The rift valley takes shape

Through the Pliocene the East African Rift deepens into the recognisable modern system of escarpments, floor basins and volcanic centres. Its geology is what makes the human fossil record possible: rapidly subsiding basins bury bones quickly, interbedded volcanic ash layers can be dated by argon-argon to within a few thousand years, and later uplift and erosion cut the sequences open again. Almost every early hominin fossil comes from this narrow strip of Africa — not because hominins lived only there, but because that is where the rocks preserve and reveal them.