Age of the oldest known Homo sapiens from eastern Africa
Efforts to date the oldest modern human fossils in eastern Africa, from Omo-Kibish1,2,3 and Herto4,5 in Ethiopia, have drawn on a variety of chronometric evidence, including 40Ar/39Ar ages of stratigraphically associated tuffs. The ages that are generally reported for these fossils are around 197 thousand years (kyr) for the Kibish Omo I3,6,7, and around 160–155 kyr for the Herto hominins5,8. However, the stratigraphic relationships and tephra correlations that underpin these estimates have been challenged6,8. Here we report geochemical analyses that link the Kamoya’s Hominid Site (KHS) Tuff9, which conclusively overlies the member of the Omo-Kibish Formation that contains Omo I, with a major explosive eruption of Shala volcano in the Main Ethiopian Rift. By dating the proximal deposits of this eruption, we obtain a new minimum age for the Omo fossils of 233 ± 22 kyr. Contrary to previous arguments6,8, we also show that the KHS Tuff does not correlate with another widespread tephra layer, the Waidedo Vitric Tuff, and therefore cannot anchor a minimum age for the Herto fossils. Shifting the age of the oldest known Homo sapiens fossils in eastern Africa to before around 200 thousand years ago is consistent with independent evidence for greater antiquity of the modern human lineage10.
Only eight sites in Africa have yielded possible early anatomically modern Homo sapiens fossils from the late Middle Pleistocene (approximately 350–130 thousand years ago (ka))11. Most of these have considerable age uncertainty or debatable H. sapiens apomorphy11. A principal method for constraining the fossil ages is the use of single-crystal 40Ar/39Ar isotope dating applied to stratigraphically associated volcanic ash (tephra) beds12,13,14. However, many distal tephra deposits consist largely of glass and lack suitable crystals for dating. In this case, geochemical fingerprinting can be used to match a tephra layer to more readily dated proximal deposits with larger, more abundant phenocrysts. The most widely accepted fossils that are interpreted as possessing unequivocal modern cranial apomorphies (that is, a tall cranial vault and a chin) and classified as H. sapiens are two Ethiopian finds11,15,16, namely the Omo I1 and Herto specimens4. Accordingly, the evidence that constrains their ages assumes particular importance but is a topic of considerable geochronological controversy3,6,8.
