Scientists now believe that much of our planet’s atmosphere was generated by carbon dioxide, water vapor, nitrogen, argon, and methane spewing out of volcanoes. When life began to form as primitive plant cells, the carbon dioxide issued from volcanoes was absorbed by these plants and then released as oxygen. At first, the oxygen reacted with iron and other metals in the Earth’s crust, creating iron oxides that form the commonly seen reddish earth in the ground. Eventually, though, there was enough oxygen that it became part of the atmosphere, and breathable air was created.