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Oxygen
From PESWiki
Oxygen is a chemical element with the chemical symbol O and atomic number 8. On Earth, it is usually covalently or ionically bonded to other elements.
Unbound oxygen (also called molecular oxygen, or dioxygen, O2, a diatomic molecule) first appeared in significant quantities on Earth during the Paleoproterozoic era (between 2.5 billion years ago and 1.6 billion years ago) as a product of the metabolic action of early anaerobes (archaea and bacteria). The presence of large amounts of free oxygen may have driven most of the organisms then living to extinction.Template:Citation needed The atmospheric abundance of free oxygen in later geological epochs and up to the present has been largely driven by photosynthetic organisms; roughly three quarters of the free element being produced by algae in the oceans, and one quarter from terrestrial plants.
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