1. Department of Cell and Developmental Biology, University College London, London WC1E 6BT, UK
2. Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Leipzig 04103, Germany
3. Turkana Basin Institute, PO Box 24926, Nairobi 00502, Kenya
4. Department of Anthropology, Stony Brook University, Stony Brook, NY 11794, USA
5. Centre for Anatomical and Human Sciences, Department of Archaeology and Hull York Medical School, University of York, York YO10 5DD, UK