The study investigates the assembly of plant diversity in the Cerrado, a tropical savanna in South America, by examining the evolution of adaptations to fire. Time-calibrated phylogenies suggest that Cerrado lineages began diversifying less than 10 million years ago, with most lineages diversifying around 4 million years ago, coinciding with the rise of flammable C4 grasses and the expansion of the savanna biome. The phylogenies show that Cerrado lineages are strongly associated with adaptations to fire, and their sister groups are found in nearby wet forest, seasonally dry forest, subtropical grassland, or wetland vegetation. These findings indicate that the Cerrado formed through recent and frequent adaptive shifts to resist fire rather than through dispersal of already adapted lineages. The Cerrado's location among diverse biomes and the modest adaptive barrier posed by fire likely contributed to its high species richness. The study highlights the idiosyncratic nature of the origins and historical assembly of species-rich biomes, driven by unique regional and continental-scale geohistory and evolutionary processes.The study investigates the assembly of plant diversity in the Cerrado, a tropical savanna in South America, by examining the evolution of adaptations to fire. Time-calibrated phylogenies suggest that Cerrado lineages began diversifying less than 10 million years ago, with most lineages diversifying around 4 million years ago, coinciding with the rise of flammable C4 grasses and the expansion of the savanna biome. The phylogenies show that Cerrado lineages are strongly associated with adaptations to fire, and their sister groups are found in nearby wet forest, seasonally dry forest, subtropical grassland, or wetland vegetation. These findings indicate that the Cerrado formed through recent and frequent adaptive shifts to resist fire rather than through dispersal of already adapted lineages. The Cerrado's location among diverse biomes and the modest adaptive barrier posed by fire likely contributed to its high species richness. The study highlights the idiosyncratic nature of the origins and historical assembly of species-rich biomes, driven by unique regional and continental-scale geohistory and evolutionary processes.