The history of our planet as recorded in its rocks and fossils, is reflected in the composition the biochemical peculiarities of its present biosphere. With a little imagination one can reconstruct from the appearance and subsequent evolution of gaseous oxygen in the earth's air and water, the changing pathways in the metabolism of living things.
Differentiated multicellular life (consisting of tissue and organs) evolved only after free oxygenappeared inthe atmosphere. The cells of animals that are truly multicellular in this sense, the Metazoa, obtain their energy by breaking down fuel (produced originally by photosynthesis) in the presence оf oxygen in the process called respiration. The evolution of advanced forms of anima life would probably not have been possible without the high levels of energy release that are characteristic of oxidative metabolism. At the same time free oxygen is potentially destructive to all forms of carbon-based life and we know no other kind of life. Most organisms have therefore had to "learn" to conduct their oxidations anaerobically, primarily by removing hydrogen from food-stuff rather than by adding oxygen. Indeed, the anaerobic process called fermentation is still the fundamental way of life, underlying other forms of metabolism.
The origin of life and its subsequent evolution was contingent on the development of systems that shielded it from or provided chemical defenses against ordinary molecular oxygen (О2) and atomic oxygen (0). Yet the energy requirements of higher life forms can be met by oxidative metabolism. The simple sugar glucose oxidation e. g. yields 686 kilocalories per mole.
Free oxygen supports life: it arises from life. The oxygen now in the atmosphere is probably of biological origin. Some of it is converted to ozone, causing certain high energy wavelengths to be filtered out of the radiation reaching the earth's surface. Oxygen also combines with a wide range of other elements in the earth's crust. The result of these and other processes is evolutionary interaction among the biosphere, atmosphere, hydrosphere and lithosphere.
Consider where the oxygen comes from to support the high rates of energy release in multicellular organisms and what happens to it and to C02 that is respired. The oxygen comes fromthe air of which it constitutes roughly 21%. Ultimately, however, it originates with the decomposition of water molecules by light energy in photosynthesis. The 1,5 cubic kilometers оf water on the earth are split by photosynthesis and reconstituted once every two million years or so. Photosynthetically generated oxygen temporarily enters the atmospheric bank, whence it is self recycled once every 2000 years at current rates.
Thus oxygen in free state plays the major role in the evolution and present functioning of the biosphere. The carbon dioxide that is respired joins the small amount already present in the atmosphere which is in balance with the carbon dioxide in the oceans and other parts of the hydrosphere. Through other interactions it may be removed from circulation as a part of the carbonate ion in calcium carbonate precipitated from solution. CO2 sequestered in this way is eventually be returned to the atmosphere when limestone, formed by the consolidation of calcium carbonate sediments, emerges from under the sea and is dissolved by some future rainfall. In this eternal cycle do sea, air and life interact and exchange components.