Scientists suggest that the ability to use water as a source of electrons for photosynthesis appeared once in the common ancestor of modern cyanobacteria. The geological record indicates that this event took place in the early stages of Earth’s history, at least 2,450–2320 million years ago, and as some researchers believe, it was much earlier. Geo-biological studies of Archean sedimentary rocks (> 2500 million years ago) indicate that life existed 3,500 million years ago, but the question of when oxygenic photosynthesis remains without a clear answer. The first fossils, which are believed to have imprinted filamentous photosynthetic organisms, were dated to 3.4 billion years. Overall, it is accepted that the paleontological age of cyanobacterial evolution was about 2000 million years ago - a period characterized by a variety of blue-green algae.