Abstract
Efforts to date the oldest modern human fossils in eastern Africa, from Omo-Kibish1–3 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.
Highlights
The Omo I remains were discovered in the late 1960s in the lower Omo valley of southern Ethiopia[1,14], at the surface of a siltstone near the top of Member I of the Omo-Kibish Formation (Fig. 1a, b)
The maximum age of Omo I was derived from the 40Ar/39Ar age of 196 ± 4 kyr (2σ)[3,6,17] obtained for alkali feldspar phenocrysts from the three youngest pumice clasts that were sampled from a heterogeneous tuffaceous deposit correlated with the Nakaa’kire Tuff[3], which is reported to lie “near, but probably slightly below” the fossils[3] (Fig. 1b)
Owing to the uncertain stratigraphic relationship between this tuff and the hominin fossils[19], much attention has been focused on dating the Kamoya’s Hominid Site (KHS) Tuff—a widespread, more-than-2-m-thick deposit of fine ash fallout at the base of Member II of the Omo-Kibish Formation (Fig. 1b)
Summary
Efforts to date the oldest modern human fossils in eastern Africa, from Omo-Kibish[1,2,3] and Herto[4,5] in Ethiopia, have drawn on a variety of chronometric evidence, including 40Ar/39Ar ages of stratigraphically associated tuffs. 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 apomorphy[11]. Owing to the uncertain stratigraphic relationship between this tuff and the hominin fossils[19], much attention has been focused on dating the KHS Tuff—a widespread, more-than-2-m-thick deposit of fine ash fallout at the base of Member II of the Omo-Kibish Formation (Fig. 1b). Relating the sediment flux in the Omo-Kibish basin with high lake levels that
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call