Despite a large number of Homo fossil finds, chimpanzee fossils (genus Pan) were not described until 2005. Existing chimpanzee populations in West and Central Africa do not overlap with the major human fossil sites in East Africa. However, chimpanzee fossils have now been reported from Kenya. This would indicate that both humans and members of the Pan clade were present in the East African Rift Valley during the Middle Pleistocene.
According to John Gribbin and Jeremy Cherfas in their books The Monkey Puzzle: Reshaping the Evolutionary Tree and The First Chimpanzee: In Search of Human Origins, chimps and bonobos may be descended from Australopithecus. Chimpanzees were amongst the animals affected during the Toba eruption, hinting that Toba ash extended far west as Africa.