The decline of atmospheric carbon dioxide over the last 65 million years (Ma) resulted in the ‘carbon dioxide–starvation’ of terrestrial ecosystems and led to the widespread distribution of C4 plants, which are less sensitive to carbon dioxide levels than are C3 plants. Global expansion of C4 biomass is recorded in the diets of mammals from Asia, Africa, North America, and South America during the interval from about 8 to 5 Ma. This was accompanied by the most significant Cenozoic faunal turnover on each of these continents, indicating that ecological changes at this time were an important factor in mammalian extinction. Further expansion of tropical C4 biomass in Africa also occurred during the last glacial interval confirming the link between atmospheric carbon dioxide levels and C4 biomass response. Changes in fauna and flora at the end of the Miocene, and between the last glacial and interglacial, have previously been attributed to changes in aridity; however, an alternative explanation for a global expansion of C4 biomass is carbon dioxide starvation of C3 plants when atmospheric carbon dioxide levels dropped below a threshold significant to C3 plants. Aridity may also have been a factor in the expansion of C4 ecosystems but one that was secondary to, and perhaps because of, gradually decreasing carbon dioxide concentrations in the atmosphere. Mammalian evolution in the late Neogene, then, may be related to the carbon dioxide starvation of C3 ecosystems.