Oxygen first entered earths atmosphere when one called organisms resembling modern cyanobacteria began to carry out the process of what

Cyanobacteria were the first autotrophic organisms to evole in this universe. They are also known as blue green algae and contain photosynthetic pigments which carried out the process of photosynthesis and produce oxygen as a by products. In the begining the level of oxygen was not enough to form a part in the atmosphere. As cyanobacteria stared their life cycle the level of oxygen increased to 21%.