Genetic evidence shows that the chimpanzee (Pan) is the closest living relative to humans. Diligent prospecting by dozens of researchers over the last 30 years has resulted in a fossil record for the human lineage that now numbers in the thousands of specimens. But prior to McBrearty's discoveries in the Kapthurin Formation, Kenya, reported in the Journal Nature in 2005, there were no known fossils of the chimpanzee. The lack of a fossil record for our closest relative had been the object of much speculation. Because living chimpanzee populations inhabit wooded West and Central Africa, while most hominin fossil sites occur in the semiarid East African Rift Valley, a shift from forest to grassland vegetation, perhaps driven by climate change, was thought to have caused the divergence between the human and chimpanzee lineages 5 to 8 million years ago. The Rift Valley itself was thought to present an obstacle to chimpanzee occupation. The Kapthurin Formation fossils show that representatives of Pan were present in the East African Rift Valley about 500,000 years ago, where they were contemporary with an extinct species of Homo. This suggests that the ancestors of humans and chimpanzees may have occupied quite similar habitats, and calls for a reevaluation of the role of vegetation and climate change in the initial divergence between the two lineages. The proposed project will further explore and excavate in the Kapthurin Formation to recover more complete chimpanzee fossil specimens. The research will also clarify the habitat in which the ancient chimpanzees lived, through study of associated fossil remains of other animals, detailed examination of the fossils' geologic context, and stable isotope analysis of carbonates in ancient soil and fossil tooth enamel. Broader impacts include collaboration of University of Connecticut graduate students, local Kenyan workers, and personnel from the Palaeontology Division of the National Museums of Kenya (NMK), all of whom will take part in the research. Their roles will include prospecting for fossils, mapping, and excavation, as well as fossil identification, preparation, and curation. The project is part of a long-term collaboration with the NMK, where the fossils will ultimately be housed.
