Out of Afar: the first hominin migration? Long-term landscape changes in the Afar region and implications for hominin bipedalism

Ian George Smith , Sally Christine Reynolds

Tectonic and geomorphological processes in East Africa have long shaped the environments within which hominins evolved, with the Afar Depression and adjacent Main Ethiopian Rift, created through rifting and plate reorganisation, providing an important setting in which landscape isolation may have influenced evolutionary opportunity. Climate change in Africa has traditionally been regarded as the main driver of adaptations such as hominin bipedalism, often considered separately from the tectonic and geomorphic setting in which it arose. This study evaluates a refugial scenario for bipedalism that situates the evolution of hominin bipedalism within the tectonic and geomorphic history of the Afar region.

A GIS-assisted reconstruction of successive tectonic stages, basin development, and changing drainage patterns, integrating published geological syntheses, DEM-derived topographic context, and selected source-checked locality records, provides a spatial framework for evaluating isolation, faunal migration, and the opening of dispersal corridors. The reconstruction indicates that geological isolation may have altered faunal exchange and predation exposure, contributing to refugial conditions in which upright behaviours could emerge in concert with climate-driven pressures. By ca. 4.2 Ma, the Main Ethiopian Rift corridor, established by ca. 5 Ma, may have become sufficiently permeable to enable southward dispersal, coinciding with the earliest secure evidence for bipedal hominins in the Turkana record. Their emergence in the region, after earlier gaps in the fossil record, may reflect the first hominin migration ‘out of Afar’ and into wider Africa. By combining fossil distributions with GIS-assisted landscape history, this study argues that tectonic and topographic isolation may have been a major, underappreciated component of the context in which early bipedalism evolved, alongside climate-driven pressures.