A global increase in C4 photosynthetic plants occurred between 8 and 6 million years ago, as indicated by changes in carbon isotope ratios in fossil tooth enamel from Asia, Africa, North America, and South America. This increase may be linked to a drop in atmospheric CO2 levels below a threshold that favors C3 photosynthesis. The shift was more pronounced at lower latitudes, where the threshold for C3 photosynthesis is higher at warmer temperatures. C3 and C4 plants have distinct carbon isotope ratios, with C3 plants ranging from -22‰ to -30‰ and C4 plants from -10‰ to -14‰. Fossil tooth enamel reflects the C3/C4 composition of mammalian diets. Soil organic matter preserves this distinction with minimal isotopic fractionation, while soil carbonate and biogenic apatite from large mammals show significant enrichment in 13C compared to source carbon. Cerling et al. studied fossil soils and tooth enamel from Pakistan and North America, concluding that C4 ecosystems expanded globally between 7 and 5 million years ago. However, others found no global expansion of C4 biomass in the late Miocene and no link between C3/C4 biomass and atmospheric chemistry. Hill suggested that observed dietary changes in Africa 7 million years ago may be due to faunal immigration rather than vegetation change. This study analyzed over 500 equids and other hypsodont mammals from Asia, Africa, North America, South America, and Europe. Results showed that large mammals 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 mammals in low latitudes had a C4-dominated diet in Africa, South America, North America, and southern Asia. No evidence of a significant C4 component was found in the diets of large mammals from western Europe. The study also found that C4 grasses are favored at lower atmospheric CO2 levels (below 500 ppmv) when accompanied by high growing-season temperatures. The persistence of significant C4 biomass from 6 to 8 million years ago is compatible with declining CO2 levels in the late Miocene below the crossover point where C4 grasses are favored over C3 grasses. The study shows that C4 grasses dominate in tropical and subtropical regions, while C3 grasses dominate at higher latitudes. The transition to C3 grasses occurs between 30° and 45° latitude. The model explains the temporal and spatial changes in diets, showing that the transition from C3 to C4 diets occurred earlier in tropical regions than in higher latitudes. The expansion of C4 grasses was a global phenomenon beginning in the late Miocene and continuing to the present. It was accompanied by important faunal changes worldwide. The expansion was not solely due to higher temperatures or arid conditions butA global increase in C4 photosynthetic plants occurred between 8 and 6 million years ago, as indicated by changes in carbon isotope ratios in fossil tooth enamel from Asia, Africa, North America, and South America. This increase may be linked to a drop in atmospheric CO2 levels below a threshold that favors C3 photosynthesis. The shift was more pronounced at lower latitudes, where the threshold for C3 photosynthesis is higher at warmer temperatures. C3 and C4 plants have distinct carbon isotope ratios, with C3 plants ranging from -22‰ to -30‰ and C4 plants from -10‰ to -14‰. Fossil tooth enamel reflects the C3/C4 composition of mammalian diets. Soil organic matter preserves this distinction with minimal isotopic fractionation, while soil carbonate and biogenic apatite from large mammals show significant enrichment in 13C compared to source carbon. Cerling et al. studied fossil soils and tooth enamel from Pakistan and North America, concluding that C4 ecosystems expanded globally between 7 and 5 million years ago. However, others found no global expansion of C4 biomass in the late Miocene and no link between C3/C4 biomass and atmospheric chemistry. Hill suggested that observed dietary changes in Africa 7 million years ago may be due to faunal immigration rather than vegetation change. This study analyzed over 500 equids and other hypsodont mammals from Asia, Africa, North America, South America, and Europe. Results showed that large mammals 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 mammals in low latitudes had a C4-dominated diet in Africa, South America, North America, and southern Asia. No evidence of a significant C4 component was found in the diets of large mammals from western Europe. The study also found that C4 grasses are favored at lower atmospheric CO2 levels (below 500 ppmv) when accompanied by high growing-season temperatures. The persistence of significant C4 biomass from 6 to 8 million years ago is compatible with declining CO2 levels in the late Miocene below the crossover point where C4 grasses are favored over C3 grasses. The study shows that C4 grasses dominate in tropical and subtropical regions, while C3 grasses dominate at higher latitudes. The transition to C3 grasses occurs between 30° and 45° latitude. The model explains the temporal and spatial changes in diets, showing that the transition from C3 to C4 diets occurred earlier in tropical regions than in higher latitudes. The expansion of C4 grasses was a global phenomenon beginning in the late Miocene and continuing to the present. It was accompanied by important faunal changes worldwide. The expansion was not solely due to higher temperatures or arid conditions but