Copyright © 1998 by McGraw-Hill, Inc. All rights reserved. Printed in the United States of America. The entire contents or parts of this Update may be reproduced for use with Physical Anthropology, Sixth Edition, or Physical Anthropology: The Core, second edition, by Philip L. Stein and Bruce M. Rowe, provided each reproduction bears the copyright notice. The publisher's written permission must be obtained for other use.
See Physical Anthropology, 6th edition, Chapter 19, page 492; Physical Anthropology: The Core, 2nd edition, Chapter 13, page 327.
When set beside a series of fossil skulls, the skull of anatomically-modern Homo sapiens sapiens is very distinct, set apart by features such as a large globular cranium, a vertical forehead, no or very slight brow ridges, and a well-developed chin.
Humans are also unique among mammals in lacking facial projection: the face of the adult H. sapiens lies almost entirely beneath and anterior to the cranial fossa, whereas the face in all other adult mammals, including Neandertals, projects to some extent in front of the braincase. Daniel Lieberman uses x-rays and computered tomography to show that many of these unique human features stem partly from a single, ontogenetically-early reduction in the length of the sphenoid, the central bone of the cranial base from which the face grows forward. Sphenoid reduction, through its effects on facial projection and cranial shape, may account for the apparently rapid evolution of modern human cranial form, and suggests that Neandertals and other archaic Homo should be excluded from H. sapiens.
The sphenoid is a very complex bone found between the bones of the cranium and the bones of the facial skeleton. It actually touches 12 other bones of the skull.
Shortening of the sphenoid influences human cranial shape primarily by altering the spatial relationships between the face, cranial base and neurocranium in the sagittal plane, that together determine the degree of facial projection. Statistical analysis of the measurements of the anterior sphenoid body length provide the main structural basis for the differences in facial projection between archaic and anatomically-modern Homo sapiens. This measurement is about 30 percent shorter in anatomically-modern H. sapiens than it is in archaic H. sapiens, including Neandertals.
Facial reduction in modern humans, that results primarily form sphenoid shortening, contributes to other features of the modern human skull including reduction of the browridge and increased globularity of the cranium. Thus, many of the features that distinguish the two groups of hominids are actually the result of a single shift in growth pattern. This also explains how modern humans may have evolved rapidly from more archaic hominids.
While the reason for this change is unknown, Lieberman suggests that it may be an adaptation for speech since the proportions of the human vocal tract would also change as a result of the shortening of the sphenoid.
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Reference: D. E. Lieberman, "Sphenoid Shortening and the Evolution of Modern Human Cranial Shape," Nature, 393 (14 May 1998), 158-162.