1. Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, P.O. Box 37012, Washington, DC 20013, USA
2. Human Origins Program, National Museum of Natural History, Smithsonian Institution, P.O. Box 37012, Washington, DC 20013-7012, USA
3. Department of Earth Sciences, National Museums of Kenya, P.O. Box 40658-00100, Nairobi, Kenya
4. Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA