oxygen. But during the Carboniferous period some 300 mil lion years ago, very high levels of plant activity shot oxygen levels up to 35 percent of the atmosphere.