Significance Dietary change among hominins is a critical aspect of human evolution. Here we use carbon isotope data from fossil teeth of hominins, monkeys, and other mammals from Ethiopia to document C ₄ food consumption by both hominins and the baboon, Theropithecus oswaldi , during the early Pliocene. The expansion of hominin diet and the appearance of the Theropithecus oswaldi lineage as early as 3.76 Ma mark a major ecological change within African primate communities. The ability to eat a range of C ₃ and C ₄ foods indicates that early Pliocene hominins were likely generalists who could thrive in different and perhaps varying environments.