Cyanobacteria, also known as "blue-green algae", are the unicellular organisms which are able to produce oxygen.
Approximately 540 years ago, the atmosphere was very poor on oxygen. Cyanobacteria are thought to be responsible for the oxidization of the atmosphere back then, in an event known as "The Great Oxygenation Event". As a result, Earth's life forms suffered drastic changes in their composition, as they evolved from being anaerobic to becoming organisms highly dependant on oxygen. The oxygenation of the atmosphere would eventually provide the adequate environment in order for the Cambrian Explosion, or the first diversification of living organisms, to happen.