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The distribution of C3 versus C4 plants in the modern world is related to temperature. However, because of the sensitivity of C3 to the CO2/O2 ratio of the atmosphere it is expected that global changes in the distribution of C3 versus C4 plants should accompany atmospheric CO2 changes in geological time. The period from 6 to 8 million years ago was a time of C4 expansion in many parts of the globe.