How did oxygen gas accumulate in Earth's atmosphere following the evolution of photosynthesis?

The atmosphere of early Earth contained no oxygen, so organisms at the time were necessarily anaerobic. When the noncyclic pathway of photosynthesis evolved, oxygen gas (O₂) released from uncountable numbers of water molecules began seeping out of photosynthetic prokaryotes. O₂ easily removes the electrons from (oxidizes) the molecules that carry electrons during metabolism in living cells, thereby interfering with their function and also producing dangerous free radicals in the process. Most cells had no way to counter these effects of oxygen and so were wiped out everywhere except deep water, muddy sediments, and other habitats that remained anaerobic. A few types of cells were already making antioxidants that could prevent oxidative damage cause by O₂. They were the first aerobic organisms-they could live in the presence of oxygen. As these organisms evolved in environments with abundant oxygen, so did their metabolic pathways. Today, most cellular respiration uses oxygen.