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The Endocast of MH1, Australopithecus sediba

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Science  09 Sep 2011:
Vol. 333, Issue 6048, pp. 1402-1407
DOI: 10.1126/science.1203922

Abstract

The virtual endocast of MH1 (Australopithecus sediba), obtained from high-quality synchrotron scanning, reveals generally australopith-like convolutional patterns on the frontal lobes but also some foreshadowing of features of the human frontal lobes, such as posterior repositioning of the olfactory bulbs. Principal component analysis of orbitofrontal dimensions on australopith endocasts (MH1, Sts 5, and Sts 60) indicates that among these, MH1 orbitofrontal shape and organization align most closely with human endocasts. These results are consistent with gradual neural reorganization of the orbitofrontal region in the transition from Australopithecus to Homo, but given the small volume of the MH1 endocast, they are not consistent with gradual brain enlargement before the transition.

The relative importance and timing of two critical processes in the evolution of the human brain—cortical reorganization and size increase—has been debated since the discovery of Australopithecus (1, 2). Recent incorporation and validation of computer-based techniques for reconstructing and comparing endocranial casts (endocasts, proxies of brains from fossilized crania) have substantially improved the quality of data on this issue (3, 4). Eight endocasts [MLD 1, MLD 37/38, Sts 5, Sts 19, Sts 60, Sts 71, StW 505, and Taung (5, 6)] show that Au. africanus had an average cranial capacity of 459 cm3 (37.7 SD). Three endocasts [AL 162-28, AL 333-45, and AL 444-2 (6)] show that Au. afarensis had an average cranial capacity of 481 cm3 (75.6 SD). The earliest representative of the “robust” australopith lineage (KNM-WT 17000), on the other hand, had a comparatively small endocast—410 cm3 (5)—whereas later members of the lineage such as Paranthropus boisei [KGA 10-525, KNM-ER 406, KNM-ER 407, KNM-ER 732, KNM-ER 23000, KNM-WT 13750, KNM-WT 17400, OH 5, and Omo L338y-6; mean = 485 cm3, SD = 45.6 (6)] and P. robustus [SK 54, SK 859, and SK 1585; mean = 493 cm3, SD = 40.4 (6)] had slightly larger average cranial capacities (5). Considering these data, Falk and colleagues (5) hypothesized that australopith brain size might have begun to increase gradually, and cortical reorganization might have begun well before 2.0 million years ago (Ma) and the emergence/evolution of Homo, although confirming data are sparse between 2.0 to 2.5 Ma.

The partial cranium of the holotype juvenile male from Au. sediba, Malapa Hominin 1 (MH1), is dated to 1.977 Ma (7, 8) and thus provides crucial data for evaluating the pace of brain evolution in early hominins. On the basis of epiphyseal closure patterns in the associated postcranial elements and development of the unerupted third molars, MH1 was at a developmental stage at death equivalent to that of a human child of 12 to 13 years, with brain growth essentially complete. The MH1 partial cranium has a virtual reconstructed cranial capacity estimate of 420 cm3 (7), which is higher than one estimate for the Taung specimen [adult size-corrected cranial capacity = 406 cm3 (4)], but more than 1 SD below the Au. africanus mean. The estimated volume of the MH1 endocast is 33 cm3 higher than the smallest estimate reported for Au. afarensis (AL 288-1) but below the Au. afarensis mean by nearly 1 SD (6). Thus, the MH1 estimated cranial capacity is at the lower end of the australopith spectrum of variation. Given its younger date relative to other australopiths (8) and possibly closer phylogenetic relationship to Homo (7), the MH1 endocast is difficult to reconcile with a proposed gradual trend in brain enlargement leading from australopiths to Homo (5) if Au. sediba is ancestral to Homo.

Retained australopith (primitive) brain size in Au. sediba is intriguing given the appearance of derived morphology elsewhere in the cranium (7) and postcranial skeleton, particularly within the pelvis (9) and hand (10). Presumed selective pressures (such as complex object manipulation and tool use) favoring more derived hand morphology in a species that may be ancestral to H. erectus sensu lato would seem inconsistent with retention of a small brain size. The possibility exists, however, that neural reorganization independent of overall size increase could explain such discrepancies (1, 11, 12).

Over the course of growth the endocranial surface, and to a lesser extent cranial form, comes to reflect the form of the expanding brain (13). Yet, brain, craniofacial, and basicranial morphology are clearly integrated to some degree (14, 15). For example, craniofacial size reduction has been linked to a rostral shift of the cribriform plate (16). Impressions retained on the endocranial surface, thought to mirror convolutional patterns on the brain surface, also are useful in extracting additional information (2, 5, 6, 13). Here, we assess basic morphological features of the MH1 endocast, focusing on the orbitofrontal region, to determine whether the small brain of MH1 was morphologically similar to earlier australopiths or whether it exhibited changes absent in these earlier australopiths, and perhaps even foreshadowing those eventually expressed later in Homo.

We reconstructed surface morphology of the endocast of the MH1 cranium using phase contrast x-ray synchrotron microtomography at the European Synchrotron Radiation Facility (ESRF) and a specific acquisition protocol developed for high-quality imaging of large fossils [supporting online material (SOM) materials and methods S1]. Representation of convolutional morphology of the MH1 endocast is striking compared with most hominin endocasts, in part because of the exquisite preservation of material from Malapa and also because the individual was relatively young at death (17).

The MH1 endocast is missing the entire right hemisphere posterior to the coronal suture, posterior portions of the left occipital and temporal lobes, and the cerebellum (Fig. 1, A to D). Post-mortem medial displacement of the right temporal pole appears likely, accompanied by inward displacement of the right posterior inferior frontal lobe internal to pterion (Fig. 1B). Neither displacement appears accompanied by visible distortion or warping of relative proportions in these areas (Figs. 1, B to D, 2, and 3, B to D, and fig. S1). The shape of the MH1 endocast cannot be fully characterized because of its incompleteness (SOM material and methods S2), but on the basis of the frontal and left parietal regions, it bears greater resemblance to the rostrocaudally elongated shape of modern human and Sts 5 endocasts rather than the mediolaterally broadened shape of Sts 60 and chimpanzee endocasts (Fig. 3A). Reconstruction of missing regions in the MH1 endocast, however, will be necessary to confirm this. The MH1 endocast cannot exhibit a classic “tear-drop” shape, as described and illustrated for those of Paranthropus (5), because of its rostrally squared-off frontal lobes (Fig. 3, A and B). Similar squaring-off is present in other Australopithecus [such as Sts 5, Sts 60, and Sterkfontein Type 2 (5)] and modern human endocasts (Fig. 3A).

Fig. 1

Virtual endocast of MH1 in (A) superior, (B) inferior, (C) left lateral, and (D) anterior views. Scale bar is 2 cm.

Fig. 2

Left lateral view of the MH1 virtual endocast with major surface features indicated. (A) Precentral sulcus (inferior). (B) Precentral sulcus (superior). (C) Inferior frontal sulcus. (D) Superior frontal sulcus. (E) Fronto-orbital sulcus. (F) Anterior inferior frontal gyrus. (1) Middle branch of the middle meningeal artery. (2) Posterior branch of the middle meningeal artery. (3) Coronal suture. Scale bar, 2 cm.

Fig. 3

Comparisons of virtual endocasts in (A) superior, (B) inferior, (C) anterior, and (D) left lateral views. MH1 is in the center of each cluster surrounded by a representative modern human at the top, then proceeding clockwise, Sts 60 (Australopithecus africanus), a representative chimpanzee, and Sts 5 (Au. africanus). All endocasts are scaled to the estimated volume of the MH1 endocast (420 cm3) for illustration purposes. Scale bars, 2 cm.

A right frontal petalia is present. Within this part of the MH1 endocast, the impression representing the right frontal lobe appears larger than that representing the left frontal lobe (Fig. 1A). Such visually apparent (qualitative) right frontal petalias are observed in nonhuman primates but at a much lower frequency than in humans (18), in whom there is some support for linking right frontal petalia and right-handedness (19). Both temporal poles of the MH1 endocast are anteriorly expanded (Figs. 1, B and C, and 3, B and D), which is a condition similar to that of other Australopithecus endocasts, but unlike that described and illustrated for Paranthropus endocasts (5). Temporal poles of MH1 do not appear as laterally projecting as those of other australopiths (such as Sts 5 and Sts 60) but appear more centrally projecting as in modern humans and chimpanzees (Figs. 1B and 3B and fig. S1). Other features useful for distinguishing between endocasts of early hominin taxa, such as parietal lobe asymmetry, posteriorward occipital expansion over the cerebellum, and presence/position of the lunate sulcus, cannot be assessed on the MH1 endocast.

In the left hemisphere of the MH1 endocast, the coronal suture and the middle and posterior branches of the middle meningeal artery are visible (Figs. 1, A and C, and 2). The missing right parietal appears to have detached cleanly along the coronal suture, but the relationship of the midline fractured edge to the sagittal suture is less clear. A narrow crack, which is visible on the exterior cranial surface, forms a subtly protruding ridge on the endocast (Fig. 1, A and D). This ridge runs from the left orbitofrontal surface diagonally and medially across the frontal (Fig. 1D) and backward to the approximate vicinity of bregma (Fig. 1A). It widens slightly along its course and partially encompasses the superior sagittal sinus, ultimately obscuring potential evidence of a metopic suture.

Identification of cortical convolutions from endocranial evidence is problematic because of issues of preservation, individual variation, and ambiguous homology (2, 6, 17, 20). Of more consistently expressed and identifiable features, the MH1 endocast preserves clear traces of the anterior Sylvian fissure bilaterally and of the left inferior and superior precentral sulci (Figs. 1A and 2). Two large horizontal furrows are present on the left dorsolateral prefrontal surface (Fig. 2), producing a general configuration that is similar to that seen in other South African australopith endocasts, including Sterkfontein Type 2 and Sts 60 (21). The superior furrow probably corresponds to the superior frontal sulcus commonly observed in modern humans and other apes (17, 22). Identification of the inferior furrow (Fig. 2C) is more problematic because of extreme variability of secondary sulci on the lateral frontal lobes of modern humans and other apes. We consider the inferior furrow of MH1 to most likely represent the inferior frontal sulcus because of its position, orientation, and close association with the superior portion of the fronto-orbital sulcus (Fig. 2). This is in keeping with many researchers’ interpretation of similarly positioned sulci on chimpanzee brains (fig. S2) (20, 2225); however, some consider this sulcus on chimpanzee brains and australopith endocasts to be homologous to the middle frontal sulcus on human brains (17, 21). Dissimilar preservation and/or the presence of a more differentiated and discontinuous sulcal pattern on the left versus right hemisphere may contribute to less distinct details on the right side of the MH1 endocast (Fig. 1, A and D), however, major organizational differences between the sides are not visible, and convolutional patterns of the MH1 endocast appear comparable with those on Sterkfontein Type 2 and Sts 60 endocasts.

As in other australopith endocasts, sulcal anatomy of the MH1 endocast corresponding to the inferior frontal cortex appears generally “ape-like.” A distinct fronto-orbital sulcus incises the lateral margin of the caudal portion of the left inferior frontal lobe and courses along the orbital surface toward the temporal pole (Figs. 1C, 2, and 3D). This is a primitive condition that is present in apes (17) and some other South African australopiths [(21), but see (6)] but not typically expressed in Homo. Also similar to endocasts of apes and other australopiths, the MH1 endocast exhibits no evidence of a vertical ramus of the Sylvian fissure, which typically divides anterior [pars triangularis; Brodmann’s area (BA) 45] and posterior (pars opercularis; BA 44) Broca’s area in humans (26). These two sulcal features have been previously used to identify presence or absence of “human-like” inferior frontal morphology in fossil hominins, with possible implications for the evolution of speech and language (27, 28). In this respect, the inferior frontal region of MH1 clearly appears more ape-like than human-like. However, because homologs of BA 44 and BA 45 are present in chimpanzees (25)—despite the absence of human-like sulcal anatomy—recognition of intermediate gradations may be necessary.

Modern human BA 44 and 45 are regularly localized on the free surfaces of pars opercularis and pars triangularis, respectively, although their boundaries within the adjoining sulci display substantial variation (26, 29). In contrast, chimpanzee area 44 is usually located immediately anterior to, or partially within, the inferior precentral sulcus, whereas chimpanzee area 45 most commonly occurs anterior to the fronto-orbital sulcus and inferior to the inferior frontal sulcus (25). It is unknown whether extant chimpanzees exemplify the ancestral hominin condition, or through what intermediate stages this unknown ancestral condition evolved to produce the modern human range of variation. One hypothesis (17) is that the expansion of the human frontal operculum (BA 45 and 47) displaced the ape fronto-orbital sulcus posteriorly to become part of the anterior limiting sulcus of the insula in humans.

In MH1, the region of the left inferior frontal gyrus anterior to the fronto-orbital sulcus displays a distinct ventrolateral bulge (Fig. 2). This contrasts with the typical ape condition in which the fronto-orbital sulcus sharply defines the anterior extent of a (variably) protuberant “orbital cap” [(17), p. 326] and suggests an intermediate stage in the emergence of a true frontal operculum. Furthermore, this rostral protuberance is asymmetric in MH1, being more pronounced on the left (Fig. 1D). In combination with a well-developed notch of the anterior Sylvian fissure separating the inferior frontal cortex from the temporal lobe, this bulge produces a convex profile of the lateral margin that contrasts with the straight anteroposterior slope commonly seen in chimpanzee endocasts (Fig. 3, C and D). Similar ventrolateral protrusion is not evident on the Sts 5 endocast (Fig. 3, C and D) or in published images and descriptions of endocasts of StW 505 (6) and Taung (12). The Sterkfontein Type 2 endocast is missing the relevant area of the left inferior frontal and right orbitofrontal surfaces, but anteriorly the right inferior frontal gyrus does appear comparable with that of MH1. The inferior frontal surface of Sts 60 is missing on the right and damaged on the left (6, 21) but appears more similar to that of Sts 5 than MH1. Thus, the shape of the MH1 inferior frontal gyrus clearly differs anteriorly from the ape condition and also from other South African australopith endocasts, except perhaps for Sterkfontein Type 2, which does not preserve the comparable area. In light of the tension-based folding theory on neural morphogenesis (30), the bulge on the MH1 endocast could represent early stages of bolstering local neural interconnectivity in area 45, but not yet to the point that more advanced interconnectivity had established true outward folding, thus reconfiguring gyral and sulcal patterns (development of a human-like pars triangularis). A parallel scenario for early increased interconnectivity preceding reconfiguration of sulcal patterns has been suggested with respect to the posterior parietal/occipital morphology of Australopithecus and Paranthropus endocasts [(31), p. 25].

In providing an interpretative framework for surface morphology on the MH1 virtual endocast, we compiled a sample of virtual endocasts generated from 18 modern human crania selected from the Dart Collection of the School of Anatomical Sciences at the University of the Witwatersrand, 18 crania of free-ranging Liberian chimpanzees (Pan troglodytes verus) from the Peabody Museum (United States), and two australopith endocasts from South Africa (Sts 5, Sts 60) (SOM materials and methods S3 for details). Although additional hominin endocasts could offer valuable insights (such as Sterkfontein Type 2 and KNM-WT 17000), they were excluded for lacking the same extent of homologous orbitofrontal regions or for lacking comparable published dimensions.

A series of landmark-based linear dimensions on the orbitofrontal and temporal regions of the MH1 and comparative endocasts were digitally measured on renderings (Fig. 4 and fig. S3); we used a principal component analysis (PCA) of these measurements to compare the endocasts (table S1). Size was removed from comparisons by scaling linear dimensions of individual endocasts (Fig. 5 and figs. S4 to S7) by the geometric mean of all endocast dimensions (32) then logging the ratios by using the common logarithm. The first three principal components (PCs) account for 94.6% of the explained variance (table S1). The first principal component (PC 1, 61.4% of variance explained) easily distinguishes endocasts of modern humans and fossil hominins (MH1 and Sts 5) from those of chimpanzees (Fig. 6, fig. S8, table S1, and SOM text S4). The Sts 60 endocast sits firmly between chimpanzee and modern human/MH1/Sts 5 clusters.

Fig. 4

Illustration of measurements taken on endocasts by using landmark definitions in (5), unless noted otherwise (SOM materials and methods S3). Data are reported in Table 1. Because the occipital pole in the MH1 endocast is missing, we measured absolute dimensions rather than projected dimensions in basal view (5). Mbat is the midpoint of a line drawn between the most rostral points of the temporal lobes (bat) in basal view. Mat occurs at the lateral extension of the bat-bat line to the edge of the frontal lobe in basal view. Rof is a landmark at the most rostral position on the orbital surface of the frontal lobe in basal view. This is not necessarily the same point as the frontal pole. We use the three-dimensional (3D) coordinates of the left (rof-l) and right (rof-r) landmarks to establish a mid-point (rof) on the line joining rof-l and rof-r (fig. S3). 3D coordinates of the mid-point (rof) were used to generate a landmark that was used in measuring absolute 3D linear dimensions (such as mbat-rof). Cob is the caudal boundary of the olfactory bulbs, corresponding to the cribriform plate in basal view. Rob is the rostral boundary of the olfactory bulbs, corresponding to the cribriform plate in basal view.

Fig. 5

Bivariate plots of endocast dimensions. (A) Frontal breadth at rob versus mat-mat. (B) Rob-rof versus mat-mat. (C) Mbat-cob versus mat-mat. (D) Frontal breadth at cob versus mat-mat. All units are millimeters. Open circles, adult Homo sapiens; solid circles, H. sapiens with M3 erupting or not yet erupted (same condition as in MH1); solid triangles, adult Pan troglodytes verus; open triangles, P. t. verus with M3 erupting or not yet erupted (same condition as in MH1); asterisk, MH1 (Australopithecus sediba); “Y” symbol, Sts 5 (Au. africanus); cross, Sts 60 (Au. africanus). Bracketed numbers correspond to measurements in Table 1 and definitions in Fig. 4. For comparative fossil measurements (also absolute rather than projected dimensions), see Table 1.

Fig. 6

Bivariate plot of first and second principal components. Open circles, adult Homo sapiens; solid circles, H. sapiens with M3 erupting or not yet erupted (same condition as in MH1); solid triangle, adult Pan troglodytes verus; open triangles, P. t. verus with M3 erupting or not yet erupted (same condition as in MH1); asterisk, MH1 (Australopithecus sediba); “Y” symbol, Sts 5 (Au. africanus); cross, Sts 60 (Au. africanus). The dashed lines (ellipse area) indicate 95% group membership and are computed from the bivariate normal distribution fit of principal component (PC) 1 and 2. If the ellipse collapses diagonally, the correlation between variables strengthens; a more circular ellipse indicates relatively uncorrelated variables. Dimensions driving PC 1 and PC 2 are discussed in the text and in greater detail in SOM text S4. MH1 fits just within the 95% ellipse for modern humans, whereas Sts 5 falls just beyond it, and Sts 60 is situated between modern human and chimpanzee 95% ellipses.

Negative values along PC 1 in the bivariate score plot for modern humans, MH1, and Sts 5 (Fig. 6) reflect a fairly complex, coordinated pattern involving broadening of the anterior frontal region, medial movement of the temporal poles, and posterior repositioning of the olfactory bulbs (cribriform plate) relative to the temporal poles and the orbitofrontal surface (SOM text S4). The region corresponding to the anterior orbitofrontal cortex in modern humans, MH1, and Sts 5 is broader relative to that in chimpanzees (Figs. 3A and 5, A and D, and Table 1), corroborating previous qualitative observations of modern human, chimpanzee, and australopith endocasts (5). The Sts 60 endocast is comparatively less broad in this region than MH1 and Sts 5 endocasts (Figs. 3A and 5, A and D, and Table 1). Anterior orbitofrontal broadening manifests as an inverse relationship between mat-mat and frontal breadth at rob dimensions (Fig. 4 and table S1). A medial shift of the temporal poles of modern human endocasts compared with those of chimpanzees is reflected in decreased bat-bat relative to mat-mat dimensions (Table 1 and fig. S4). Posterior repositioning of the olfactory bulbs (cribriform plate) on modern human endocasts is indicated by increased rob-rof (Fig. 5B) and decreased mbat-cob dimensions (Fig. 5C and Table 1). Such repositioning is most evident on the MH1 endocast among the australopiths that we examined (Table 1). This posterior repositioning on the MH1 endocast contrasts with the anterior repositioning relative to basicranial landmarks that accompanies facial reduction in australopiths and modern humans compared with chimpanzees (16).

Table 1

Measurements on MH1 and comparative endocasts. Values for modern human and chimpanzee endocasts are means, with SDs in parentheses underneath. Landmarks defined following (5), except when indicated otherwise (Fig. 4 and SOM materials and methods S3). Frontal breadths at cob and rob are illustrated in Fig. 4. Numbers in brackets correspond to measurement definitions in Fig. 4.

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Posterior repositioning of the olfactory bulbs is consistent with known differences in cortical anatomy of modern humans and other apes. Reduction of mbat-cob in the australopith and modern human endocasts relative to chimpanzee endocasts might reflect a decrease in the relative size of the posterior orbitofrontal cortex, which is consistent with evidence of the relatively decreased anteroposterior extent of posterior orbitofrontal area 13 in modern humans (33), and/or the anterior extension of the temporal poles, which is consistent with evidence of temporal lobe expansion in modern humans (15, 34). An increased rob-rof dimension among australopith and modern human endocasts relative to chimpanzee endocasts (Table 1), reflected by the negative loading of this variable on PC 1 (table S1 and SOM text S4), is intriguing because it is consistent with evidence of frontopolar cortical expansion in modern humans (35). The rostralmost orbitofrontal region in modern humans and other apes comprises Brodmann’s areas 10 and 11 (35, 36), both of which are associated with higher cognitive functions such as information encoding and retrieval, relational reasoning, and multi-tasking (37, 38). In modern humans, the frontopolar cortex sits at the apex of frontal executive systems and is thought to have a role in “meta-level” processing required to maintain two simultaneously ongoing tasks (39)—an ability that may contribute to the distinctive human capacities for long-term planning and behavioral innovation (38). The MH1 endocast has a greater rob-rof dimension than do Sts 5 or Sts 60 endocasts (Fig. 5B and Table 1), and thus MH1 more closely resembles the modern human condition.

The MH1 endocast also exhibits prominent, paired convolutions in the region corresponding to the anterior limit of the frontal lobes immediately bordering the sharp depression made by the frontal crest (Fig. 1, A, B, and D). These curl around the rostral limits of the endocast to continue onto the orbital surface of the frontal lobes, ending near the position of the foramen cecum. Some chimpanzee brains (fig. S2) and endocasts (Fig. 3A) show rudimentary comparable structures, and these are somewhat more apparent on the Sts 5 (Fig. 3, A to C) and the Sterkfontein Type 2 endocasts, although not to the same degree as those on the MH1 endocast. These convolutions usually are described as indistinguishable in modern human endocasts but have been recognized as exceptionally developed and clearly defined in the H. floresiensis (LB 1) endocast (40). At some point after the human-chimpanzee split, horizontal spacing distance between neurons in BA 10 shifted in Homo to become relatively greater than that observed in other cortical areas, probably signaling greater interconnectivity of the human brain in the frontal pole region (41). Following the tension-based folding theory on neural morphogenesis (30), greater prominence of the polar convolutions on South African australopith endocasts (such as MH1, Sts 5, Sts 60, and Sterkfontein Type 2) relative to chimpanzees—and of these convolutions of MH1 relative to those of the other australopiths—could indicate relatively increased neural interconnectivity of BA 10 in Au. sediba (MH1).

The second principal component (PC 2, 26.3% of variance explained) does not separate fossil hominins and modern humans from chimpanzees but does cluster fossil hominins (MH1, Sts 5, and Sts 60) together at the lower range of modern human variation (Fig. 6, table S1, and SOM text S4). Higher values along PC 2 in the bivariate score plot reflect a relatively posterior position of cob, indicated by increased length of the orbitofrontal region anterior to this landmark (cob-rof) (fig. S6) and decreased length of the region posterior to it (mbat-cob) (Fig. 5C). Associated with this repositioning is an increase in frontal breadth at cob and at rob (Figs. 4 and 5, A and D). The position of australopiths in the extreme low range of modern human PC 2 scores (Fig. 6) suggests that modern humans may have experienced moderate expansion of orbitofrontal cortex anterior to cob (in relation to a reduced posterior portion) relative to the putatively ancestral australopith condition.

One of the most striking characteristics of the MH1 endocast is its small volume, comparable with early Paranthropus (KNM-WT 17000) and smaller than other australopiths at similar developmental stages or older, except for well-known examples such as the earlier and extensively reconstructed AL288-1 and the AL 162-28 specimens (6). Given the proposed evolutionary relationship between Au. sediba and Au. africanus (7), it is not surprising that there are features (apart from volume) that group endocasts of MH1, Sts 5, Sts 60, and Sterkfontein Type 2 to the exclusion of modern human endocasts (for example, form of lateral inferior frontal cortex). Other features may group MH1 and modern human endocasts to the relative exclusion of other australopiths (such as rob-rof) (Table 1), as indicated by the PCA analysis (Fig. 6). Critically, however, there are no frontal lobe characteristics that we identified grouping endocasts of modern humans and other australopiths to the exclusion of the MH1 endocast (Fig. 5, A to D, Table 1, and figs. S4 to S7).

Thus, the MH1 endocast (Au. sediba), although decidedly australopith-like in cranial capacity and convolutional patterns, shows some evidence for changes in the orbitofrontal region beyond that observed in other relatively complete australopith endocasts (such as Sts 5 and Sts 60), possibly foreshadowing elements of the development of a human-like frontal lobe across the transition from Australopithecus to Homo. This supports the notion that some neural reorganization preceded brain size expansion in the hominin lineage (1, 2, 6) and provides evidence for size-independent morphological change, particularly within the orbitofrontal region, that is more apparent than in previously available fossils.

Supporting Online Material

www.sciencemag.org/cgi/content/full/333/6048/1402/DC1

Materials and Methods

SOM Text

Figs. S1 to S10

Table S1

References (42, 43)

References and Notes

  1. Acknowledgments: We thank the South African Heritage Resource Agency for permits to work at the Malapa site; the Nash family for granting access to the Malapa site and their continued support of research on the Malapa and John Nash Nature Reserves; the South African Department of Science and Technology, the South African National Research Foundation (particularly the African Origins Platform Initiative), the Institute for Human Evolution (IHE), the University of the Witwatersrand, the University of the Witwatersrand’s Vice Chancellor’s Discretionary Fund, the National Geographic Society, the Palaeontological Scientific Trust (PAST), the Andrew W. Mellon Foundation, the Ford Foundation, the United States Diplomatic Mission to South Africa, the French Embassy of South Africa, the Research Office of the University of the Witwatersrand, the Ray A. Rothrock ’77 Fellowship, the Program to Enhance Scholarly and Creative Activities, and International Research Travel Assistance Grant of Texas A&M University, the Oppenheimer and Ackerman families, and Sir Richard Branson for funding; the University of the Witwatersrand’s Schools of Geosciences and Anatomical Sciences and the Bernard Price Institute for Palaeontology for support and facilities; the Gauteng Government, Gauteng Department of Agriculture, Conservation and Environment, and the Cradle of Humankind Management Authority; and our respective institutions for ongoing support. For access to comparative specimens, we thank S. Potze, L. Kgasi, and the Ditsong National Museum of Natural History; B. Billings, D. Lieberman and the Peabody Museum of Archeology and Ethnology (Harvard University); E. Mbua, P. Kiura, V. Iminjili, and the National Museums of Kenya; and the University of Zurich 2009 and 2010 Field Schools. Numerous individuals assisted with ongoing preparation and excavation of the Malapa fossils, including C. Dube, S. Jilah, C. Kemp, M. Kgasi, M. Languza, J. Malaza, G. Mokoma, P. Mukanela, T. Nemvhundi, M. Ngcamphalala, S. Tshabalala, and C. Yates. Other individuals, contributing substantial support to this project include L. Backwell, D. Conforti, B. de Klerk, C. Dlamini, V. Fernandez, J. Kretzen, B. Kuhn, W. Lawrence, B. Louw, B. Nkosi, M. Peltier, L. Pollarolo, C. Steininger, F. Thackeray, H. Visser, and B. Zipfel. T.J. also would like to thank the Claude Leon Foundation for awarding her a postdoctoral fellowship. We acknowledge the ESRF ID17 beamline team, and the ESRF for granting beamtime under proposal ec521. We thank Charlotte Maxeke Johannesburg Academic Hospital and J. Smilg for facilitating computed tomography (CT) scans and Q. Letsoalo for assistance in conducting them; D. Falk, R. Holloway, and R. Clarke for discussions on hominin fossils in South Africa; T. Preuss and J. Rilling for images and discussions of chimpanzee cortical anatomy; M. Dowdeswell for statistical guidance; S. Hurst for permitting use of CT data from Sts 60 and Sterkfontein Type 2 endocasts; and the editor and three anonymous reviewers for their comments. Image data of the MH1 cranium and endocast are curated in the IHE at the University of the Witwatersrand and at ESRF.
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