We explored backgrounds of the cladogenesis of the living African apes and human as well as adaptations of their last common ancestor by the fieldwork of the late Miocene Nakali Formation, which yields a rich primate fauna including a great ape Nakalipithecus. During 2013-15, about 500 fossil specimens were collected, 11 new fossil sites were spotted, and the geological mapping of this area was almost completed. While primate taxa of Nakali were quite diversified, they were small-bodied except apes. The paleoenvironment of Nakali was certainly forest elements-dominated. The primate fauna of Nakali consists of primitive and modern groups. This suggests that the transition to the genuine modern primate fauna occurred in East Africa after the age of the Nakali Formation, probably triggered by the environmental changes and the radiation of modern cercopithecoids.