Talk abstract: This talk will review current isotopic evidence for primate dietary evolution in Africa. Carbon isotopes have revealed a major dietary transition revealed by carbon isotopes in fossil teeth after 4 million years ago from a concentration of C3 plants (leaves, fruit and nuts from trees, shrubs, and herbs) to diets extended to include tropical C4 plants (grasses and sedges) and/or animals. By the early Pleistocene, multiple primate lineages were consuming significant amounts of C4 foods, and some species relied almost exclusively on these resources. What is the biological significance of this innovation, and its relationship to environmental changes? How did diets change over seasonal to geologic time-scales? How were foods partitioned among hominins, primates, and other mammals? What is the extent of C4 feeding among recent and historical primate populations? I will discuss ongoing projects seeking to address these questions.