There are thousands of fossils of hominins, but no fossil chimpanzee has yet been reported. The chimpanzee (Pan) is the closest living relative to humans1. Chimpanzee populations today are confined to wooded West and central Africa, whereas most hominin fossil sites occur in the semi-arid East African Rift Valley. This situation has fuelled speculation regarding causes for the divergence of the human and chimpanzee lineages five to eight million years ago. Some investigators have invoked a shift from wooded to savannah vegetation in East Africa, driven by climate change, to explain the apparent separation between chimpanzee and human ancestral populations and the origin of the unique hominin locomotor adaptation, bipedalism2,3,4,5. The Rift Valley itself functions as an obstacle to chimpanzee occupation in some scenarios6. Here we report the first fossil chimpanzee. These fossils, from the Kapthurin Formation, Kenya, show that representatives of Pan were present in the East African Rift Valley during the Middle Pleistocene, where they were contemporary with an extinct species of Homo. Habitats suitable for both hominins and chimpanzees were clearly present there during this period, and the Rift Valley did not present an impenetrable barrier to chimpanzee occupation.
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We wish to thank B. Kimeu, N. Kanyenze and M. Macharwas, who found the chimpanzee fossils reported here. Research in the Kapthurin Formation is carried out with the support of an NSF grant to S.M., and under a research permit from the Government of the Republic of Kenya and a permit to excavate from the Minister for Home Affairs and National Heritage of the Republic of Kenya. Both of these are issued to A. Hill and the Baringo Paleontological Research Project, an expedition conducted jointly with the National Museums of Kenya. We also thank personnel of the Departments of Palaeontology, Ornithology and Mammalogy of the National Museums of Kenya, Nairobi; A. Zihlman; and Y. Hailie-Selassie, L. Jellema and M. Ryan for curation and access to specimens. We express gratitude to A. Hill for his comments on the manuscript. We also thank G. Chaplin for drafting Fig. 1, B. Warren for preparing Figs 3 and 4, and A. Bothell for help with submission of the figures. We are grateful to J. Kelley, J. Kingston, M. Leakey, R. Leakey, C. Tryon, A. Walker and S. Ward for discussions. We thank G. Suwa for his remarks.
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Primate phylogeny: molecular evidence for a pongid clade excluding humans and a prosimian clade containing tarsiers
Science China Life Sciences (2012)