Jeffrey H. Schwartz The human chin revisited: what is it and Department of Anthropology, who has it? University of Pittsburgh, Pittsburgh, PA 15260, Although the presence of a ‘‘chin’’ has long been recognized as U.S.A. E-mail: unique to Homo sapiens among mammals, both the ontogeny and the jhs+@pitt.edu morphological details of this structure have been largely overlooked. Here we point out the essential features of symphyseal morphology in Ian Tattersall H. sapiens, which are present and well-defined in the fetus at least as Department of Anthropology, early as the fifth gestational month. Differences among adults in American Museum of Natural expression of these structures, particularly in the prominence of the History, Central Park West at mental tuberosity, are developmental epiphenomena and serve to 79th Street, New York, emphasize the importance of studying this region in juveniles when- NY 10024, U.S.A. ever possible. A survey of various middle to late Pleistocene fossil E-mail: [email protected] hominids for which juveniles are known reveals that these features are present in some late Pleistocene specimens assigned to H. sapiens, but Received 7 May 1998 not in all of the presumed anatomically modern H. sapiens (i.e., Revision received 28 April Qafzeh 8, 9, and 11). The adult specimens from Skhu¯l, as well as the 1999 and accepted 26 May adult Qafzeh 7 specimen, are similarly distinctive in symphyseal 1999 morphology. Neanderthals are quite variable in their own right, and they as well as other middle to late Pleistocene fossils lack the Keywords: chin, Homo sapiens, symphyseal features of H. sapiens. Some of the latter are, however, Neanderthals, Skhu¯l, seen in the Tighenif (Ternifine) mandibles. Qafzeh. 2000 Academic Press Journal of Human Evolution (2000) 38, 367–409 doi: 10.1006/jhev.1999.0339 Available online at http://www.idealibrary.com on Introduction characteristics became incorporated into scenarios of evolution. With regard to the More than two centuries ago, Blumenbach chin, which essentially became synonymous (1775) identified the ‘‘chin’’ as being among with ‘‘a prominence at the front of the the most fundamental of the features that he mandible,’’ the general concern among considered uniquely human. Of course, paleoanthropologists was with how an ape- when Blumenbach presented his discussion like jaw could be converted to that of Homo of ‘‘what distinguished humans,’’ it was in sapiens (Hrdlicˇka, 1911; Robinson, 1913; the context of ‘‘compared to other extant Wallis, 1917; Gregory, 1922; Howells, primates.’’ But with the acceptance in the 1959; Enlow, 1982; Stringer et al., 1984; nineteenth century of hominid fossils as real Daegling, 1993). entities, and the subsequent documentation Discussion of the human chin has been of a well-represented and diverse human approached from several different perspec- fossil record, features such as the chin, tives. Anatomical descriptions have focused which had a specific kind of significance on the protrusiveness of the symphyseal when the primary goal of the taxonomist region, with the mounded structure being was to distinguish between humans and identified in extant humans as the ‘‘mental other extant primates, became cast in a protuberance’’ (Johnston & Willis, 1954), different light. Rather than being static enti- the ‘‘mental eminence’’ (Lieberman, 1995), ties, as would be expected of organisms that the ‘‘mental osseum’’ (Bra¨uer, 1984; Rosas, had been specially created, morphological 1995), or the ‘‘tuber symphyseos’’ (Hublin 0047–2484/00/030367+43$35.00/0 2000 Academic Press 368 . . & Tillier, 1981; Inke, 1967). Thus defined, It is the basal protrusion which has, of the mental protuberance may continue course, been the primary criterion for laterally on each side to some extent as a identifying the mental protuberance; and thickening of the inferior margin of the this, in turn, has become the primary ele- corpus. In some individuals, each lateral ment in the identification of a chin. From extremity of the mental protuberance may this perspective, it is perfectly reasonable to bear a blunt ‘‘corner,’’ which has been conclude that ‘‘chins are a notoriously vari- referred to as a ‘‘mental tubercle’’ (Johnston able character, as many modern humans & Willis, 1954; Rak, 1998)or‘‘tubercula lack them’’ (Lieberman, 1995:174). And lateralia’’ (Hublin & Tillier, 1981; Inke, when the focus of attention in this region 1967). Above each lateral extremity and to is the degree of symphyseal protrusion each side of the mental protuberance, lies a anteriorly—which is often judged against the depression, or ‘‘mental fossa’’ (Wolpoff, vertical plane of the lips or incisors (see 1996), which would appear to exist as a Enlow, 1982)—there might, indeed, appear default of the midline field of bone being to be ‘‘chinless’’ and ‘‘almost chinless’’ elevated or protrusive. Collectively, the human beings. mental protuberance and lateral extremities, The evolution of the human chin has been especially if they bear tubercles, have been discussed in various ways. Hrdlicˇka (1911) identified as the ‘‘mental trigon’’ (Rak, ventured the suggestion that this structure 1998), the ‘‘mental trigone’’ (Smith, 1984), evolved as a result not of the specific devel- or, if the structure is restricted quite opment of a protrusion, but of the reduction inferiorly along the corpus, the ‘‘trigonum of the dental arch (which essentially left the basale’’ (Bra¨uer, 1984). Just below the protrusion behind as the anterior margin alveolar margin, the bone may be curved retreated). This idea was elaborated upon by inward, presenting a shallow depression that Robinson (1913), who proposed that it was has been referred to as the ‘‘incurvatio the demands of the tongue and laryngeal mandibularis’’ (Hublin & Tillier, 1981), the musculature that maintained mandible ‘‘incurvatio mandibulae’’ (Rosas, 1995), or length inferiorly as the region above it the ‘‘impressio subincisiva externa’’ (Inke, supposedly retreated posteriorly. Gregory 1967). Although there is obviously much (1922) added to these notions the possibility morphological detail to be recognized in the that the chin provides space at the front of symphyseal region of H. sapiens, the human the jaw that is necessary for the enlarging chin has frequently been identified solely crowns and roots of incisors and canines. on the basis of the general protrusiveness of Howells (1959) reiterated Robinson’s the symphyseal region (Lieberman, 1995; (1913) proposition and suggested further Smith, 1984; Wolpoff, 1980; Lam et al., that bone thickening, which ultimately pro- 1996). duced the chin, must have evolved in order The development of the human chin is to reinforce the front of the jaw against typically described as a postnatal phenom- masticatory stresses (which, in apes, were enon, a simple matter of bone resorption absorbed by the internal buttress known as near the alveolar margin, with concomitant the ‘‘simian shelf’’). Reisenfeld (1969) deposition of bone more inferiorly (Enlow, thought that he had refuted this kind of 1982; see also Aiello & Dean, 1990; functional explanation by inducing in mice Daegling, 1993; Lieberman, 1995). Individ- what he believed to be a chin by extracting ual differences or variations in chin develop- the continually growing anterior teeth and ment result essentially from differential rates severing the muscles of mastication (which of resorption vs. deposition (Enlow, 1982). did, of course, have the effect of eliminating ‘‘’’ 369 masticatory stress at the symphyseal region). a conspicuous incurvatio mandibulae’’ Wolpoff (1980) adopted a different stance (Rosas, 1995:550). on the subject by speculating that the The impetus for the present review came appearance of the chin was correlated with from two different and unexpected sources. expansion of the neurocranium and a con- First, in the course of conducting a twenty- comitant shift of the temporal muscles to a year analysis of the skeletal remains of peri- more medial angulation. Most recently, natal individuals from the Punic site of Daegling (1993) surveyed the various func- Carthage (n>650 individuals; Schwartz & tional interpretations put forth to explain the Houghton, in preparation), one of us (JHS, presence of the chin, and concluded that the unpublished data) discovered that it was best was ‘‘the hypothesis that the human possible to identify very small fragments chin represents a structural response to of the symphyseally unfused mandibles of resist vertical bending in the coronal plane.’’ these highly immature individuals on the Regardless of the proposed mechanisms basis of specific features of this region: behind the appearance of the chin as a namely, a continuous everted margin that structure, the language of the literature is proceeds along the edge of the symphysis invested with the notion of an evolutionary and laterally around along the inferior edge trend, in which a perceived increase in swell- and that, consequently, subtends a deep ing of the symphyseal region ultimately leads fossa. Second, as a result of realizing that the to the formation of the typical human chin. configuration of the brow in some of the Thus, it is common to read of the supposed most familiar of the supposed early ‘‘ana- existence of incipient chins in various tomically modern’’ H. sapiens (i.e., from Neanderthals (themselves considered Skhu¯l and Qafzeh) was not bipartite (cf. merely an archaic, primitive version of H. Stringer et al., 1984), as it is in late sapiens). These include adult specimens Pleistocene and extant representatives of from Guattari, S{ipka, and Vindija (Lam this species (Schwartz & Tattersall, 1999b), et al., 1996; Lieberman, 1995; Smith, 1984; we came to realize that these same fos- Wolpoff, 1980; Wolpoff et al., 1981) and the sils were also not H. sapiens-like in Archi Neanderthal child (Ascenzi & Sergi, mandibular symphyseal morphology. For 1971a,b; Mallegni & Trinkaus, 1997; the record, we must admit that when we first Stringer et al., 1984). Hence, because dis- studied the Skhu¯l specimens, we were not cussions of later human evolution are often initially struck by these disparities.