Cyanobacteria have an extremely long fossil record, starting at least 3,500 million years ago. They were the main organisms in the stromatolites of the Archaean and Proterozoiceons.
The ability of cyanobacteria to perform oxygenic photosynthesis is highly significant. The early atmosphere on Earth was largely reducing, that is, without oxygen. The cyanobacteria in stromatolites were the first known organisms to photosynthesise and produce free oxygen. After about a billion years, the effect of this photosynthesis began a huge change in the atmosphere. The process, called the Great Oxygenation Event, took a long time. Eventually, it killed off most of the organisms which could not live in oxygen, and led to the kinds of environment we know today, where most organisms use and need oxygen.