Endocranial Capacity in Early Human Cranium from Sterkfontein, South Africa

Endocranial Capacity in Early Human Cranium
from Sterkfontein, South Africa

Summary of article appearing in Science Vol. 280; 12 June 1998

Article by Glenn C. Conroy, Gerhard W. Weber, Horst Seidler, Phillip Tobias, Alex Kane, Barry Brunsden

In 1989, the late Allen Hughes discovered the partial cranium of a large early human at Sterkfontein Member 4, in South Africa. The find has been dated between 2.8 and 2.6 million years old, and the individual has been assigned to the species Australopithecus africanus. This specimen is the most complete cranium recovered at Sterkfontein since the discovery of "Mrs. Ples" in 1947. Presumed male because of its size, the fossil has been affectionately dubbed "Mr. Ples." The fossil is designated Stw 505. The specimen preserves much of the face and the left side of the cranium.

Anecdotally, the large Stw 505 cranium has been said to have a cranial capacity in excess of 600cc. This would, essentially, rewrite our understanding of Australopiths, and early human cranial evolution in general. This reading, if correct, would exceed the largest known cranial capacity of an A. africanus by 100cc, and would exceed or approach the capacities of several early Homo specimens, such as OH 7, OH 24, KNM ER 1813 and others. Additionally, this would raise the average cranial capacity for A. africanus from 440cc to over 460cc.

It is necessary to examine such a dramatic claim.

In this study, a team of researchers first put together a series of highly accurate 1mm thick CT scans of the fossil. The CT scan was able to precisely determine the bone structure of the fossil, despite heavy mineralization. Layering these sections one on top of another, they formed a three dimensional digital model of the Stw 505 specimen. From this model of the fossil, the team reconstructed the missing right side of the skull using the principle of bilateral symmetry (that the right side of the skull should be the mirror image of the left). After the reconstruction, the interior of the skull was "filled" on the computer, creating a separate 3D object using the points on the inside of the model. This "virtual endocast" was then tested by the modeling program for volume. A cranial capacity of 513cc was obtained.

To verify the results of their "virtual endocast" model, the researchers repeated the same method nine times obtaining measurements ranging from 482cc to 536cc with an average of 515cc. Additionally, they decided to look at the 106 original 1mm thick "slabs" created in the CT scan which showed the brain case. By adding all of the cranial measurements contained therein, they arrived at a cranial capacity of 518cc, validating the "virtual endocast" model.

The team concluded that the claim of an Australopithecus africanus individual with a cranial capacity of 600cc was incorrect, although at 515cc Stw 505 still represents the largest A. africanus cranial capacity to date. Another implication of the study, however, is to call into question the estimates for cranial capacity of several early human fossils that were measured using the same method that provided the 600cc estimate for Stw 505, for example OH 24, KNM ER 1813 and KNM ER 732. Additionally, new research into the STS 71 cranium shows a possible cranial capacity of only 378cc -- not only much smaller than the accepted average for A. africanus, but inside the range of modern female chimpanzees. The problem for paleoanthropology appears to be not that we have underestimated the cranial capacities and, therefore, the evolution of early human crania, but that we may have overestimated.

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