The Oxygen Catastrophe was the appearance of dioxygen in the atmosphere at around 2.3bya. This caused the mass extinction of anaerobic monerans because the dioxygen is toxic to anaerobic prokaryotes. The oxygen in the air reacted with large amounts of methane and greatly cooled down the atmosphere of Earth. Glaciation occurred and possibly caused a snowball Earth, or an Earth that was almost completely covered in ice. Since the Great Oxygen Catastrophe, life on Earth would not be possible without dioxygen.