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 pd èBssion must be obtained for other use.
See Physical Anthropology, 6th edition, Chapter 16, pages 427-430; Physical Anthropology: The Core, 2nd edition, Chapter 11, pages 279-280.
One of the most significant differences between the hominids and other animals is brain size. Paleoanthropologists have always been interested in reconstructing the evolution of the brain. Since brains are not preserw á in the fossil record, one of the most important attributes available for the study of brain evolution is cranial capacity, the volume of the cranium or braincase. Needless to say, the idea that most currently published cranial capacities may be wrong could result in a major rewriting of the story of hominid evolution.
Sts 505 is an Australopithecus africanus cranium discovered in 1989 at Sterkfontein. It dates from around 2.8 to 2.6 million years B.P. This relatively large cranium, presumably! íLt of a male, has been named "Mr. Ples," a reference to one of the earliest and most complete A. africanus crania, Sts 5, discovered by Robert Broom in the 1930s and named "Mrs. Ples."
At the time of its discovery, the cranial capacity of Sts 505 was given as exceeding 600 cubic centimeters. This would make it the largest australopithecine cranial capacity known, one that even exceeds the cranial size of several early members of the genus Homo.
There is an!çXious need to verify this figure, and this has been accomplished by a very new technique that involves the production of a series of CT (transaxial computer tomography) scans. The series of scans, taken at one millimeter intervals, enables a computer program to produce a three-dimensional computer model of the cranium. Through the computer program, distortions caused by the process of fossilization were corrected and missing areas were filled in by creating mirror images of the same area on the opposite ráJ of the cranium.
After the cranium was reconstructed by the computer the cranial cavity was visualized as a separate three-dimensional object or "virtual endocast." The volume was then calculated from this "virtual endocast" and from other methods. All methods were consistent with a cranial capacity of around 515 cubic centimeters, although the new calculation of 515 cubic centimters is still larger than the estimated capacities of other specimens of A. africanus, but consh àBably smaller than the original estimate.
This entire exercise brings up the possibility that many other cranial capacity measurements are inaccurate and need to be reevaluated. Dean Falk writes: "The implications of Sts 505's surprising cranial capacity are that something is very wrong with the published record of early hominid cranial capacities. If so, the ramifications for hominid brain evolution may be profound."1
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1 D. Falk, "HominieXÇCain Evolution: Looks Can Be Deceiving," Science, 280 (12 June 1998), 1714.
Reference: G. C. Conroy et al., "Endocranial Capacity in an Early Hominid Cranium from Sterkfontein, South Africa," Science, 280 (12 June 1998), 1730-1731.