Evolution of Earth's atmosphere

Early volcanic activity is thought to have made an atmosphere of carbon dioxide and nitrogen billions of years ago. Carbon dioxide can dissolve in oceans and form carbonates that, over millions of years, can form sedimentary rock. The amount of carbon dioxide was reduced by the formation of sedimentary rocks and photosynthesis by algae and plants. Algae and plants produced the oxygen that is now in the atmosphere by photosynthesis. Phytoplankton in the oceans are responsible for roughly half of all oxygen production that takes place today.