Louis Leakey, writing in Nature in 1966 as part of a defense of the Homo habilis definition:
Even though H. erectus and the australopithecines share a number of cranial characters in their occipital bones, and in having a maximum width at the base of the skull, I have never been able to accept the view that Australopithecus represented a distinct ancestral stage leading to H. erectus, and I disagree even more strongly with the present suggestion of placing H. habilis between them. It places a hominid type which has a wholly distinct cranial morphology and one recalling the H. sapiens type between these two. It is possible that H. habilis may prove to be the direct ancestor of H. sapiens, but this can be no more than a theory at present. If true, it would mean that there had been two distinct species of the genus Homo at a very early stage—one leading to H. erectus and subsequent extinction and the other to H. sapiens.
John Robinson had in 1965 published an attack on H. habilis, claiming that the remains could not be distinguished from Australopithecus on the one hand, and that the H. habilis remains provided a possible intermediate toward H. erectus on the other hand. In his view, the situation was best handled by recognizing the gradual transformation of Australopithecus into Homo erectus, and not by erecting a new species between them. Phillip Tobias had responded to Robinson’s attack, but did not disagree with Robinson’s central point that H. habilis was a logical intermediate between Australopithecus africanus and Homo erectus.
Leakey demurred from this point of view, and chose to write his own response to Robinson, in which he pointedly marked his disagreement with Tobias. Leakey could not see H. habilis as an intermediate between Australopithecus and H. erectus. His main argument was that the cranial form of H. habilis shared some derived features with Homo sapiens, whereas H. erectus was much more like Australopithecus in these same respects.
Leakey today is often associated with the concept of a very deep phylogenetic split between humans and the australopithecines. This short article is a very nice illustration of the morphological evidence that drove him toward that hypothesis.