Dictionary of Human Evolution and Biology

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Atmospheric Oxygen Concentration

Proportion of oxygen in the atmosphere relative to particulate matter and, especially, other gases such as carbon dioxide. In the fossil record, at the time when photosynthetic organisms seem to have radiated, there was an increase in the oxygen concentration of the earth’s atmosphere. Gas ratios are affected by the relative proportions of photosynthetic organisms and herbivores, which increase and decrease atmospheric oxygen, respectively.

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