Two chimpanzee fossils, KNM-TH (Kenya National Museum-Tugen Hills) 45519 and KNM-TH 45520, were found in surface context within an area of 12 m 2 within Loc. 99 consists of 80 m 2 of exposures at an outcrop 1 km northeast of site GnJh-19 where hominin mandible KNM-BK (Kenya National Museum-Baringo Kapthurin) 8518 was found. The new chimpanzee fossils were discovered at Locality (Loc.) 99 in K3', the lacustrine facies of the same geological member. Hominin fossils attributed to Homo erectus or Homo rhodesiensis have been found in the fluvial sediments of K3 (refs 11, 13, 14). 7), and the sequence is well calibrated by 40 Ar/ 39 Ar dating. It is divided into five members informally designated K1-K5 (ref. It consists of a package of fluvial, lacustrine and volcanic sediments 125 m thick, exposed over 150 km 2 (refs 7-9) that contains numerous palaeontological and archaeological sites. The Kapthurin Formation forms the Middle Pleistocene portion of the Tugen Hills sequence west of Lake Baringo (Figs 1 and 2). 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. 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. Here we report the first fossil chimpanzee. The Rift Valley itself functions as an obstacle to chimpanzee occupation in some scenarios. 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. This situation has fuelled speculation regarding causes for the divergence of the human and chimpanzee lineages five to eight million years ago. 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. The chimpanzee ( Pan ) is the closest living relative to humans. There are thousands of fossils of hominins, but no fossil chimpanzee has yet been reported. erectus is thought to have lasted around a million years.Author(s): Sally McBrearty (corresponding author) Nina G. In contrast, modern Homo sapiens has been around for only some 200,000 years. If the teeth do belong to the same species as modern chimps, this would mean the species is quite long-lived. "It wouldn't surprise me if there are lots of extinct chimp species," McBrearty says. So far it is impossible to say whether they belonged to the same species as modern chimps, Pan troglodytes, or to some unnamed, now extinct ancestor. The teeth are around 500,000 years old, McBrearty and Jablonski report in Nature 1. The discovery that chimps were living in semi-arid conditions as well as in the jungle seems to blow apart the simplistic idea that it was the shift to savannah that led to humans walking upright. This means we need a better explanation of why and how chimps and humans went their separate evolutionary ways, McBrearty says. But now McBrearty has stumbled on chimp remains east of this divide. Some even suspected that this physical separation was what set the earliest chimp and human ancestors on contrasting evolutionary voyages. Previous theories suggested that chimps never crossed east of the Rift Valley, but instead stayed in the jungles of western and central Africa. Sally McBrearty University of Connecticut Humans, on the other hand, are thought to have lived for millennia on the savannah, where bones are less likely to rot. Part of the problem, McBrearty explains, is that chimps tend to live in hot, wet jungle conditions that are not good for the preservation of remains. Set against the many human fossils found in East Africa, the lack of specimens documenting the chimp's evolutionary story was exasperating. That no one had previously found a chimpanzee fossil had long been a frustrating puzzle, comments Sally McBrearty, an anthropologist at the University of Connecticut, who made the find near Lake Baringo, Kenya, with her colleague Nina Jablonski. The modest haul of just three teeth is the first hard evidence of the evolutionary path that led to today's chimpanzees.Īs well as shedding light on chimps, the find throws up new questions about human evolution it seems that chimpanzees may not have been physically separated from humans as was once thought. Palaeontologists digging in the dusty wastelands of East Africa have discovered the first known chimpanzee fossil. Sally McBrearty unearthed some chimp teeth where no one expected to find them.
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