Why types of organism were hypothesized to be inhabiting earth prior to the rise of oxygen

The organism Cyanobacteria was the first organism to inhabit t he Earth prior to the rise oxygen in the atmosphere.

Explanation:

The single-celled Cyanobacteria were known to be the first oxygen-producing organisms even before the rise of oxygen in the atmosphere or the "Great Oxidation Event," way back 3.5 million years ago. They were able to survive the harsh conditions of the Earth by photosynthesis. During this process, the Cyanobacteria would use sunlight, water and organic carbon dioxide  to produce sugar for their energy. As a waste product, oxygen was released to the atmosphere. Accumulation of the oxygen in the atmosphere was increased by the presence of the volcanic gases like hydrogen, reacting with the oxygen. This caused a reduction reaction and made a more viable oxygen-rich atmosphere.

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