The discovery of Australopithecus anamensis fossils1 from strata lying between tephra dated at 4.17 and 4.12 million years ago, and from slightly higher strata not well constrained in age by overlying dated units, provoked the claim that more than one species might be represented: it was suggested that the stratigraphically higher fossils, which include the important tibia, humerus and a large, presumed male, mandible (KNM-KP 29287), might belong to a later, more derived hominid2. We have recovered new fossils from Kanapoi and Allia Bay, Kenya, during field work in 1995–1997 that confirm the primitive status of Australopithecus anamensis, the earliest species of Australopithecus. Isotope dating confirms A.anamensis' intermediate age as being between those of Ardipithecus ramidus3,4 and Australopithecus afarensis5,6. New specimens of maxilla, mandible and capitate show that this species is demonstrably more primitive than A. afarensis. A lower first deciduous molar (dm1) is intermediate in morphology between that reported for Ardipithecus ramidus4 and A.afarensis7. Single-crystal 40Ar–39Ar age determinations on the Kanapoi Tuff show that, except for a large mandible, all of the hominid fossils from Kanapoi are from sediments deposited between 4.17 ± 0.03 and 4.07 ± 0.02 million years ago.
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We thank the Government of Kenya for permission to carry out this research and the National Museum of Kenya for logistical support. The National Geographic Society funded the field work; the National Science Foundation and the Leakey Foundation awarded grants to C.F. and A.W. Neutron irradiations were facilitated by the Australian Institute of Nuclear Science and Engineering and the Australian Nuclear Science and Technology Organization. We thank Caltex (Kenya) for fuel and Rover Group and the NGS for vehicles, and T. White and F. Spoor for their help. The expedition included K.Cheboi, S. Dominic, G. Ekalale, M. Eregae, J. Erus, N. Ewalan, J. Kaatho, C. Kiarie, K. Kimeu, F. Kyalo, M. Kyeva, B. Kyongo, L. Leakey, N. Lowis, S. Makathimo, B. Malika, W. Mangao, S. Milledge, L. Moru, M.Muoka, J. Mutaba, D. Mutinda, N. Mutiwa, S. Ngui, B. Onyango and J. Wynn; S. Hagemann and L.Hlusko assisted at Allia Bay; J. Mya undertook the mineral separations; R. Maier assisted with handling of samples for irradiation; and A. Ibui, F. Kyalo, E. Mbua, M. Muungu and A. Mwai provided curatorial assistance.
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Human Evolution (2001)