Australopithecus afarensis is the earliest known hominin, for which we have incontrovertible fossil evidence. This fossil evidence is about 3 to 4 million years old. The most famous example of Australopithecus afarensis is a fossil skeleton called Lucy, who was found at Hadar in Ethiopia. Paleoanthropologists believe that chimpanzees and hominins first began to separate off from one another around 5 to 6 million years ago. So, if there were other hominin species between the separation and the appearance of Australopithecus afarensis, then we haven't found evidence of it yet.