The answer is - apes.
According to the evolutionary theory the hominids originate from the apes. The first transitional species appeared in the Eastern African plateaus that created the needed evolutionary pressure for certain species to start using their legs more and to become bipedal, and also to develop their mobility of their thumb that had a new role in the newly created environment. The transitional species (from what is known so far) is the Australopithecus, which later on gave rise to all other hominid species like Homo habilis, Homo erectus, and at the end Homo sapiens...