The fact that the oldest known fossils from the Archaean rocks of western Australia are of cyanobacteria is a very strong proof of them being the oldest lifeform on the Earth.
These fossils date back to 3.5 billion years and are found in rocks that are themselves 3.8 billion years old.
These shreds of evidence support the theory stating that cyanobacteria are the oldest living lifeform son the Earth.