Currently I am translating a book by Richard Leakey. One of the first things to be obvious is that Leakey has a bias; he does not consider any australopithecines to be directly ancestral to Homo sapiens. I can't help but feel that he may be wrong, however he cites studies of their middle ear, posture, rib cage, etc. to back up his position.
On the other hand, Donald Johanson, discoverer of Lucy, seems to be on the opposite camp, by holding that australopithecines did become humans. However I must tell you (embarrassedly) that I haven't read much from him, much less his views against Leakey.
Leakey pointed that there should be another Homo species which gave rise to Homo habilis, which he thought could not have descended from australopithecines. This is maybe his weak point for no such fossil have been found.
Does anybody know more about this?