According to Leakey, the purpose of the chin eminence is to strengthen the front portion of the jaw. In apes this is accomplished by the simian shelf, a ridge of bone running between the two sides of the forward part of the lower jaw. In Neanderthals, Homo erectus, Homo habilis,
and Australopithecus, none of which have a simian shelf, the strengthening is accomplished by thickening the entire front portion of the jaw.
In making his case, Leakey also considered the presence of a feature of the facial skeleton called the canine fossa. Leakey (1960d, pp. 165–166) stated: “If we look at the facial region of different types of Homo sapiens
we find that . . . there is always present a depression or hollow in the bone beneath each eye, which is called the ‘canine fossa.’. . . In the great apes and in the skulls of human species other than Homo sapiens it is only very rarely seen and is more commonly replaced by a convexity or puffing out of the bone in that region.”
Other anatomical differences between Homo sapiens
and its presumed ancestors, as discussed by Leakey (1960d), involved the tympanic plate around the ear hole, the mastoid process, the articulation of the jaw, and the position of the foramen magnum.
Time, said Leakey, was another problem. Not only was Homo habilis
contemporary with Australopithecus, thus eliminating the latter, in Leakey’s mind, as a human ancestor—there was also trouble with the supposed transition from Homo erectus to Homo sapiens. Leakey (1971, p. 27) wrote: “The textbooks, on the whole, still suggest that Homo sapiens stems from Homo erectus; this view can no longer be sustained. The time interval between Java and Peking man in Asia, or the Olduvai form of Homo erectus in Tanzania, and the appearance of Homo sapiens over a wide area from Europe and east Africa is far too short.” The later specimens of Homo erectus in Java and China, and in the upper levels of Olduvai Gorge, existed from 200,000 to 500,000 years ago in the Middle Pleistocene. Early Homo sapiens, is said to have appeared 300,000 to 400,000 years ago. In other words, Homo erectus and Homo sapiens were roughly contemporary, and this, to Leakey, seemed to eliminate Homo erectus as a human ancestor, although others might suggest that humans branched from Homo erectus far earlier.
Here we are, of course, restricting ourselves to conventionally accepted fossil evidence. In previous chapters, we have argued that the totality of evidence—including the fully modern human skeleton found in a Pliocene formation at Castenedolo in Italy, the advanced stone artifacts and human skeletal remains found in Eocene formations in the California gold country, and much else—does not support an evolutionary origin of the modern human type. If this is correct, then we should not expect the various hominid finds in Africa and elsewhere to line up neatly in an evolutionary sequence. And they do not.
If Australopithecus, Homo erectus,
and the Neanderthals were not human ancestors, then how were they to be explained in terms of evolution? As far as the australopithecines were concerned, Leakey (1960d, p. 180) said it was likely that “they represent a very aberrant and specialized offshoot from the stock which gave rise to man.”