The challenge of direct dating old human fossils
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This paper reviews the possible approaches for dating of human bones and teeth, older than 500,000 years. Skeletal tissues can, in principle, be dated with an open-system U-series approach, which requires that the Th/U activity ratio is still below equilibrium. This indicates that the sample experienced continuing U-uptake. To gain insights about the state of U-series isotopes in old samples, laser ablation ICP-MS analysis was used for the detailed mapping of U-series isotopes in a 1.2 Ma old horse tooth from the archaeological site of Fuente Nueva III (Orce, Andalusia, Spain). The maps show a complex system of U-migration in and out of the dental tissues. There are clearly domains in the dentine of the tooth where the Th/U ratios indicate continuing U-accumulation. These domains could be extracted and used for open-system U-series dating Firstly, the diffusion-adsorption model could be applied for open-system U-series dating of bones well beyond one million years. The practical difficulty of this approach is whether there are actually any old samples that conform to the D-A model. The horse tooth from Fuente Nueva III, for example, was definitely not suitable for D-A dating. Secondly, a combination of Th/U-Pa/U dating could be applied on bones. When calculating open-system Th/U-Pa/U isotope evolution diagrams with isochrons, it turns out that dating is theoretically possible to perhaps one million years. However, even the presently most precise analytical results would be associated with errors that will prevent dating beyond about 600-700 ka, because the isochrons beyond this time range become very tightly spaced. Lastly, a combination of ESR and U-series analyses can be used in ideal circumstances for dating teeth up to several millions of years of age.