Blue green algae are also called cyanobacteria. They are a very ancient organism and are not related to the more modern green algae. Blue green algae are more similar to bacteria in the internal cell structure. They are classified with the bacteria into a classification grouping called the Protcista.

Blue green algae are thought to be the first organisms on Earth capable of taking energy from Sun and using it to combine carbon dioxide and water to form simple sugars in the biochemical process of photosynthesis. The blue green algae that flourished in the seas of prehistory are thought to be responsible for all of the oxygen in the Earth's atmosphere. If they hadn't existed, it is doubtful that life on Earth would have evolved.

The tiny fossilised remains of blue green algae have been found in rocks that are 3.5 billion years old. They still exist today, virtually unchanged, distributed in aquatic habitats and on the damp surfaces of rocks, trees and soil.