Climate change and the evolution of C4 photosynthesis

Plants assimilate carbon by one of three photosynthetic pathways, commonly called the C3, C4, and CAM pathways. The C4 photosynthetic pathway, found only among the angiosperms, represents a modification of C3 metabolism that is most effective at low concentrations of CO2. Today, C4 plants are most common in hot, open ecosystems, and it is commonly felt that they evolved under these conditions. However, high light and high temperature, by themselves, are not sufficient to favor the evolution of C4 photosynthesis at atmospheric CO2 levels significantly above the current ambient values. A review of evidence suggests that C4 plants evolved in response to a reduction in atmospheric CO2 levels that began during the Cretaceous and continued until the Miocene.