Explanatiion : Beginning of the the Archean Eon, was a major time of ocean formation and availability of water molecules which were bonded to hydrogen atoms.
There was a lack of oxygen in the atmosphere eventhough it existed in water molecules. The available organisms were cyanobacteria which were blue- green algae. Eventhough oxygen wasn't there cyanobacteria were able convert sunlight to energy. This is when the photosynthesis took place , when they made their own food in the ocean and they secreted oxygen .
