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As cyanobacteria created more free oxygen, the amount of oxygen in the atmosphere reached one percent of today’s level, which is 21 percent.

Earth’s early atmosphere contained only small amounts of free oxygen

The oxygen-rich atmosphere that evolved later, and upon which oxygen-breathing life now depends, was a result of the origin of photosynthesis.

During the Precambrian, vast numbers of single-celled algae and cyanobacteria living in the seas eventually released enough oxygen to transform the environment.