The Late Miocene emergence and proliferation of C4 grasses across eastern Africa is central to current evolutionary interpretations of many mammal lineages, including our hominin ancestors. This model of C4 expansion is supported by densely sampled late Neogene to Quaternary localities, yet a relative paucity of data from Early to Middle Miocene sites has inhibited a full understanding of the origins of C4 grasses in eastern Africa. This study reports the results of a decade-long investigation into nine Early Miocene mammal site complexes across Kenya and Uganda. Our multi-proxy approach combined analyses of plant silica microfossils (phytoliths) with stable carbon isotopic data from soil organic matter, plant waxes, and pedogenic carbonates to document vegetation structure within a robust geochronologic and sedimentologic framework. The results demonstrate clear, multi-proxy evidence for locally abundant C4 grasses at multiple sites between 21 and 16 Ma. Variability in proxy signals both within and between sites suggests that habitat heterogeneity was a common feature across multiple spatial and temporal scales. We applied a Monte Carlo approach to quantify C4 grass abundance using the stable carbon isotope data and demonstrate with >95% confidence that C4 plants have been present since the earliest Miocene (~21 Ma). This finding conflicts with decades of prior work from paleosols and paleodiets but supports molecular clock studies of C4 origins, new phytolith records, and Neogene paleo-CO2 reconstructions. These results should inspire a reanalysis of deep-time dietary reconstructions from fauna in the region, including our hominoid ancestors and their associated faunal communities.
