Forests typically don't produce fossils, and chimpanzees were assumed to always have been forest-dwelling. Turns out, that's not true:
Nature volume 437, pages105–108 (2005)
First fossil chimpanzee
Sally McBrearty & Nina G. Jablonski
...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.