Answer:
They released oxygen that is essential for the formation of the atmosphere that allows us to be alive.
Explanation:
Cyanobacteria are bacteria that, like plants, photosynthesize, a process in which plants absorb carbon dioxide (CO2) and release oxygen (O2). Millions of years ago the first cyanobacteria were responsible for the formation of the atmosphere that allows the formation of more complex life, since they released the oxygen gas by doing photosynthesis. Oxygen allowed more complex life forms to emerge.
