For roughly the first two billion years there was almost no oxygen in the air. About 2.7 billion years ago, algae in the oceans began to photosynthesise, and plants followed later. Photosynthesis uses carbon dioxide and water to make glucose and releases oxygen.
carbon dioxide + water → glucose + oxygen
6CO2 + 6H2O → C6H12O6 + 6O2
At first the oxygen reacted with materials such as iron in the oceans and rocks, so very little reached the air. Once these were used up, the oxygen level rose slowly over hundreds of millions of years until it reached about 21%, as there were more and more plants and algae. At the same time photosynthesis removed carbon dioxide, so its level fell. The extra oxygen allowed animals that need oxygen for respiration to evolve.