The origin of the African savanna has been traced back to 10–6 million years (Ma) ago, but the mechanisms driving its evolution are hotly debated, and include global atmospheric CO2, regional fire activity, herbivore competition, and hydrological climate change. Here, we present the first microcharcoal-based fire activity records covering the last ~7 Ma at four International Ocean Discovery Program (IODP) sites near southern Africa. The records show that fire activities in both savanna and non-savanna regions were stable during this interval. Grass vegetation in burnt biomass continued to expand from 6 Ma into the present savanna region, whereas no grass expansion was observed in non-savanna regions. A compilation of regional data suggests asynchronous C4 grass expansion since 6 Ma on the African Continent. We consider that CO2 concentrations and wildfires might have caused the first appearance of C4 plants at ~10 Ma and ~ 7–6 Ma, respectively. Since 6 Ma, the regional climate promoted expansion of C4 plants into the present-day savanna habitat. •Microcharcoals in four IODP boreholes surrounding the southern Africa are studied.•Microcharcoal influxes show the stable wildfire trends since 6 Ma ago.•Microcharcoal morphologies show the C4 vegetation expanded along specific routes.•The C4 expansion is attributed to the regional climate rather than CO2 or wildfire.