Context

• Great Oxidation Event refers to a time period when the Earth’s atmosphere and the shallow ocean experienced a rise in oxygen,approximately 2.4 billion years ago (2.4 Ga) to 2.1–2.0 Ga during the Paleoproterozoic era.
• The appearance of free oxygen in Earth’s atmosphere led to the Great Oxidation Event.

More details

• This was triggered by cyanobacteria producing oxygen that was used by multicellular forms.
• The biologically produced molecular oxygen started to accumulate in Earth’s atmosphere and changed Earth’s atmosphere from a weakly reducing atmosphere to an oxidizing atmosphere, causing many existing species on earth to die out.
• Cyanobacteria belong to Earth’s oldest organisms and are still present today in oceans and waters and even in hot springs.
• By producing oxygen and evolving into multicellular forms, they played a key role in the emergence of organisms that breathe oxygen.