The Early Origins of Terrestrial C₄ Photosynthesis

The C 4 photosynthetic pathway is a series of structural and biochemical modifications around the more primitive C 3 pathway that improve the photosynthetic efficiency under specific climatic conditions. Hence, the origin and subsequent geographical expansion of the C 4 plants likely reflects a record of climate change. Multiple paleoatmospheric pCO 2 proxies indicate a critical CO 2 threshold was breached ∼30 Ma, that potentially selected for CO 2 -concentrating mechanisms to overcome photorespiratory stresses imposed on the basic C 3 pathway. Details of the C 4 pathway's earliest origins remain enigmatic given the paucity of the geologic record. Nonetheless, δ 13 C proxy records from paleosol carbonates, ungulate teeth, and plant-derived compounds indicate C 4 plants likely represented an important component of plant biomass by the Early Miocene. Low CO 2 levels appear to be a precondition for the development of the C 4 photosynthetic pathway; however, comparisons of δ 13 C proxy records indicate that the timing of C 4 geographical expansion was not globally synchronous, and thus point toward more regional controls on the development of C 4 -dominated ecosystems. Terrestrial and marine records indicate that continental aridity and wind strength increased during the Late Miocene. These conditions would have likely increased fire occurrence and fire intensity leading to the clearing of large tree stands and the expansion of C 4 grasses in warm-season precipitation regimes.