Great Oxygenation Event
Oxygen was toxic to most of the Earth's anaerobic inhabitants at the time. As cyanobacteria produced oxygen, and built their stromatolites, they changed the environment for other protists. Since the other protists had no way to deal with oxygen, most became extinct. Another consequence was the effect of free oxygen on atmospheric methane, a greenhouse gas. Their reaction removed the methane, and caused the Huronian glaciation. This was perhaps the longest snowball Earth episode ever. Free oxygen has been an important part of the atmosphere ever since.