Letters to Nature

Nature 433, 733-736 (17 February 2005) | doi:10.1038/nature03258; Received 22 September 2004; Accepted 8 December 2004

Stratigraphic placement and age of modern humans from Kibish, Ethiopia

Ian McDougall1, Francis H. Brown2 & John G. Fleagle3

  1. Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
  2. Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA
  3. Department of Anatomical Science, Stony Brook University, Stony Brook, New York 11794, USA

Correspondence to: Ian McDougall1 Correspondence and requests for materials should be addressed to I.McD (Email: ian.mcdougall@anu.edu.au).

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 plusminus 14 kyr (weighted mean age 196 plusminus 2 kyr) on the hominids. A younger age limit of 104 plusminus 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 plusminus 5 kyr, making them the earliest well-dated anatomically modern humans yet described.

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