The percentage of carbon dioxide in the atmosphere decreased over time for several reasons. Algae and plants took in carbon dioxide for photosynthesis. When algae and plants died, some of the carbon in them was locked away.
Carbon dioxide was also decreased by the formation of sedimentary rocks and fossil fuels, which contain carbon. Limestone formed from the shells and skeletons of marine organisms. These were made of calcium carbonate, and they built up in layers on the sea bed over millions of years.
Coal formed from dead plant material that was buried and compressed over millions of years. Crude oil and natural gas formed from the remains of tiny marine organisms, which were buried in mud and changed by heat and pressure.
Overall, the main changes were that carbon dioxide fell, oxygen rose, and nitrogen built up to about four-fifths of the air.
