The uplift of the Tibetan Plateau plays a critical role in controlling global climate, yet the history of the Tibetan uplift is still a contentious issue. In particular, the elevation of the plateau during the Neogene—a crucial period in the development of the Asian monsoons and C4 ecosystems—remains uncertain. Here we present carbon isotopic evidence, preserved in tooth enamel from 7-m.y.-old horses and rhinos from the high Himalayas, which indicates that, unlike modern herbivores in the area, these ancient mammals ate substantial amounts of C4 grasses. The presence of significant amounts of C4 grasses in the diets of these ancient mammals indicates that the climate in the area was much warmer and the elevation was much lower in the late Miocene than today. The carbon isotope data from the high Himalayas, after accounting for late Cenozoic global cooling and paleoatmospheric CO2 levels, indicate that this part of southern Tibet was less than 2900–3400 m above sea level in the latest Miocene. This implies that the present elevation of the area must have been attained after 7 Ma, much later than generally believed.