<P> Early chemosynthetic organisms likely produced methane, an important trap for molecular oxygen, since methane readily oxidizes to carbon dioxide (CO) and water in the presence of UV radiation . Modern methanogens require nickel as an enzyme cofactor . As the Earth's crust cooled and the supply of volcanic nickel dwindled, oxygen - producing algae began to out - perform methane producers, and the oxygen percentage of the atmosphere steadily increased . From 2.7 to 2.4 billion years ago, the rate of deposition of nickel declined steadily from a level 400 times today's . </P> <P> Another hypothesis posits a model of the atmosphere that exhibits bistability: two steady states of oxygen concentration . The state of stable low oxygen conentration (0.02%) experiences a high rate of methane oxidation . If some event raises oxygen levels beyond a moderate threshold, the formation of an ozone layer shields UV rays and decreases methane oxidation, raising oxygen further to a stable state of 21% or more . The Great Oxygenation Event can then be understood as a transition from the lower to the upper steady states . </P> <P> Another theory credits the appearance of cyanobacteria with suppressing hydrogen gas and increasing oxygen . </P> <P> Some bacteria in the early oceans could separate water into hydrogen and oxygen . Under the Sun's rays, hydrogen molecules were incorporated into organic compounds, with oxygen as a by - product . If the hydrogen - heavy compounds were buried, it would have allowed oxygen to accumulate in the atmosphere . </P>

Where does the bulk of atmospheric oxygen come from