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Recent efforts to determine the cranial cavity size of a 2.6 million year old Australopithecine from Sterkfontein, South Africa have returned a consistent finding from two methods: volumetric analysis and computer simulation.These show that the partly complete skull of an Australopithecus africanus known as "Mr. Ples" (Stw 505) had an endocranial size of 515 cc, somewhat smaller than recent estimates of about 600 cc. The latter size would have been a strong anomaly, since Australopithecines of this date range of 3-2 million years ago (myr) have all been less than 500 cc, and the earliest Homo specimens (also 3-2 myr) from East Africa are often less than 600 cc. These include OH 24 from Olduvia Gorge (594 cm3), KNM-ER 1805 (582 cm3), and KNM-ER 1813 (510 cm3) , the latter two from Kenya.
The actual figure of 515 cc for Mr. Ples reported now in Science ( Vol.280, 12 June 1998, pp. 1730-1) by Glenn C. Conroy, Gerhard W. Weber, Horst Seidler, Phillip V. Tobias, Alex Kane, and Barry Brunsden still remains the largest endocranial capacity measured for the A. africanus. This small, gracile australopithecine species is thought to have branched off, between 3 and 2 myr, from the main hominid line of A. afarensis in Ethiopia, which includes the famous Lucy skeleton. A. africanus was the first of the Australopithecines to be identified, in 1924-5, by Raymond Dart who reported the Taung skull from South Africa. Thereafter, in the 1930s through '50s, Robert Broom and his coworkers also found examples of a larger austraolopithecine (A. robustus) at Sterkfontein and other South African sites. Since these discoveries in the late 1950's, A. robustus in southern Africa and an even larger taxa A. boisei in East Africa have been found to overlap early Homo fossils in time.
The large, probably adult male cranium of Stw 505 was found at Sterkfontein in 1989. It retains the face and left side, and as the most complete skull from the site since a female skull (Stw 5) nicknamed Mrs. Ples was found by Broom 50 years ago, it was accordingly called "Mr. Ples." To arrive at their measurement of the brain size of Mr. Ples, Conroy et al. created a "virtual endocast" through sophisticated computer imaging techniques including tomography scans of the fossil cranium, which can record internal details of even mineralized structures. A 3-dimensional model was generated which showed an endocast of the brain cavity of 513 cc. This compared closely with a series of tomographic image sections taken at 1 cm. intervals, which averaged 518 cc. Finally, the researchers filled a facsimile cast with water, which also produced an average volume of 515 cm (ranging from 482 to 536 cm3).
The researchers point out that if their current measurement techniques are accurate, some earlier proposals for hominid brain sizes may have been overestimated, and reappraisals are probably in order for those early specimens complete enough to yield accurate data on endocranial capacity, since theories on human brain evolution depend on such estimates. Using the water volume method, they tested the endocranial capacity of another A. africanus skull (Sts 71), and found the brain size to be 370 cm3 (in the range of female chimpanzees), rather than 428 cm3 as had been recorded. One implication may be that major increases in brain size began earlier than had previously been considered likely, perhaps involving the gracile australopithecines. Further testing and revisions of theories of human evolution should directly result from these findings.
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