1. Department of Anthropology, University of Connecticut, Box U-2176, Storrs, Connecticut 06269, USA
2. Department of Anthropology, California Academy of Sciences, 875 Howard Street, San Francisco, California 94103, USA

Correspondence to: Sally McBrearty¹ Correspondence and requests for materials should be addressed to S.M. (Email: mcbrearty@uconn.edu).

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, bipedalism^{2, 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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