Oxygen began to accumulate in the atmosphere during the Proterozoic Era, which began about two billion years ago and ended about 700 million years ago. Much of this oxygen arose from the actions of the cyanobacteria.Continue Reading
During the Hadean Era, which started 4.6 billion years ago, Earth's atmosphere consisted mainly of hydrogen and helium. About a billion years later in the Archaean Era, oxygen was found in the gases coming from volcanoes.
Earth then began to cool down, allowing water to form. Ultraviolet radiation from the sun caused water to break apart into its constituent hydrogen and oxygen atoms, and oxygen levels in the atmosphere rose to about 1 to 2 percent. Organisms called cyanobacteria started appearing on Earth and making their own food through photosynthesis. Oxygen gas is a byproduct of photosynthesis. By the end of the Proterozoic Era, oxygen levels had increased to about 10 percent.Learn more about Atmosphere