THE origin of our own genus, Homo, has been tentatively correlated with worldwide climatic cooling documented at about 2.4 Myr (million years) (refs 1-5). It has also been conjectured that members of Homo made the first stone tools, currently dated at 2.6 - 2.4 Myr (refs 6-8). But fossil specimens clearly attributable to Homo before about 1.9 Myr have been lacking. In 1967 a fossil hominoid temporal bone (KNM-BC1) from the Chemeron Formation of Kenya was described as family Hominidae gen. et sp. indet. 9. Although a surface find, its provenance within site JM85 (BPRP site K002) was established and a stratigraphic section provided indicating the specimen's position 9. This evidence has been affirmed (see for example refs 10-12) but the exact age of the fossil was never determined, and the absence of suitable comparative hominid material has precluded a more definitive taxonomic assignment. Here we present Ar-40/Ar-39 age determinations on material from the hominid site indicating an age of 2.4 Myr. In addition, comparative studies allow us to assign KNM-BC1 to the genus Homo, making it the earliest securely known fossil of our own genus found so far.