What historical event dramatically changed atmospheric chemistry due to the Biosphere's activity?

Answer

The Great Oxidation Event, where photosynthetic organisms produced free oxygen.

Early life, specifically photosynthetic organisms, fundamentally altered the atmosphere by producing massive quantities of free oxygen, a process known as the Great Oxidation Event.

What historical event dramatically changed atmospheric chemistry due to the Biosphere's activity?
Earthenvironmentsupportsystemlife