Answer
The great oxygenation event was triggered by the evolution of blue green algae.
Work Step by Step
Prior to the evolution of blue green algae, the earth's environment was reducing. By releasing oxygen into the atmosphere through photosynthesis, the atmosphere on earth became oxygen rich. The blue green algae used chlorophyll and could utilize photosynthesis as photo-autotrophs, making oxidative metabolism favorable for the future of evolution on earth.
