Between 8 and 6 million years ago, a global increase in the biomass of plants using C₃ photosynthesis was observed, as indicated by changes in carbon isotope ratios in fossil tooth enamel from Asia, Africa, North America, and South America. This abrupt and widespread increase in C₄ biomass may be linked to a decrease in atmospheric CO₂ concentrations below a threshold that favored C₃ photosynthesizing plants. The change occurred earlier at lower latitudes, as the threshold for C₃ photosynthesis is higher at warmer temperatures. The study analyzed over 500 equids and other hypsodont large mammals from these regions, finding that all samples older than 8 million years had diets compatible with a pure C₃ or C₃-dominated diet. By 6 million years ago, equids and some other large mammals from low latitudes had a C₃-dominated diet in Africa, South America, North America, and southern Asia. No significant C₄ component was found in the diets of large mammals from western Europe at any time. The persistence of significant C₄ biomass from about 6 to 8 million years ago and continuing to the present is consistent with atmospheric CO₂ levels declining below the 'crossover' point where C₄ grasses are favored over C₃ grasses. The expansion of C4 grasses was a global phenomenon, accompanied by significant faunal changes in many parts of the world. The model suggests that the 'C4-world' became established in the late Miocene due to a gradual decrease in atmospheric CO₂ levels, which favored C4 plants over C3 plants in many environments.Between 8 and 6 million years ago, a global increase in the biomass of plants using C₃ photosynthesis was observed, as indicated by changes in carbon isotope ratios in fossil tooth enamel from Asia, Africa, North America, and South America. This abrupt and widespread increase in C₄ biomass may be linked to a decrease in atmospheric CO₂ concentrations below a threshold that favored C₃ photosynthesizing plants. The change occurred earlier at lower latitudes, as the threshold for C₃ photosynthesis is higher at warmer temperatures. The study analyzed over 500 equids and other hypsodont large mammals from these regions, finding that all samples older than 8 million years had diets compatible with a pure C₃ or C₃-dominated diet. By 6 million years ago, equids and some other large mammals from low latitudes had a C₃-dominated diet in Africa, South America, North America, and southern Asia. No significant C₄ component was found in the diets of large mammals from western Europe at any time. The persistence of significant C₄ biomass from about 6 to 8 million years ago and continuing to the present is consistent with atmospheric CO₂ levels declining below the 'crossover' point where C₄ grasses are favored over C₃ grasses. The expansion of C4 grasses was a global phenomenon, accompanied by significant faunal changes in many parts of the world. The model suggests that the 'C4-world' became established in the late Miocene due to a gradual decrease in atmospheric CO₂ levels, which favored C4 plants over C3 plants in many environments.