What primary chemical reaction do scientists suggest for the evolution of an aerobic environment to an anaerobic one?

Oxygenic photosynthesis

Explanation:

The process of oxygenic photosynthesis in which organisms use atmospheric carbon dioxide and fix it in the presence of sunlight into the organic nutrients led to the transition of anaerobic conditions on the primitive earth into the aerobic ones.

The oxygenic photosynthesis uses a water molecule as an electron donor. The water molecule is oxidized and oxygen gas is released as a by-product. This process led to the accumulation of oxygen in the atmosphere of the primitive earth.  

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