Broad ethnolinguistic grouping (Somali, Oromo, Afar, Sidama, Beja and related peoples) descended from Proto-Cushitic speakers established on the Ethiopian Highlands by 5000-4000 BCE; pastoralist and highland-agricultural populations of the Horn across the whole span covered by the atlas's Aksum/Dʿmt/Adal/Ajuran-era records.
Not an ending: Cushitic-speaking peoples — Somali, Oromo, Afar, Beja and kin — remain the great nations of the Horn. The map hands off to the states and colonies of the region.
The same author's accessible general history of early Africa, covering the Cushitic-speaking peoples and their East African neighbors, in print from University of Virginia Press.