Evolution of C4 photosynthesis in response to changes in carbon and oxygen concentrations in the atmosphere through time.

The hypothesis is presented that C4 photosynthesis has evolved at lease in part in response to reduced efficiency of C3 photosynthesis due to increased oxygen partial pressures in our recent atmosphere. Oxygen competition with carbon dioxide for the active site on ribulose diphosphate carboxylase was of little consequence when atmospheric levels of CO2 were high and those of O2 were low. However at the present low CO2 and high O2 partial pressures, adaptations for protecting the enzyme from oxygen have been selected.