Free Oxygen is inferred to have been absent from Earth's atmosphere during the Early Archean Eon.
Explanation:
During the early Archean Eon the free Oxygen was found to be absent. Though water molecules were found and the water molecules contained Oxygen it was found combined with the Hydrogen molecule not as free Oxygen.
Due to the absence of Oxygen only blue green algae was found. But they started to release Oxygen as a waste product. Eventually the Oxygen started to infer into the atmosphere. Due to organic photosynthesis Oxygen accumulated in the atmosphere
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