carbon dioxide + water → glucose + oxygen
6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g)
Scientists think that algae first evolved approximately 2.7 billion years ago, and soon after this oxygen began to exist in the atmosphere. Photosynthesis by primitive plants and algae released oxygen, which gradually built up in the atmosphere. Eventually, the amount of oxygen present in the atmosphere enabled animals to evolve.