The supraorbital region
A broad survey of anthropoid primates reveals that, regardless of the specific morphology of the adult supraorbital region, in very young individuals (i.e., even as late as M1 eruption) the generally smooth supraorbital region invariably gives little or no hint of the morphology that will eventually characterize the adults (dimorphic or otherwise) of that species (Schwartz, 1997 and unpublished data). Indeed, it is virtually impossible to predict from the neonate what its adult supraorbital morphology will be.
Adult conformations of this region vary widely. In papionins (see Schwartz, 1997), the adult supraorbital region presents a distinctly defined, superoinferiorly thin, and anteriorly projecting bar-like torus that runs essentially straight across from side to side. In African apes (Pan troglodytes, P. paniscus, and Gorilla spp.), the supraorbital region grows into an anteroposteriorly thick torus, with a markedly vertical component that produces a post-toral sulcus behind and which is indented over glabella to varying degrees (Fig. 4). The list of examples is endless, but from the undistinguished supraorbital region of the juvenile, myriad distinctive adult configurations emerge developmentally, ranging from the projecting, “goggle-like” circumorbital region of gibbons and siamangs, through the superomedially- and laterally raised partial circumorbital rims of Pongo, to the more fully but differently rimmed orbits of various New World monkeys, such as Cebus and Alouatta (Schwartz, 1997).
Figure 4. Emergence of supraorbital morphology in Gorilla, from the nondescript to the specific (counterclockwise, from top left to top right), and in H. sapiens (right column), from 7 month fetus to adult (in the adult, arrows point to the medial “glabellar butterfly,” which, in this specimen, extends into the field of the laterally flatter plane). Not to scale. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Download figure to PowerPoint
Even though one might expect living H. sapiens, the poster-child of paedomorphosis, to be the most neotenic of anthropoids in supraorbital morphology, it is not. Rather, one also finds little or no supraorbital embellishment not only in the small marmosets and tamarins, but also in various colobine monkeys (thereby ruling out size as a factor in lack of supraorbital morphology). Still, in H. sapiens, albeit closer to the onset of adulthood than in other anthropoids, the previously featureless region of glabella (Fig. 4; also Fig. 12) swells anteriorly (even if only slightly), and from each side of this mounded midline a wing-like swelling may also emerge, its inferolateral extremity terminating generally at the supraorbital foramen/notch (i.e., near the midpoint of the superciliary arch) and the superolateral extremity extending somewhat beyond this point laterally (Fig. 4). Altogether, this mounded protrusion forms a “butterfly”-like shape, which elsewhere we have described as a “glabellar butterfly” (e.g., Schwartz and Tattersall, 1996b, 1999a, 2002b; Tattersall and Schwartz, 1998; Antunes et al., 2000). On each side, the superciliary region lateral to the “butterfly wing” is flatter and more plate-like, with perhaps also a slight posterior declination to its surface (Fig. 4). This then constitutes the “bipartite” brow, the development of which is unique to H. sapiens compared with all living and almost all fossil primates.
Among fossil hominids, the available sample is adequate to allow us to track the emergence of the thick, double-arched, and laterally continuous brow seen in H. neanderthalensis (Schwartz and Tattersall, 1996b, 2002b, 2003). As in extant anthropoids, the supraorbital region of 3–4-year-old specimens (Engis, Pech de l'Azé, Roc de Marsal, and Subalyuk) is featureless in this species (Fig. 5). Only in slightly older individuals (La Quina child and Teshik Tash) can one discern with any confidence the beginning of supraorbital swelling, which developmentally expands bilaterally from the region of glabella (Fig. 5). In the Le Moustier adolescent, the laterally continuous brow typical of Neanderthal adults can already be detected (Fig. 5). What cannot be known, though, is whether the brow of this individual would have become much more anteriorly distended as it matured, as in La Ferrassie I and especially Guattari, or if it would have remained relatively low as in Gibraltar I or Krapina C (Skull 3) (Fig. 5).
Figure 5. Growth sequence demonstrating emergence of the “double arched” and smoothly rolled supraorbital region typical of adult Neanderthals (La Quina child, top left; Teshik Tash juvenile, bottom left; Le Moustier adolescent, top right; and Krapina C (skull 3), bottom right). Not to scale. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Download figure to PowerPoint
Nevertheless, what is important about these specimens is that they are consistent with a picture of taxon-specific, postnatally achieved, supraorbital morphology. Consequently, although we cannot reliably infer adult supraorbital form from the study of juvenile hominids, such as those from Herto, Modjokerto, Taung, Skhūl I, and Dikika, we can state with some confidence that the specific supraorbital morphology of any adult was acquired during growth from a previously featureless frontal bone. With this in mind, we turn to fossil specimens that have been considered anatomically modern H. sapiens, in search of those that present a bipartite brow.
Among the fossils that most of us were taught were uncontestable early representatives of our species are specimens from Qafzeh and Skhūl. From Qafzeh, the specimen most frequently cited and illustrated is the fairly complete skull Qafzeh 6. Yet this specimen lacks a bipartite brow, possessing instead a superoinferiorly somewhat tall brow that is anteriorly low and mounded, and continuous across an equally tall glabellar, region. Thus, although the neurocranium of Qazeh 6 is rather globular, and relative to it the face is not massive, this specimen conspicuously lacks the one particular apomorphy that would cement its allocation to H. sapiens (Schwartz and Tattersall, 1996b, 2000b) (Fig. 6).
Figure 6. Comparison of Qafzeh 9 (left column), Qafzeh 11 (middle column), and Qafzeh 6 (right column). The former two specimens appear to have had a bipartite brow, which is clearly lacking in Qafzeh 6. See text for detail. Not to scale. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Download figure to PowerPoint
Less frequently discussed and illustrated are the broken, but reasonably reconstructed, adult Qafzeh 9 and the less complete and subadult Qafzeh 11 crania (Fig. 6). Yet for our discussion here, these specimens are interesting and frustrating in equal measure. The adult Qafzeh 9 specimen is so damaged that we can only surmise that the fragment that is apparently correctly placed medially in the superciliary arch on the left side is mounded or somewhat swollen, suggesting the “butterfly” configuration. Bone laterally in both superciliary regions is more clearly flat and plate-like, also suggesting that Qafzeh 9 possessed a bipartite brow. Although the subadult Qafzeh 11 is less damaged, and thus presents a more pristine supraorbital region than Qafzeh 9, crucial morphological detail in this region was not yet fully developed. Nevertheless, we feel confident in identifying the glabellar “butterfly-shape” swelling characteristic of a bipartite brow.
Along with Qafzeh 6, Skhūl V has often been presented as a representative of “early anatomically modern” H. sapiens (Fig. 7). Like Qafzeh 6, the skull is generally rounded and vaulted in profile, and the largely reconstructed face does not present itself as excessively massive relative to the neurocranium. Also as in Qafzeh 6, what is preserved of the supraorbital region of Skhūl V does not present a bipartite configuration. Interestingly, the brow of Skhūl V is less tall superoinferiorly, and much more anteriorly protrusive in the form of a torus, than its counterpart in Qafzeh 6 (Fig. 7). The Skhūl II frontal fragment (which retains the glabellar region together with some of the left supraorbital region and most of the right) and the small portion of the left supraorbital region of Skhūl IV are strikingly similar to Skhūl V (Fig. 7). However, in Skhūl IX, the preserved right supraorbital region, with most of glabella, is superoinferiorly thin but arced rather than relatively straight, and barely protrudies anteriorly (Fig. 7). Skhūl VII retains the lateral portion of the right orbital region, but the anterior surface of the superciliary arch is missing; the brow appears to have been arcuate and may have been taller than in the other Skhūl specimens. Predictably, the supraorbital region of the juvenile cranium Skhūl I is featureless. Thus, whatever the exact morphology of each of the Skhūl specimens, there is no trace of a glabellar “butterfly” in any of them.
Figure 7. Comparison of Skhūl V, Skhūl IX, and Qafzeh 6 (top, left to right), and LH18 (Ngaloba), Skhūl II (lower middle, anterior and oblique views), and Liujiang (bottom, left to right). Although the superciliary region in Skhūl V is the most protrusive anteriorly (followed by Skhūl II) and is the least tall superoinferiorly, neither it nor the other specimens present a bipartite brow. The “crease” in the right supraorbital region of Skhūl V is due to damage. Note also the distinctly teardrop-shaped bulge, rather than inverted T-shape of the H. sapiens “chin.” See text for detail. Not to scale. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Download figure to PowerPoint
With regard to other specimens that have been identified as “early anatomically modern” H. sapiens, we could confidently detect a glabellar butterfly only in the Liujiang cranium (>67 ka, possibly 101–227 ka) (Fig. 7). In the otherwise distinctive LH 18 (Ngaloba) calotte (108–129 ka) (Fig. 7), there appears to be something resembling this structure, the more robust and superoinferiorly thicker lateral portion forming an antero-obliquely facing plane (Schwartz and Tattersall, 2003). The variably complete crania of Omo Kibish I and II, Singa, and Jebel Irhoud I, and the Klasies River Mouth frontal fragment (Figs. 8 and 9), are broadly contemporaneous with, or older than, the Liujiang and LH 18 specimens and have been suggested as at least representing a precursor to anatomically modern H. sapiens. None of these specimens, however, displays a supraorbital configuration that could be described as bipartite, or as possessing a butterfly-shaped glabellar region.
Figure 8. Comparison of Omo Kibish I [left and middle columns; frontal, mandible (anterior, right profile, inferior views), and lateral view of occipitoparietal region] and Omo Kibish II (anterior and right lateral cranial views, right column). The two may represent different morphs, but they are similar in lacking a bipartite brow. The symphyseal region of Omo Kibish 1 clearly lacks H. sapiens's features. See text for detail. Not to scale. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Download figure to PowerPoint
Figure 9. Variably complete crania of Jebel Irhoud 1 (top left), Dar es Soltane II (bottom left), Border Cave 1 (top right), Singa (bottom right), and Klasies River Mouth frontal 6103 (middle). None displays evidence of a bipartite brow. Not to scale. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Download figure to PowerPoint
We have not been able to examine the adult cranial specimen (BOU-VP-16/1) from the somewhat older site of Herto, Middle Awash. However, we need to mention it because it has been allocated not only to H. sapiens, but also to a new subspecies, H. sapiens idaltu (White et al., 2003). As is clear from the excellent published images, BOU-VP-16/1 has a more robust and superoinferiorly taller brow (including the lateralmost extremity) than any fossil in which we can confidently describe a bipartite brow replete with glabellar “butterfly.” As seen in the published photographs, the more completely preserved right superciliary arch of BOU-VP-16/1 presents a slightly postero-obliquely oriented “crease” that delineates medial and posterior moieties, with the medial portion more anteriorly facing and the posterior portion inclining posteriorly. Atypical of any bipartite brow, however, is that the anterior surface of the medial supraorbital moiety of BOU-VP-16/1 is vertically flat from top to bottom and appears to “twist” toward its medial extremity so that it ultimately faces rather laterally. The superior margin of this anterior moiety also bears a distinct margin that continues onto the glabellar region, thus partitioning the two supraorbital sections as separate entities.
Among chronologically younger specimens that have been considered definitively anatomically modern H. sapiens are the incomplete crania Border Cave 1 and Dar es Soltane II (Fig. 9). Although we have in the past agreed with this interpretation (Schwartz and Tattersall, 2003), our reassessment of these specimens has made us much more tentative now in both cases. Among the variably complete Pleistocene crania that we also viewed as morphological H. sapiens in our 2003 study, we still confidently include in our species the relatively recent specimens from Abri Pataud, Brno, Chancelade, Combe Capelle, Cro-Magnon, Dolni Věstonice, Engis (the adult), Grimaldi, Isturitz, Mladeč, Pavlov, Predmostí, Svitavka, Tuinplaas, Velika Pécina, Vogelherd, Wajak, Zhoukoudian Upper Cave, and Zláty Kůn (Fig. 10). Unaligned with typical H. sapiens on supraorbital conformation are the very recent specimens from Fish Hoek and Boskop (Schwartz and Tattersall, 2003) (Fig. 11). The latest estimate of 6891 ± 37 BP for Fish Hoek (Stynder et al., 2009) makes this atypicality all the more intriguing.
Figure 10. Crania and associated mandibles of various fossil H. sapiens: Abri Pataud, Mladeč 1, and Dolni Věstonice XV (top, left to right); Grimaldi 5, Cro-Magnon 2, and Wadjak 4 frontal and Wadjak 23 mandible (bottom, left to right). Note variable expression both of a bipartite supraorbital configuration and of an inverted T-shaped chin. Not to scale. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Download figure to PowerPoint
Figure 11. Boskop (left column, including left partial mandible in symphyseal, left lateral, and inferior views, and Fish Hoek (middle and right columns). Note nonbipartite supraorbital configuration. Note in anterior and lateral views of the mandible the smoothness of the symphyseal region and, in inferior view, the relatively uniform anteroposterior thickness of the bone from the symphyseal region onto the corpus. Not to scale. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Download figure to PowerPoint
From at least the time of Blumenbach's (1969) treatise on features that distinguish H. sapiens from other living animals, the human “chin” has received particular attention from comparative biologists (Schwartz and Tattersall, 2000a). Unfortunately, the focus has typically been on the presence of some (any) anterior protrusion in the region of the mandibular symphysis. This has led to such unhelpful comments as that the only living mammals that develop a chin are humans and elephants (Enlow, 1982). In the search for evidence of the emergence of anatomically modern from more archaic H. sapiens, any three-dimensional perturbation of a symphyseal surface that in other mammals is typically smooth or flat (whether vertical or posteroinferiorly slanted) tends to be taken as evidence of an incipient chin. In light of this emphasis on simple anterior protrusion, rather than on morphological detail, even modern-day humans who fail to achieve the requisite anterior growth of the mandible have been seen as anatomical curiosities (Enlow, 1982). Nevertheless, although the form and development of the feature that is truly unique to H. sapiens, the chin, has been illustrated and described in textbooks for centuries, its systematic and phylogenetic significance has been obscured by the endeavor to create a sequence of morphological transformations from extinct to extant humans.
As reviewed elsewhere (Schwartz and Tattersall, 2000a; Schwartz, 2007b), the major features of the human chin are visible prior to the fifth fetal month, well before the right and left sides of the mandible fuse along the symphysis (Fig. 12). Specifically, early on, there is an anteriorly raised inferior symphyseal margin that continues laterally for some distance, creating the depression that in adults is identified as a mental fossa. Before birth, and continuing afterward until coalescence is complete, the right and left raised symphyseal margins fuse from top to bottom (Fig. 12). The result is an inverted “T” configuration in which the stem of the “T” is represented by the raised but now joined right and left symphyseal margins, and the arms on either side of the fused symphysis are the thickened inferior margins. The mental fossae lie on either side of the stem of the inverted “T” and above the thickened inferior margin.
Figure 12. Growth series illustrating the configuration of the symphyseal region in H. sapiens. Five-month fetus (upper left): note everted symphyseal and inferior margins and large and deep bilateral mental foramina; also, the symphyseal sides have only begun to fuse superiorly, a process that will continue inferiorly. Two- to 3-year old (bottom left): anteriorly, note everted inferior margins with mental fossae above and in the midline a modest triangular swelling of bone; inferiorly, note that the symphyseal region is thicker anteroposteriorly than the corpora immediately lateral to it. In the 5-year-old (middle) and adult (Abri Pataud) (right), note that that the thickness is maintained inferiorly, whereas, in the symphyseal regions, the inverted T is variably expressed. Pre-adult specimens are in uncatalogued teaching collections, American Museum of Natural History. Not to scale. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Download figure to PowerPoint
With continued growth and bone remodeling, and the emergence of the first set of anterior teeth, the mandibular alveolar margin grows superiorly away from the tip of the stem of the inverted “T.” The crisp and thin rami of the inverted “T” also thicken, especially along the inferior margin and at the juncture of the stem and the arms, forming what in the adult is commonly identified as a “mental trigon” (Fig. 12). Sometimes the lateral extremities of the arms also thicken and are then referred to as mental tubercles. Although the superior limit of the stem often becomes less distinct, it never extends to the alveolar rim because its development is from the basilar, Meckel's cartilage-derived bone of the mandible, whereas alveolar bone derives from the neural crest-derived mesenchymal cells that give rise to the teeth and their attendant soft and hard tissue structures (Ten Cate and Mills, 1972). Of further note in H. sapiens is that, from the juvenile through the adult, the symphyseal region is noticeably thicker anteroposteriorly than the bone of the corpora on either side when viewed from below (Fig. 12). Thus, in H. sapiens, the anterior region of the mandible differentiates early on in fetal development into the basic configuration that will be retained to varying degrees of crispness in the adult.
In anthropoid primates, the anterior region of the neonate mandible is essentially as featureless as it will remain in the adult (Schwartz, 1997, 2007b; Schwartz and Tattersall, 2000a). Indeed, as seen for instance in the Taung child and Swartkrans SK 3978, the featureless symphyseal region is consistent with the equally featureless symphyseal regions of australopith adults, regardless of the morph they represent (Fig. 13). Consequently, it is reasonable to conclude that if the adults of a species present a morphologically blank symphyseal region, the juveniles did too.
Figure 13. Anterior views of mandibles of young and adult australopiths illustrating their characteristically featureless symphyseal regions. Taung child, Swartkrans SK3978 child, and Makapansgat MLD 2 subadult (top, left to right); Swartkrans SKW 5, Peninj, and SK 12 (bottom, left to right). Note also, as in juvenile anthropoids generally, the supraorbital region of Taung is essentially featureless. Not to scale. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Download figure to PowerPoint
Although some adult Neanderthal specimens may exhibit some anterior symphyseal topography, it is significantly absent in both the Le Moustier adolescent and known juveniles, Gibraltar 2 (Devil's Tower), Pech de l'Azé, Roc de Marsal, Amud, and Teshik Tash (Schwartz and Tattersall, 2002b, 2003) (Fig. 14). Indeed, the vertically oriented symphyseal regions of these specimens are similarly broad and shallowly curved from side to side, with a smooth profile across the midline. This configuration is retained in a number of adults, notably La Ferrassie I, La Chapelle-aux-Saints, and various mandibles from Krapina. But in other adult specimens, the midline in lateral profile may slope down and back (e.g., Tabūn C1); the anterior teeth may protrude anteriorly farther than the bone below (e.g., some Krapina specimens); or a subincisal fossa may produce apparently protruding anterior teeth and, immediately below (but well above the inferior margin), a gentle bulge or swelling (e.g., Spy 1, Shanidar 1) (Fig. 14) (also illustrations in Schwartz and Tattersall, 2002b). Also of note, from juvenile into adult, is that when viewed from below the Neanderthal symphyseal region not only is broad and variably straight across or shallowly arced from side to side, but is also typically (though not invariably, in Regourdou and Kebara, for example, bone thickness is consistent) thinner anteroposteriorly than the bone of the corpora on either side (Fig. 14). In any event, from a developmental perspective, the variability in details of adult Neanderthal symphyseal configuration clearly emerged with growth from a morphologically undistinguished symphyseal surface. Thus, no Neanderthal adult specimen with a bulge (invariably well above the inferior margin) provides any insight into the “evolution” of the human chin.
Figure 14. Growth series from Neanderthal children to adult in anterior and inferior views to illustrate characteristic symphyseal configuration. Amud infant, Pech de l'Azé child, and Le Moustier adolescent (top, left to right); Krapina 58 and La Ferrassie 1 adults (bottom, left and right). Note variability in anterior “overhang” of anterior teeth versus smooth surface in adults; note inferiorly, especially in pre-adults, that the symphyseal region may be thinner anteroposteriorly than the bone of the corpora to either side. See text for detail. Not to scale. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Download figure to PowerPoint
Given the obvious differences between H. sapiens and Neanderthals, the mandible of the Skhūl I child is of particular interest (Figs. 14 and 15). For, although partially reconstructed in the symphyseal region, the preserved bone on the inner surface demonstrates that it was very broad and gently arced from side to side; it was also thin anteroposteriorly and thicker farther along the corpora. Although not extending across the midline, the bone preserved externally on the right side is smooth and shows no sign either of a mental fossa or of a rise toward the symphysis. In addition to this atypical (for H. sapiens) symphyseal morphology, the exposed right M1 presents not only the peripherally placed cusps and broad and long talonid basin characteristic of Neanderthals, but also a well-developed centroconid, as in the Tabūn C1 M1 (Schwartz and Tattersall, 2003) (Fig. 14). These comparisons are particularly interesting in light of the fact that, even though slightly distorted, the outline of the cranial vault viewed from behind and the morphological details of the occipital region are not typical of Neanderthals, as is also the case with the adult specimen Skhūl V.
Figure 15. Partial skull and mandible of Skhūl I child compared with inferior view of mandible of Tabūn C1 (lowermost right) and close-up of Tabūn C1's left M1 (insert with mandible of Skhūl I). The numerous views of the Skhūl I mandible demonstrate that reconstructed Neanderthal-like shape of the symphyseal region and its thinness anteroposteriorly relative to the thicker corpora is accurate. Also note evidence of a centroconid in the Tabūn C1 molar, which is pronounced in the Skhūl I child. Note also in Skhūl I, as in all juvenile anthropoids, the featureless supraorbital region. Not to scale. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Download figure to PowerPoint
With regard to the Skhūl adults, the Skhūl V mandible (Fig. 16; also Fig. 7) is damaged along the incisor roots, but the reconstruction of these teeth as slightly anteriorly inclined, and of the subincisal fossa immediately below, seems to be accurate. In profile, an anterior bulge emerges below the subincisal fossa, reaching its most anterior extent around the inferior margin. In front view, this bulge is teardrop-shaped, and it transitions smoothly into the surrounding bone all around it (Schwartz and Tattersall, 2000a, 2003). In inferior view, the Skhūl V mandible is clearly uniformly thick anteroposteriorly throughout the broad symphyseal region, becoming somewhat thicker more laterally along the corpora.
Figure 16. Skhūl IV, II, and V (top, left to right) and Border Caves 2 and 5 (bottom, left and right). Note variably developed teardrop-shaped bulge in symphyseal region of Skhūl specimens and anteroposteriorly uniformly thick bone from symphyseal region onto corpus in all. The symphyseal region of Border Cave 5 is essentially featureless, whereas that of BC 2 is enigmatic in presenting a bulge versus a developmental derivative of an inverted T. Not to scale. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Download figure to PowerPoint
The less well-preserved Skhūl IV mandible is broken between the right C1 and P1, but the intact symphyseal region shows the same curvature and anteroposteriorly uniform thickness as Skhūl V (Fig. 16). In Skhūl IV, the anterior teeth are not truly forwardly inclined, but are instead undercut by an extremely shallow subincisal fossa below which, in left profile, a bulge not unlike that in Skhūl V emerges. In anterior view, some damage notwithstanding, the bulge is broader than in Skhūl V and less well defined, but it too merges smoothly with the surrounding bone. Only the anterior portion of the mandible of Skhūl II (Fig. 16) is known (better along the right corpus than the left), but the relative uniformity of anteroposterior thickness throughout this region is preserved. Unfortunately, plaster reconstruction occupies much of the upper portion of the anterior surface of the symphyseal region, but it is clear, as seen particularly on the right side, that the surface between the mental foramen and the front of the jaw is not hollowed out or in any other way fossa-like. Further, rather than being thickened with some anterior distension, the inferior margin of the symphyseal region curves smoothly down and then posteriorly.
Although they do not present a configuration of the symphyseal region comparable with the inverted “T” of H. sapiens, the Skhūl adults also do not exhibit the typical anteriorly thin, and broad and gently arced, symphyseal region of Neanderthals (cf. Figs. 14 and 16). Thus, in the symphyseal region, as in the configuration of the brow, Skhūl adults are characteristic neither of H. sapiens nor of Neanderthals. What is more, although the Skhūl I mandible might on its own be interpreted as a juvenile representative of the population represented in the adult form by Tabūn C1, features of the vault make this unlikely. What can be said with some confidence, though, is that the Skhūl I mandible is not very plausibly the basis from which adult Skhūl mandibular morphology emerged. All in all, this assemblage gives us a very good example of the difficulties of sorting out the systematics of a morphologically diverse but evolutionarily close-knit group.
To add yet another wrinkle to this puzzle, the presence of a human-like chin in the Tabūn II mandible, which is larger and taller in both corpora and rami than the Tabūn C1 mandible, has been actively debated (see review in Schwartz and Tattersall, 2000a). As also seen in profile in various Neanderthal specimens (see above), the anterior teeth of Tabūn II are set well forward of the symphyseal region below. This anterior tooth “displacement” is emphasized further by a superoinferiorly tall subincisal fossa, from the lower part of which a modest bulge emerges anteriorly. Unfortunately, the inferior margin across the entire symphyseal region is reconstructed in plaster. However, on each side of the reconstructed portion, there is a distinct tubercle on the inferior margin. When viewed from below the mandible appears to be uniformly very thick anteroposteriorly throughout the somewhat tightly curved symphyseal region, and it is perhaps slightly thinner along the corpora. An additional piece of the mystery is seen in the full exposure of the third molars in front of the anterior margins of the rami (although on both sides this margin is indented posteriorly by a marked preangular sulcus that accentuates the exposure of the M3's).
Just as the cranial specimens from Qafzeh probably represent more than one morph, the mandibles from this site suggest at least two different morphs. The mandible of the juvenile Qafzeh 4 is unfortunately somewhat crushed and slightly incomplete in the symphyseal region, though its anterior portion clearly lacks the inverted “T” configuration and attendant mental fossa characteristic of H. sapiens (see images in Schwartz and Tattersall, 2000a, 2003), something that would be expected to have begun forming in a H. sapiens of this individual's age (approximately 5–6 years, given the eruption of the first molar). Instead, the anterior surface of this mandible is featureless. Yet the slight medial crushing of the partial left corpus cannot obscure the fact that, as in H. sapiens, when viewed from below the mandible would have been thicker anteroposteriorly in the symphyseal region than lateral to it.
The partial adult mandible Qafzeh 7 (Fig. 17) presents a short and shallow subincisal fossa, below which a fairly large but low teardrop-shape bulge flows smoothly and gently into the surrounding bone; the inferior margin of the symphyseal region is missing, but in lateral view it is clear that this bulge recurves posteriorly. The mandible is sufficiently intact in the symphyseal region to show in inferior view that the bone is uniformly thick anteroposteriorly in the midline, and may also have been so farther laterally. The symphyseal morphology of the Qafzeh 8 fragment is undecipherable.
Figure 17. Qafzeh 7, 9, and 11 (top, left to right). Note tear-drop shaped bulge on symphyseal region of Qafzeh 7 and apparent uniform thickness of bone anteroposteriorly from symphyseal region to corpora; apparent outward deflection of bone of symphyseal region of Qafzeh 9; and thicker symphyseal region on Qafzeh 11. Also note dental similarities between Qafzeh 9 and 11, but not between them and Qafzeh 7. See text for detail. Not to scale. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Download figure to PowerPoint
Although cracked and reconstructed, the adult Qafzeh 9 (Fig. 17) mandible is clearly different from Qafzeh 7. The anterior teeth of Qafzeh 9 are somewhat anteriorly displaced, creating a short subincisal fossa below their roots, but there is no sign of a teardrop-shaped swelling. Rather, pieces of undeformed bone on either side of the midline clearly curve anteromedially, as if rising toward a midline keel (Schwartz and Tattersall 2000a, 2003). What is preserved of the inferior margin suggests that the symphyseal region would have been thickest inferiorly. Thus, although it is impossible to describe the configuration in inferior view with confidence, we believe the most straightforward interpretation of symphyseal morphology is that it bore the inverted “T” and the associated mental fossae characteristic of H. sapiens.
The partially reconstructed mandible of the subadult, Qafzeh 11, presents a thin midline keel whose uppermost extremity lies well below the alveolar crest of the central incisors (Fig. 17). This keel is confluent with a symphyseal swelling that fans out as it approaches the inferior margin (rather than curving medially to form a teardrop shape); on each side is a shallow depression. In lateral view, this triangular swelling projects anteriorly above the inferior margin, although, primarily because the inferior margin is elevated above, a small but distinct tubercle is seen on the left side. In inferior view, the mandible is thickest anteroposteriorly in the symphyseal region, with thinner bone laterally along the corpora. We believe that Qafzeh 11 exhibits a subtly configured inverted “T,” with attendant mental fossae.
In contrast to the Skhūl mandibular specimens, among which it appears most likely that Skhūl 1 does not represent a juvenile form of the adults from that site, it is possible to hazard that Qafzeh 4 is the juvenile of whatever adult morph Qafzeh 7 represents. Certainly, it is neither a juvenile version of Qafzeh 9 and 11, nor of a Neanderthal: the symphyseal region of Qafzeh 4 lacks any of the morphological adornment characteristic of H. sapiens; yet, when viewed from below, it is not thinner anteroposteriorly than the bone lateral to it along the corpora as in Neanderthals.
The most compelling reasons for associating Qafzeh 4 and 7 come from M1 morphology (Schwartz and Tattersall, 2003). In both specimens, the protoconid sits more mesially than the metaconid; the mesiodistally long entoconid courses from just mesial to the midline of the distal margin so that it lies across from the apex of the metaconid; and the relatively small but bulbous and slightly buccally placed hypoconulid juts out from the perimeter of crown. The M1's of Qafzeh 9 and 11 are mesiodistally longer on the buccal than on the lingual side, the entoconid is smaller than and lies opposite the hypoconid, and the wedge-shape hypoconulid is positioned distally in the midline of the crown (Fig. 17). This in itself is interesting, but beyond providing insight into the hominid assemblage represented at Qafzeh, the association of Qafzeh 4 with Qafzeh 7 permits us to suggest that the adult individuals from Skhūl may also be aligned with this potential hominid morph.
Among specimens penecontemporaneous with Skhūl and Qafzeh, only one, a mandible fragment from Klasies River Mouth (AP 6222), exhibits an anteroposteriorly thickened symphyseal region bearing the inverted “T” configuration (Fig. 18). Other Klasies River Mouth mandibular specimens that preserve some, if not the full extent of the symphyseal region [KRM1/AP 6100, AP 6101 (21776), AP 6102, and AP 6223], present neither the H. sapiens structure nor the teardrop-shape configuration seen in the Levantine non-H. sapiens morphs (Fig. 18). Rather, in these Klasies River Mouth specimens, the symphyseal region tends to be rather straight and vertical in lateral profile. Interestingly, although this region [best preserved in KRM1/AP 6100, AP 6101 (21776), and AP 6223] is strongly curved laterally (and is not as wide from side to side or as straight across as in the probably chronologically older Jebel Irhoud 3 juvenile mandible or the Neanderthals), when viewed from below (Fig. 18) it is thinner anteroposteriorly than the corpora immediately to its sides, as in Neanderthals. Although weathered and slightly damaged, the yet older Omo 1 mandible is strikingly reminiscent of these latter specimens from Klasies River Mouth, i.e., it lacks morphological detail, and the strongly curved and uniformly thick symphysis is narrower than the corpora to its sides.
Figure 18. Anterior and inferior views of mandibles from Klasies River Mouth. KRM 6222, 6100, and 6101 (top and middle rows, left to right); KRM 6102 (anterior view only); and KRM 6223 (bottom row, left to right). Note in KRM 6223 a distinct triangular development in the symphyseal region and, in inferior view, that the bone of the symphyseal region is somewhat thicker anteroposteriorly than the corporal bone immediately lateral to it. In the other mandibles, although the symphyseal region in some presents various three-dimensional configurations, none approaches the inverted T-shape and, inferiorly, one sees that the bone of the symphyseal region is either thinner anteroposteriorly or of equal thickness compared with the thickness of the bone to its side. Not to scale. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Download figure to PowerPoint
Of the two Border Cave mandibles, only one [BC 2 (A1102), not BC 5] presents a symphyseal keel that swells laterally and anteriorly toward the inferior margin. However, as in BC 2, the BC 5 mandible is quite strongly curved in inferior view, and the corpora are as thick anteroposteriorly as the uniformly thick symphyseal region (Fig. 16). Thus, among the African specimens just discussed, we feel confident in identifying the morphology typical of living H. sapiens only in Klasies River Mouth AP 6222.
Among hominids of more recent age, all diagnostic features of H. sapiens mandibular symphyseal morphology are unambiguously present in specimens from Abri Pataud, Brno, Chancelade, Combe Capelle, Cro-Magnon, Dolni Věstonice, Grimaldi, Haua Fteah, Isturitz, Predmostí, Svitavka, Vogelherd, Wajak, Zhoukoudian Upper Cave, and Zlaty Kun (Fig. 10). Apparently excluded from this group are the very recent specimens from Fish Hoek and Boskop, neither of which exhibits any detailed symphyseal morphology (Fig. 11). Again, the redating of Fish Hoek to less than 7 kya (Stynder et al., 2009) adds another element to the puzzle of human diversity in the Holocene.
In this section, we have focused on two of the more striking features of cranial osteology that seem to characterize the living species H. sapiens. Still, we are fairly sure that, on close inspection, we might well discover similar patterns of distribution and variation in those other osteological features that have at one time or another been claimed as unique to our species. Clearly, whatever the structure under consideration, one might reasonably expect developmental outliers to exist within any species, and, as a result, we are unable to say with any confidence whether, for example, the lack of the modern human chin and glabellar butterfly in the Fish Hoek specimen indicates its exclusion from H. sapiens in a broader biological sense. If we were to insist on tossing out all characters that are not very tightly correlated indeed with other features used in the analysis, we might find ourselves totally bereft of any characters to work with; still, on strictly morphological considerations (which is all we have that is directly systematically relevant), we can do no more than say securely that the Fish Hoek specimen and others like it (including the Omo 1, Herto, Border Cave 5, Boskop, and Klasies specimens, except for AP 6222) belong to the same close-knit clade as modern H. sapiens.
Because at least the vast bulk of living humans are plainly derived in these features, whereas the fossils are not, on the basis of morphology, we are able neither definitively to include such specimens in H. sapiens, nor assuredly to exclude them from the living species. Our inclination is to err on the side of inclusion, which, in operational terms, should not greatly distort the overall evolutionary pattern we perceive among our closest relatives (it has the effect of simplification, leading to a more robust result than potentially needless complication). We think it is important to remember that there must be a systematic signal buried somewhere in the mass of morphologies with which our immediate clade presents us. But for the time being, it seems, we are limited to a fairly gross level of resolution in clarifying that signal.