Answer :

Photosynthesis released oxygen which transformed the earth to an oxidizing atmosphere.

What is photosynthesis?

The term photosynthesis refers to the process by which carbon dioxide and water combines in the presence of sunlight in the chlorophyll of a green plant to release glucose  and oxygen.

We know that the earth was a predominantly reducing atmosphere. However, photosynthesis released oxygen which transformed the earth to an oxidizing atmosphere.

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