In 1967 the Kibish Formation in southern Ethiopia yielded hominid cranial remains identified as early anatomically modern humans, assigned to Homo sapiens 1,2,3,4. However, the provenance and age of the fossils have been much debated5,6. Here we confirm that the Omo I and Omo II hominid fossils are from similar stratigraphic levels in Member I of the Kibish Formation, despite the view that Omo I is more modern in appearance than Omo II1,2,3. 40Ar/39Ar ages on feldspar crystals from pumice clasts within a tuff in Member I below the hominid levels place an older limit of 198 ± 14 kyr (weighted mean age 196 ± 2 kyr) on the hominids. A younger age limit of 104 ± 7 kyr is provided by feldspars from pumice clasts in a Member III tuff. Geological evidence indicates rapid deposition of each member of the Kibish Formation. Isotopic ages on the Kibish Formation correspond to ages of Mediterranean sapropels, which reflect increased flow of the Nile River, and necessarily increased flow of the Omo River. Thus the 40Ar/39Ar age measurements, together with the sapropel correlations, indicate that the hominid fossils have an age close to the older limit. Our preferred estimate of the age of the Kibish hominids is 195 ± 5 kyr, making them the earliest well-dated anatomically modern humans yet described.
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We thank J. Mya, R. Maier and X. Zhang for technical support for the geochronology; participants in the Kibish expeditions between 1999 and 2003, including Z. Assefa, J. Shea, S. Yirga, J. Trapani and especially C. Feibel, B. Passey and C. Fuller for their geological contributions; and R. Leakey, K. Butzer and especially P. Abell for providing us with information and documents about the 1967 expedition to the Kibish area. We thank the Government of Ethiopia, the Ministry of Youth, Sports and Culture, the Authority for Research and Conservation of Cultural Heritage, and the National Museum of Ethiopia for permission to study the Kibish Formation. Support was provided by the National Science Foundation, the Leakey Foundation, the National Geographic Society and the Australian National University. Neutron irradiations were facilitated by the Australian Institute of Nuclear Science and Engineering and the Australian Nuclear Science and Technology Organization.
Results of laser fusion 40Ar/39Ar dating of alkali feldspar crystals from two pumice clasts (mass ~5 g) from Member I, Kibish Formation, west of Omo River at Nakaa'kire, 1 km north of Aliyo, Ethiopia (5°24.59'N, 35°54.52'E).
Results of laser fusion 40Ar/39Ar dating of alkali feldspar crystals from three pumice clasts in a tuff of Member I, Kibish Formation, west of Omo River at Nakaa'kire, near Omo II site, Ethiopia (5°24.6'N, 35°54.5'E).
Results of laser fusion 40Ar/39Ar dating of alkali feldspar crystals from two pumice casts, 99-275A, B, from pumiceous gravels, 0.4 km SSE of KHS, and from two pumice clasts 99-274A, B, 0.5 km SE of KHS, all from Member III, Kibish Formation, Ethiopia.