Carbon dioxide was the main gas in the early atmosphere, but today it is only a small proportion of the air. The amount of carbon dioxide decreased in several ways.
First, algae and plants used up carbon dioxide in photosynthesis, which decreased the percentage of carbon dioxide in the atmosphere. Second, carbon dioxide dissolved in the oceans and carbonates were precipitated, forming sediment.
Third, carbon was locked up in sedimentary rocks and fossil fuels, which contain carbon. Limestone forms from the shells and skeletons of marine organisms, which are made of calcium carbonate, as they die and are compressed in layers on the sea floor over millions of years. Coal formed from dead plant material that was buried and compressed. Crude oil and natural gas formed from the remains of tiny marine organisms buried in sediment, which were compressed and heated over millions of years.
So the main changes over time were a fall in carbon dioxide and a rise in oxygen, caused by photosynthesis, the oceans, and the formation of sedimentary rocks and fossil fuels.