Cyanobacteria began the great oxygenation event (GOE) around 2 billion years ago, after the evolution of photosynthesis to fix carbon dioxide and release oxygen.
This was a significant stage in the development of life on Earth, allowing oxygenic respiration to take place. Oxygenic respiration produces much more ATP per glucose molecule (from photosynthesis) through the electron transport chain, when compared with anaerobic (anoxic) respiration. Higher levels of ATP allowed for more complex life, enabling the broad diversity that is now seen.
Cyanobacteria are able to multiply rapidly, enabling high levels of carbon fixation. In Earth’s early atmosphere, where high concentrations of CO2 were found, growth in the oceans was integral to the GOE.
Today, cyanobacteria provide 10-15% of Earth’s oxygen (although this depends on the weather).