Confirmation of a late middle Pleistocene age for the Omo Kibish 1 cranium by direct uranium-series dating
Introduction
The three fossil humans found in 1967 in the Kibish Formation of the lower basin of the Omo River, southern Ethiopia, come from different locations and stratigraphic contexts (Butzer, 1969; Butzer et al., 1969; Leakey, 1969). Omo 1, a partial skull and associated skeleton was excavated at site KHS in Member I. Omo 2, a calvaria, was a surface find reportedly at site PHS, 2.5 km away on the other side of the Omo River, but allocated to Member I by correlation (however, see below). Omo 3, consisting only of cranial fragments, was apparently derived from Member III. Omo 1 and 3 represent anatomically modern Homo sapiens, but Omo 2 is more archaic in cranial morphology (Day, 1969; Day and Stringer, 1991). Radiocarbon dates for the upper part of the Kibish Formation suggested that Members I–III lay beyond the practical limits of the method, and this was consistent with uranium-series age estimates of 130 ± 5 ka (thousands of years ago) on Etheria shells from Member I, correlated by Butzer (1969) with the stratigraphic position of the Omo 1 skeleton. In turn this suggested that the Omo 1 skeleton was potentially the oldest known example of the modern form of Homo sapiens. However, serious questions were subsequently raised about the reliability of these U-series dates on mollusc material (see e.g., Smith, 1992 and following discussion).
Renewed excavations at the original locality have produced further hominin fossils, including some from the Omo 1 skeleton, recovered in situ (Fleagle et al., 2008; Pearson et al., 2008a, b), and clarified the location and stratigraphic placement of the Omo 2 find (McDougall et al., 2005, McDougall et al., 2008; Brown and Fuller, 2008; Feibel, 2008). Associated dating studies correlated Ar–Ar determinations on feldspar crystals in pumice clasts from Member I with regional fluvial cycles and Mediterranean sapropels, suggesting a revised age of between 172 ka and ∼195 ± 5 ka for Omo 1 and 2 (McDougall et al., 2005, McDougall et al., 2008; Brown et al., 2012). This work was in turn re-evaluated by Millard (2008), who proposed an age somewhat younger than 190 ka, while Klein (2009) raised the possibility that the Omo 1 skeleton was in fact a much younger intrusive burial. Direct dating work on the Omo 1 skeleton is thus of critical importance in confirming or denying that this fossil may represent the oldest known example of anatomically modern H. sapiens.
We have applied U-series analysis to gain further insights to the age of the fossil. U-series dating of bones is seriously compromised by the fact that bones can accumulate large amounts of uranium following their deposition into sediments. A range of models have been developed to account for this uranium uptake and provide a basis for open system dating. The diffusion-adsorption (D-A) model was developed by Millard (1993) and Millard and Hedges (1996), and refined by Pike (2000) and Pike et al. (2002). It is based on laboratory experiments and assumes a continuous diffusion of uranium from the outside of a bone or tooth towards the interior, and that the partitioning between the bone and solution (groundwater) and the U concentration in the solution are constant. The bone is treated as a homogeneous medium. Under constant conditions, the cross sections of bones that conform to the D-A diffusion model are expected to have both u-shaped U-concentration and apparent U-series age profiles, with the apparent ages at the surface being closest to the correct age of the sample. Deviations from such ideal profiles can be explained either by leaching or changes in the U concentration in the solution. The D-A model was recently refined by Sambridge et al. (2012). Their Diffusion-Adsorption-Decay (DAD) model expands the D-A model for diffusion of 234U and its decay during the diffusion process. For a given volume in a bone, the DAD model postulates that 234U is continually resupplied by diffusion. As a result, 234U/238U ratios change little over time, consequently DAD model ages are somewhat older than comparative D-A results. All age results presented here are based on the assumptions underlying the DAD model.
Section snippets
Samples
We have dated two small (∼4 g) parietal fragments (A and B) of the Omo Kibish 1 skull that could not be fitted onto the cranial reconstruction and that were retained in London, first at St. Thomas's Hospital Medical School, and then the Natural History Museum. Fragment A was initially analysed along a single laser track while fragment B was later analysed along a series of laser track on two different planes. For more details on the analysed surfaces see Results and Discussion below.
Experimental
Laser ablation elemental U, Th and U-series isotope analyses were carried out at the Australian National University. For the experimental set-up, see Eggins et al. (2003, 2005), and for applications of U-series dating on human fossils, see Grün et al. (2005, 2006, 2008). U and Th concentrations were derived from repeated measurements of the NBS-610 standard, U-isotope ratios from the dentine of a rhinoceros tooth from Hexian (sample 1118, see Grün et al., 1998).
The ANU Neptune mass spectrometer
Age calculations
During the alpha decay of 238U, a parent atom experiences a strong recoil. In minerals, this recoil leads to a weakened bond between the remaining atoms and the crystal lattice. When minerals are exposed to weathering, 234U atoms are preferably leached. This process leads to the well-known fact that surface and ground waters have excess 234U over 238U (Cherdyntsev, 1971). This has some major implications for the dating of bones. In closed systems, for example for speleothems or corals, U-series
Results and discussion
Initially, sample A was analysed along a single laser track. The apparent U-series ages and U-distributions indicated a rapid U-uptake with a best age estimate of 98−6+8 ka (Fig. 1). We had submitted a paper containing these results in 2005, but unfortunately this coincided with the publication of the Ar/Ar results of McDougall et al. (2005), who had obtained an age estimate on a tuff from Member 1, underlying the human remains, of 195.8 ± 1.6 ka and an estimate of 103.7 ± 0.9 ka for tuff in
Implications
The results presented here illustrate that a wide range of apparent U-series ages can be generated on a particular sample and that careful evaluation of the data is required. The younger apparent ages obtained here appear to be related to higher U-concentrations pointing out to a later overprint of uranium. This is further illustrated by the higher U-concentrations and younger age estimates in the upper plane (sample B) compared with the lower plane. It also appears that the domain that yielded
Acknowledgements
Aspects of this study were supported by Australian Research Council Grants DP0664144 (Grün et al.) ‘Microanalysis of human fossils: new insights into age, diet and migration’ and DP0666084 (Roberts et al.) ‘Out of Africa and into Australia: robust chronologies for turning points in modern human evolution and dispersal’. Chris Stringer is a member of the Ancient Human Occupation of Britain project, funded by the Leverhulme Trust, and his research is supported by the Calleva Foundation.
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Present address: Centre for Archaeological Science, School of Earth and Environmental Sciences, University of Wollongong, Wollongong 2522, Australia.