Douglas C. Broadfield

New high-resolution computed tomography data of the Taung partial cranium and endocast and their bearing on metopism and hominin brain evolution

Significance The Taung Child (Australopithecus africanus) has historical and scientific importance in the fossil record as the first and best example of early hominin brain evolution. It was recently proposed that Taung exhibits adaptive morphology (e.g., persistent metopic suture and open anterior fontanelle), permitting important postnatal brain growth late into infancy. As Taung provides the only purported pre-Homo fossil evidence for the suggested adaptive mechanism, we test the hypothesis that it displays these features. Using new high-resolution images and in silico exploration, we did not observe the hypothesized form of these features. If delayed metopic suture closure were adaptive, for example, permitting substantial postnatal brain growth and alleviating an obstetric dilemma, there is no evidence this mechanism evolved before Homo. Falk and colleagues [Falk D, Zollikofer CP, Morimoto N, Ponce de León MS (2012) Proc Natl Acad Sci U S A 109(22):8467–8470] hypothesized that selective pressures favored late persistence of a metopic suture and open anterior fontanelle early in hominin evolution, and they put an emphasis on the Taung Child (Australopithecus africanus) as evidence for the antiquity of these adaptive features. They suggested three mutually nonexclusive pressures: an “obstetric dilemma,” high early postnatal brain growth rates, and neural reorganization in the frontal cortex. To test this hypothesis, we obtained the first high-resolution computed tomography (CT) data from the Taung hominin. These high-resolution image data and an examination of the hominin fossil record do not support the metopic and fontanelle features proposed by Falk and colleagues. Although a possible remnant of the metopic suture is observed in the nasion–glabella region of the Taung partial cranium (but not along the frontal crest), this character state is incongruent with the zipper model of metopic closure described by Falk and colleagues. Nor do chimpanzee and bonobo endocast data support the assertion that delayed metopic closure in Taung is necessary because of widening (reorganization) of the prefrontal or frontal cortex. These results call into question the adaptive value of delaying metopic closure, and particularly its antiquity in hominin evolution. Further data from hominoids and hominins are required to support the proposed adaptive arguments, particularly an obstetric dilemma placing constraints on neural and cranial development in Australopithecus.

Technical note: the midline and endocranial volume of the Taung endocast.

The Taung endocast is one of the best-preserved and most important known in paleoanthropology. Although the endocast is undistorted and preserves distinctive landmarks, Taung has proved a difficult endocast, because it is only about 60% complete. To reconstruct Taung it is necessary to first use the available anatomical landmarks to define the midline of the endocast. It is only with a proper description of the midline that it is possible to reconstruct the endocast and obtain an accurate measurement of Taungs endocranial volume. Holloway (Science 168 (1970) 966-968) determined a conservative estimate for Taung of 404 ml. More recently this estimate has been revised downward by Falk and Clarke (Am J Phys Anthropol 134 (2007) 529-534) to 382 ml, giving Taung the smallest endocast for A. africanus. Certain challenges exist with the reconstruction of any endocast, particularly a hemi-endocast such as Taung. A virtual reconstruction of Taung must assume perfect symmetry, a feature called into question here in Taungs most recent reconstruction by Falk and Clarke (2007). Holloways (1970) reconstruction of Taung provides a guidepost for a conservative approach to endocast reconstructions, and the most reliable measurement of Taungs true endocranial volume.

Missing Omo L338y‐6 occipital‐marginal sinus drainage pattern: Ground sectioning, computer tomography scanning, and the original fossil fail to show it

The Omo L338y‐6 occipital region has been recently studied by White and Falk ( 1999 ), who claim that it shows a readily identifiable enlarged left occipital‐marginal sinus (O/M). These observations are contrary to the direct observations of previous investigators (Rak and Howell, 1978 ; Kimbel, 1984 ; Holloway, 1981 ; Holloway, 1988 ). White and Falk ( 1999 ) further argue that the presence of this enlarged O/M strongly suggests that the Omo L338y‐6 hominid was indeed a “robust” Australopithecus. We used direct sectioning and CT scanning to analyze magnified sections of a high‐quality first‐generation cast of the newly cleaned original fossil. These methods fail to show any evidence of a morphological landmark that can be interpreted as an enlarged O/M, either as an eminence or a sulcus. In contrast, the same techniques used with both SK 1585 and OH5 (“robust” Australopithecus with an enlarged O/M) show extremely visible and palpable enlarged O/Ms. Examination of the original Omo fossil confirms that it lacks an O/M. This evidence clearly shows that an enlarged O/M cannot be identified on either the original fossil or a first‐generation cast, although this does not rule out the possibility that the Omo L338y‐6 hominid was a “robust” Australopithecus. We believe that the differences between observers regarding this feature are most probably due to displacement caused by a crack and the different source materials employed, i.e., the difference between a first‐generation cast of the original fossil and a third‐ or fourth‐generation cast of the endocast made two decades ago. Anat Rec 266:249–257, 2002.

Reply to Falk and Clarke on Taung virtual endocast midline and volume

The opening sentence of their letter (Falk and Clarke2012) is simply false: ‘‘Without providing any support fortheir claim, Holloway and Broadfield (H & B) assert that‘‘the Falk and Clarke (2007) reconstruction of Taungmiscalculates the midsagittal plane, resulting in a signif-icant reduction in cranial capacity....’’ We took their ownvirtual mirror-imaged endocast and showed that (1) themirror-imaging was asymmetrical, and that (2) theynever defined a midsagittal plane. We stand by theseclaims. Their letter now discusses in greater detail theirproposed midsagittal plane, but the Figure 1 showing adis-embodied prefrontal left petalia still does not justifychoosing a midsagittal plane lacking symmetry withoutfirst carefully and explicitly defining a midsagittal planeupon which mirror-imaging is to take place. The qualityof the dorsal image of Figure 1 is so poor that one cannoteven make out the sagittal, coronal, or lambdoid sutures.It is unfortunate that Science (not Nature as Falk andClarke have it in their Literature Cited) did not publishthe photographs provided for Holloway’s (1970) article,because in fact RH placed the pointers at maximal dis-tances, two along the sagittal suture, at roughly bregma,and lambda. The 3rd was placed in the middle of themidsagittal plane between the left and right superiorprefrontal gyri at the most anterior part of the frontallobe then available (longitudinal sulcus or fissure). Theneedle scribes lines went through the middle of the pos-terior part of the rostral bec, justifying the choice of mid-sagittal points on the dorsal surface. Implying that RH’schoice of points was incorrect because the sagittal suture‘‘meanders’’ is simply spurious.Falk and Clarke make much of the Figure 2 compositeof the Taung Wenner-Gren cast, and one of the recon-structions RH used to show three points in the midsagit-tal plane. Yes, the reconstruction piece is slightly tilted,but the point was to show three points along the midsa-gittal plane. Indeed, the region left of the midsagittalplane is an artifact of being a 3rd generation cast, andwas not a part of the reconstruction subjected to waterdisplacement. As indicated in the 1970 article, threereconstructions were made. The originals remain atWits, and what RH has remaining are casts made frommolds of those reconstructions, which is what appears inFigure 2. The tilt of the cast in our photograph is hardlyrelevant to our arguments.We are somewhat amused at their claim regardinghow much better ‘‘state-of-the-art’’ methods such astheirs are preferable over the crude methods employedin 1969–1970, before scans of Taung and segmentingsoftware were available. It is somewhat disingenuous forFalk and Clarke not to have noted in proof that Neuba-uer et al. (2012) using ‘‘state-of-the-art’’ virtual endocasttechniques found that Holloway’s midline was accurateand that all of their volume determinations for Taungwere identical to Holloway’s (1970) range, i.e., about402–405 ml. Indeed, it is almost approaching mantricproportions that somehow previous work done withoutthe aids of CT scans, virtual imagery, and modern mor-phometric statistical methods is somehow suspect andworthless. We applaud re-testing older results usingnewer methods, but here are two examples of use of vir-tual endocasts and ‘‘state-of-the-art’’ and the bottom lineis a difference of some 20 ml of volume, or roughly 5% ofthe total volume. Obviously, ‘‘state-of-the-art’’ methodsare relative to the ability of researchers to choose correctanatomical landmarks, and test their own methods,something that the Neubauer et al. paper does, but Falkand Clarke do not.

Erratum: Missing Omo L338y-6 occipital-marginal sinus drainage pattern: Ground sectioning, computer tomography scanning, and the original fossil fail to show it (Anatomical Record (2002) 266 (249-257))

Holloway RL, Yuan MS, Broadfield DC, Degusta D, Richards GD, Silvers A, Shapiro JS, and White TD. Missing Omo L338y-6 occipital-marginal sinus drainage pattern: ground sectioning, computer tomography scanning, and the original fossil fail to show it. Anat Rec 2002:266:249 257.