While C4 enhances the efficiency of RuBisCO, the concentration of carbon is highly energy intensive. This means that C4 plants only have an advantage over C3 organisms in certain conditions: namely, high temperatures and low rainfall. C4 plants also need high levels of sunlight in order to thrive. Models suggest that without wildfires removing shade-casting trees and shrubs, there would be no space for C4 plants. But wildfires have occurred for 400 million years – why did C4 take so long to arise, and then appear independently so many times? The Carboniferous period (~300 million years ago) had notoriously high oxygen levels – almost enough to allow spontaneous combustion – and very low CO2, but there is no C4 isotopic signature to be found. And there doesn’t seem to be a sudden trigger for the Miocene rise.

During the Micoene, the atmosphere and climate was relatively stable. If anything, CO2 increased gradually from 14 to 9 million years ago before settling down to concentrations similar to the Holocene. This suggests that it did not have a key role in invoking C4 evolution. Grasses themselves (the group which would give rise to the most occurrences of C4) had probably been around for 60 million years or more, so had had plenty of time to evolve C4, which in any case is present in a diverse range of groups and thus evolved independently. There is a strong signal of climate change in South Asia; increasing aridity – hence increasing fire frequency and intensity – may have led to an increase in the importance of grasslands. However, this is difficult to reconcile with the North American record. It is possible that the signal is entirely biological, forced by the fire- (and elephant?)- driven acceleration of grass evolution – which, both by increasing weathering and incorporating more carbon into sediments, reduced atmospheric CO2 levels. Finally, there is evidence that the onset of C4 from 9 to 7 million years ago is a biased signal, which only holds true for North America, from where most samples originate; emerging evidence suggests that grasslands evolved to a dominant state at least 15Ma earlier in South America.
Source: http://en.wikipedia.org/wiki/Evolution_of_plants

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