With the use of computed tomography (CT), Glenn C. Conroy et al. (1) estimate that the endocranial capacity of the Australopithecus africanus specimen Stw 505 is 515 cm. From this result, they reason that because previous estimates for apparently smaller crania are similar to or higher than that for Stw 505, the necessary downward readjustment of these capacities would require a reevaluation of early hominid brain evolution. We would like to revisit Stw 505 itself. Computerized imaging techniques, like other forms of measurement in paleontology, are inaccurate unless all sources of postmortem distortion have been taken into account. One of us (C.A.L.) is currently working with P. V. Tobias on the description and primary analysis of Stw 505. This study reveals aspects of damage that were not taken into account by Conroy et al. (1). The frontal bone has been crushed inward on the left side and the parietals have been flattened and bent downward near midline. The latter contributes to a distorted midsagittal contour that is inconsistent with that of other early hominin crania. In addition, the left half of the neurocranium (especially the temporal bone) has pivoted inward such that the distances between the structures in the middle cranial fossa and the anatomical midline of the cranium are substantially reduced. This effect is also seen beneath the posterior cranial fossa, where a remnant of the left occipital condyle has been pushed into the anatomical midline. As a result of these sources of distortion, which artificially reduce the endocranial capacity of Stw 505, the estimate by Conroy et al. of 515 cm must significantly underestimate the actual capacity, perhaps by as much as 10 to 15%. On the other hand, there is no obvious source of artificial expansion of the braincase. Would a higher cranial capacity for Stw 505 be as unexpected as Conroy et al. imply? They criticize anecdotal estimates (2) of 600 cm for the Stw 505 endocranial capacity, noting that (p. 1730) “[s]uch an endocranial capacity . . . would be astounding in any australopithecine . . . .” This statement should be placed in a statistical framework. Coefficients of variation (CV) for endocranial capacities in modern great ape and human samples range between 8 and 15% (3). The CV for the A. africanus sample without Stw 505 is only 5.1% [n 5 6; (4)] and the total range for this small sample is indeed very low (60 cm). If Stw 505 did have an endocranial capacity of 600 cm, the species sample CV would rise to only 14% with a standard error of 3.4% (4). Thus, an endocranial capacity of 600 cm in A. africanus should be neither “astounding” nor even unexpected if levels of variation in modern hominoids are any guide. While the endocranial capacity of Stw 505 remains uncertain, the value provided by Conroy et al. (1) seems an underestimate, and, in any event, an appreciably higher value would not be unusual for A. africanus. Reappraisal of data is always healthy in science, but the endocranial capacity of Stw 505 does not support the conclusion in the report by Conroy et al. (1), echoed in Falk’s commentary (5), that present views on the tempo and mode of early hominid brain evolution require “reevaluation.” Charles A. Lockwood Institute of Human Origins, Arizona State University, Tempe, AZ 85287–4101, USA, and Palaeo-anthropology Research Group, University of the Witwatersrand, Parktown 2193, South Africa E-mail: cal.iho@asu.edu William H. Kimbel Institute of Human Origins, Arizona State University, Tempe, AZ 85287–4101, USA E-mail: wkimbel.iho@asu.edu
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Apr 1, 2000
Anne-Marie Bacon 
Open Access