Basicranial Influence on Overall Cranial Shape
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Daniel E. Lieberman Basicranial influence on overall cranial Department of Anthropology, shape The George Washington University, 2110 G St, NW, This study examines the extent to which the major dimensions of the Washington, DC 20052, cranial base (maximum length, maximum breadth, and flexion) U.S.A. E-mail: interact with brain volume to influence major proportions of the [email protected] and Human neurocranium and face. A model is presented for developmental Origins Program, National interactions that occur during ontogeny between the brain and the Museum of National History, cranial base and neurocranium, and between the neurobasicranial Smithsonian Institution, complex (NBC) and the face. The model is tested using exocranial Washington, DC 20560, and radiographic measurements of adult crania sampled from five U.S.A. geographically and craniometrically diverse populations. The results indicate that while variations in the breadth, length and flexion of the Osbjorn M. Pearson cranial base are mutually independent, only the maximum breadth of Department of Anthropology, the cranial base (POB) has significant effects on overall cranial University of New Mexico, proportions, largely through its interactions with brain volume which Albuquerque, NM 87131, influence NBC breadth. These interactions also have a slight influ- U.S.A. ence on facial shape because NBC width constrains facial width, and because narrow-faced individuals tend to have antero-posteriorly Kenneth M. longer faces relative to facial breadth than wide-faced individuals. Finally, the model highlights how integration between the cranial Mowbray base and the brain may help to account for the developmental basis of Department of Anthropology, some morphological variations such as occipital bunning. Among American Museum of Natural modern humans, the degree of posterior projection of the occipital History, Central Park West at bone appears to be a consequence of having a large brain on a 79th, New York, NY 10024, relatively narrow cranial base. Occipital buns in Neanderthals, who U.S.A. have wide cranial bases relative to endocranial volume, may not be entirely homologous with the morphology occasionally evident in Received 1 June 1998 Homo sapiens. Revision received 25 March 1999 and accepted 21 May 2000 Academic Press 1999 Keywords: skull, cranium, Journal of Human Evolution (2000) 38, 291–315 basicranium, neurocranium, doi: 10.1006/jhev.1999.0335 occipital bun, integration. Available online at http://www.idealibrary.com on Introduction stimuli, the role of the cranial base in influ- encing overall cranial shape merits special It has long been known that the cranial base, consideration. Developmentally, the basi- vault and face derive from embryologically cranium differs from the neurocranium and distinct regions (the basicranium, neurocra- splanchnocranium in several important nium and splanchnocranium) but that these respects. Unlike the rest of the skull, which regions grow in a morphologically integrated develops intramembranously from neural manner through numerous developmental crest-derived tissue, the basicranium mostly and functional interactions (de Beer, 1937; grows from endochondral ossification pro- Moss & Young, 1960; Enlow, 1968, 1990; cesses in which mesodermally-derived carti- Cheverud, 1982; Sperber, 1989). Although laginous precursors (the chondrocranium) these interactions occur as the result of develop in utero and are gradually replaced many morphogenetic (e.g., neural) and by bone after birth (Sperber, 1989). The functional (e.g., masticatory, respiratory) basicranium is also the first region of the 0047–2484/00/020291+25$35.00/0 2000 Academic Press 292 . ET AL. skull to reach adult size (Moore & Lavelle, it is predicted that variations in the shape of 1974), and it is the structural foundation of the human neurocranium are influenced by many aspects of craniofacial architecture. interactions between two factors: (1) varia- The cranial base forms the platform upon tions in the shape of the basicranium upon which the rest of the skull grows and which the neurocranium grows, and (2) attaches (see Biegert, 1963), and it provides endocranial expansion driven by brain and protects the crucial foramina through growth. Because the face is displaced in a which the brain connects to the face and the forward and downward trajectory from the rest of the body. These aspects of cranial basicranium and neurocranium, we also test base growth and function may account for the related hypothesis that variations in its apparent morphological and develop- basicranial and neurocranial breadth con- mental conservatism in mammals compared strain upper- and mid-facial breadth, and to other regions of the skull (de Beer, 1937; hence influence other aspects of facial mor- Bosma, 1976 and references therein; phology, especially facial depth and height Sperber, 1989: 117). Consequently, a (Weidenreich, 1941; Enlow & Bhatt, 1984; number of recent phylogenetic studies of Enlow, 1990). These hypotheses are exam- hominids (e.g., Olson, 1981; Lieberman, ined using a pooled-sex sample of adult 1995; Lieberman et al., 1996; Strait et al., anatomically modern Homo sapiens from five 1997; Strait, 1998) have proposed that vari- geographically and craniometrically diverse ations in cranial base morphology may be populations in order to examine as wide a better indicators of taxonomy and phy- range of cranial variation as possible. This logeny than neurocranial or facial charac- study does not examine inter-population or ters. Some studies (e.g., Skelton & intra-population variability. A future goal is McHenry, 1992; Lieberman et al., 1996; to test the model within populations and Strait et al., 1997) have specifically exam- between sexes, but larger samples sizes are ined cladograms that emphasize the impor- required than presently available (see below). tance of basicranial traits by grouping The results of these analyses are also used neurocranial and facial characters into func- to examine variation in occipital ‘‘bunning’’ tional complexes. However, it must be among Upper Pleistocene hominids and stressed that whether or not the basicranium recent human populations. Occipital buns, is a better source of phylogenetic data re- which have been suggested to be important mains open to question. Basicranial, neuro- for testing phylogenetic hypotheses about cranial, and facial dimension show similar recent human evolution, are defined as levels of heritability within the primate skull posteriorly-directed projections of the (e.g., Sjøvold, 1984; Cheverud & Buikstra, occipital beyond the nuchal plane that result 1982; Cheverud, 1995) and appear to be in a distinctive swollen morphology when equally well (or poorly) integrated with viewed in norma lateralis (Ducros, 1967; other dimensions or features in primate Trinkaus & LeMay, 1982). Bunning has phylogeny (Strait, 1998). been suggested to be a derived Neanderthal This study tests the extent to which vari- character the presence of which in some ations in the major dimensions of the cranial early modern humans from Europe indicates base (length, breadth and flexion) may influ- regional continuity (Smith, 1984; Frayer, ence several aspects of the shape of the face 1992a, 1992b; Frayer et al., 1993; Wolpoff, and cranial vault in humans. After present- 1996). However, Trinkaus & LeMay (1982) ing a model for interactions between the and Lieberman (1995) have suggested basicranium, neurocranium and face during that bunning may be a developmental growth, we test two sets of hypotheses. First, consequence of posteriorly-directed cranial 293 vault expansion that occurs in very large- mainly from deposition within the cranial brained hominids, such as Neanderthals sutures, although some resorption does take or Upper Pleistocene modern humans, in place; its lower half also expands through which a relatively narrow cranial base con- drift in which the external (ectocranial) sur- strains lateral vault expansion. If this face is depository and the internal (endo- hypothesis is correct, then the degree of cranial) surface is resorptive (Duterloo & occipital projection among adult recent Enlow, 1970). Therefore, as Figure 1 illus- humans as well as Pleistocene hominids trates, the basicranium and neurocranium should be correlated with the ratio of endo- grow in tandem in a rapid neural growth cranial volume relative to cranial base trajectory, forming a highly integrated breadth. morphological unit, the neuro-basicranial complex (NBC). This model of cranial growth assumes that Background overall shape of the NBC has two primary In order to investigate the relationship influences: the shape of the brain, and the between basicranial dimensions and overall shape of the basicranium. This integrated skull shape, it is useful to review several growth occurs through many processes, the aspects of craniofacial development, focusing most important of which are sutural expan- on Enlow’s (1990) model of the ontogen- sion, synchondroseal deposition, drift, and etic interactions between the basicranium, flexion. As the brain expands, it generates neurocranium, and splanchnocranium tension along the endocranial surface of the (illustrated in Figure 1). The basicranium is neurocranial cavity, activating osteoblast defined here as the portion of the skull deposition within intra-sutural periosteum which derives from the chondrocranium and throughout the upper portion of the vault, which grows through endochondral ossifi- drift in the lower portions of the vault and cation. As