How oxygen increased

Plants make their own food by photosynthesis. In this process, carbon dioxide is reacted with water to produce glucose, with oxygen as a by-product:

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.

Green algae cells, viewed through using electron microscope
Green algae cells viewed using an electron microscope