Photosynthesis is a chemical reaction that occurs in the cells of the plant leaves. The process requires carbon dioxide from the atmosphere and water as reactants. These reactants get converted into carbohydrates (glucose) and oxygen as byproducts in the presence of sunlight. For the first time on earth the oxygen was produced by the photosynthesizing cyanobacteria. As these organisms used to fix carbon dioxide in water.
Therefore, oxygen was introduced into the atmosphere as a byproduct by the process of photosynthesis.
