The Earth was a suffocating place until oxygen in the atmosphere slowly began to climb from almost nonexistent levels about 2.5 billion years ago.
Breathable air is thought to have been created by cyanobacteria, single-celled microbes living in the sea.
These bacteria harnessed the energy of the sun through photosynthesis-the biochemical process used by plants-producing oxygen as a by-product.
The oxygen-rich ozone layer was also established, shielding the Earth's surface from harmful solar radiation.
Photosynthesis Unleashes Chemical Superweapon: Oxygen
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