DETAILED palaeorecords of terrestrial environments are critical for interpretations of the late Miocene mammalian evolution. Here we assess the development of modern tropical grassland (C4) biomes, an important ecosystem for modern faunas. We present enamel apatite C-13/C-12 isotope ratios of fauna from two sequences, the Siwaliks from the Potwar Plateau in northern Pakistan, and the Tugen Hills Succession in Kenya (Fig. 1). Our data do not support previous suggestions of a restricted global date for C4 emergence associated with decreased atmospheric CO2 around 7 Myr ago1. Instead, C4 grasses are first recorded as a dietary component in Pakistan at 9.4 Myr and increase as a foraging resource over the next few million years. In Kenya, C4 grasses are present by 15.3 Myr but are not the primary dietary resource of herbivores until 7 Myr. A wide range of C3 and C4 dietary signals characterizes the latest Miocene/Pliocene of Kenya.