1. Laboratory for Human Evolutionary Studies, Department of Anthropology, University of California, Berkeley, CA 94720, USA.
2. Institut Royal des Sciences Naturelles de Belgique, 1040 Brussels, Belgium.
3. Department of Anthropology, Indiana University, Bloomington, IN 47405, USA.
4. Department of Geology, Miami University, Oxford, OH 45056, USA.
5. EES-1/D462, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
6. Geochronology Center, Institute of Human Origins, Berkeley, CA 94709, USA.
7. Department of Anthropology, University of Tokyo, 113 Tokyo, Japan.
8. Addis Ababa, Ethiopia.
9. Department of Geology and Geophysics, Yale University, New Haven, CT 0651 1, USA.