The Japan Times, May 18, 1998
Nature News Service

One small bone key to 'modern' face

A single developmental change in one of the bones of the braincase may have been sufficient to produce the skull of anatomically modern humans Homo sapiens.
In the May 14 issue of the science journal Nature, Daniel Lieberman of Rutgers University provides this elegant answer to a much debated problem in hominid paleontology: What makes modern human skulls anatomically modern?
To an anatomist, modern human skulls differ from those of archaic Homo species, including Neanderthals, in relatively few features. The skulls of modern humans have a more rounded braincase or cranium, a vertical forehead, small brow ridges, and a pronounced chin. Humans are also unique among mammals, including other hominids, in that the face does not project forward, but lies entirely beneath the front of the cranium.
After analyzing X-rays and computed tomography scans of many fossil hominid and recent human and chimpanzee skulls, Lieberman concludes that most of the major craniofacial differences between humans and other hominids, including the Neanderthals, derive from just one developmental change, a reduction in the length of the spheroid, the central bone of the base of the braincase (the cranium) from which the bones of the face grow forward.
The spheroid is the bone you would reach if you take a horizontal line straight back from the bridge of the nose into the interior until you reach the base of the cranium. Along this dimension, the spheroid in anatomically modern humans is 30 percent shorter than that of Neanderthals, and Lieberman's statistical analysis shows that this is the only important dimension determining face shape that differs significantly between these two groups.
The length of the spheroid determines the position of the back of the facial bone complex relative to the base of the cranium. The reduction in length brings the anatomically modern human face into a more vertical alignment so that it lies entirely under the braincase.
Length of the spheroid is determined early in human development, its growth being almost complete at about 4-6 years of age, and thus it is easy to see how a change in it would be able to affect the growth of other parts of the face. In contrast, other important dimensions of the face are not finally determined until 15-18 years of age.
Decreased facial projection has resulted in the other changes characteristic of modern human skulls, including a reduction in the brow ridges, and a shortening in the length of the oropharynx (throat).
Why a shorter spheroid might have evolved is still pretty much a mystery. The author speculates that the shortened vocal tract could have contributed to the development of speech. The human vocal tract is unique among primates, having almost equal horizontal and vertical dimensions; in other primates the horizontal component is considerably longer than the vertical. This unique configuration improves the ability to produce acoustically distinct speech sounds.
Whatever the cause of the change, Lieberman's analysis suggests that Neanderthals and other archaic Homo species should be excluded from Homo sapiens. It supports the "out of Africa" view of modern human evolution, which holds that all modern humans are more closely related to each other than they are to the archaic Homo species that preceded them in the various regions of Europe and Asia.