Savanna tree evolutionary ages inform the reconstruction of the paleoenvironment of our hominin ancestors

International audienceIdeas on hominin evolution have long invoked the emergence from forests into open habitats as generating selection for traits such as bipedalism and dietary shifts. Though controversial, the savanna hypothesis continues to motivate research into the palaeo-environments of Africa. Reconstruction of these ancient environments has depended heavily on carbon isotopic analysis of fossil bones and palaeosols. The sparsity of the fossil record, however, imposes a limit to the strength of inference that can be drawn from such data. Time-calibrated phylogenies offer an additional tool for dating the spread of savanna habitat. Here, using the evolutionary ages of African savanna trees, we suggest an initial tropical or subtropical expansion of savanna between 10 and 15 Ma, which then extended to higher latitudes, reaching southern Africa ca. 3 Ma. Our phylogenetic estimates of the origin and latitudinal spread of savannas broadly correspond with isotopic age estimates and encompass the entire hominin fossil record. Our results are consistent with the savanna hypothesis of early hominin evolution and reignite the debate on the drivers of savanna expansion. Our analysis demonstrates the utility of phylogenetic proxies for dating major ecological transitions in geological time, especially in regions where fossils are rare or absent or occur in discontinuous sediments. The emergence of savannas and other tropical grassy biomes has been a topic of intense research interest, not least because it coincides with early hominin evolution. The savanna hypothesis of human evolution suggests that the transition from a predominately arboreal lifestyle in forest to one in open habitats favoured an upright posture and selected for bipedalism along with a shift in diet that necessitated travel over greater distances across the landscape 1. The early support for the savanna hypothesis waned, in part, due to confusion as to the definition of prehistoric savannas-as open grassland or as a grassland-tree mosaic-nonetheless, it continues to influence thinking about the selective landscape that shaped human evolution, and generate large interest in the palaeo-environments of Africa where our ancestors emerged 2-6. However, while our understanding of hominin evolution is continually updated by new fossil finds, the palaeontological reconstruction of the ancient African environment has been greatly limited by the sparse record of fossil bones and palaeosols that capture the signature of these past ecosystems 7. open