Synchronous rise of African C4 ecosystems 10 million years ago in the absence of aridification

Grasslands expanded globally during the late Cenozoic and the development of these ecosystems shaped the evolution of many faunal groups, including our hominin ancestors. The emergence of these ecosystems has been dated in many regions, but the origins of the iconic African C-4 savannah grasslands remain poorly known, as do the causal factors that led to their establishment. Here we document their origins with the distinct carbon isotope signature from the hot-, arid- and low-CO2-adapted C-4 grasses that dominate modern savannahs and grasslands. We use the carbon isotope values of leaf-wax molecules in deep-sea drill cores to measure the rise of African C-4 ecosystems. We also reconstruct African palaeohydroclimate change from leaf-wax hydrogen isotope values and dust deposition rates in these cores. We find that C-4-dominated ecosystems expanded synchronously across Northwestern and East Africa after 10 million years ago. This was not accompanied by substantial changes in palaeohydrology or dust deposition, precluding aridification as a causal factor. The expansion of C-4 grasses was coincident, however, with dramatic high-latitude cooling and increased pole-Equator temperature gradients. We suggest that declining atmospheric CO2 levels were a direct cause of the C-4 grassland expansion.