February 16, 2005

Older dates for Omo

This story in Nature neatly summarizes the redating of the Omo skulls from Ethiopia. The two skulls have now been dated to 195+/-5 thousand years ago, which makes them the earliest known fossils of Homo sapiens. This new discovery comes to upset the recent discovery of the Homo sapiens idaltu subspecies which was date to 160 thousands years ago. Unlike the idaltu fossils which were primitive, and were postulated to have belonged to an earlier form of the lineage leading to modern humans, the Omo skulls are fully modern anatomically, and this places them as the oldest members of our species.

Nature 433, 733 - 736 (17 February 2005); doi:10.1038/nature03258

Stratigraphic placement and age of modern humans from Kibish, Ethiopia


In 1967 the Kibish Formation in southern Ethiopia yielded hominid cranial remains identified as early anatomically modern humans, assigned to Homo sapiens. However, the provenance and age of the fossils have been much debated. 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 II. 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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