Biomechanical analysis of masticatory system configuration in Neandertals and Inuits.код для вставкиСкачать
AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 91:l-20 (1993) Biomechanical Analysis of Masticatory System Configuration in Neandertals and Inuits M.A. SPENCER AND B. DEMES Doctoral Program in Anthropological Sciences ( M A S . ) , and Department of Anatomical Sciences (B.D.), State University of New York at Stony Brook, Stony Brook, New York 11794 KEY WORDS Neandertals, Masticatory biomechanics, Dietary reconstruction, Anterior tooth use ABSTRACT Considerable debate has surrounded the adaptive significance of Neandertal craniofacial morphology. Numerous unique morphological features of this form have been interpreted as indicating an adaptation to intense anterior tooth use. Conversely, it has been argued that certain features related to muscle position imply a reduced mechanical advantage for producing bite forces on the incisors and canines. In this study, hypotheses about morphological specializations for anterior tooth use have been derived from a biomechanical model of Greaves (1978).These hypotheses were tested by performing separate pairwise comparisons of Neandertals and early Homo sapiens, and Inuits and Native Americans from Utah. Inuits are known to have produced repeated and high magnitude forces on their anterior dentition and therefore serve as a good model for a hominid adapted to intensive anterior tooth use. Biomechanically relevant dimensions of the masticatory system were measured using a computer-driven video analysis system and compared between the two taxa in each comparison. The results of this study reveal a number of similarities between the morphological specializations exhibited by Neandertals and Inuits that can be related to intensified anterior tooth use. The hypothesis that Neandertals were poorly designed for producing masticatory forces is rejected. Specializations that differ between the two groups are interpreted as being the result of differential functional demands placed on the postcanine dentition in Neandertals and Inuits. It is suggested that many of the unique morphological features of the Neandertal face are a response to intensified use of the anterior dentition and the need to retain a sufficiently large postcanine occlusal area necessary for a relatively high attrition diet. o 1993 Wiley-Liss, Inc. The adaptive significance of Neandertal craniofacial morphology has been a source of continued interest, particularly with regard to the unique facial features exhibited by this group. These features include: relatively large anterior dentition; spatulate, vertically implanted incisors; anterior dentition flattened into a coronal plane, resulting in a “squared-ofr’ appearance of the dental arcade; marked midfacial prognathism; zygomatic root positioned relatively posteriorly; and a so-called retromolar space between M, and the ascending ramus of the 0 1993 WILEY-LISS. INC. mandible (Brace, 1962; Howells, 1974; Trinkaus and Howells, 1979; Brace et al., 1981; Smith, 1983; Rak, 1986; Trinkaus, 1987; Smith and Paquette, 1989).Various of these features have been cited in support of hypotheses regarding the possible adaptive significance of Neandertal facial form. The majority of these hypotheses relate Nean- Address reprint requests to Mark A. Spencer, Department of Anthropology, S U N Y ,Stony Brook, NY 11794. Received May 11,1992; accepted September 30,1992 2 M.A. SPENCER AND B. DEMES dertal facial morphology to masticatory specializations (however, other explanations have been offered, e.g., related to cold adaptation [Coon, 19621).It h a s been argued that the Neandertal facial skeleton was well designed to resist heavy and/or repeated stresses resulting from extensive anterior tooth use (Smith, 1983; Trinkaus, 1983, 1987; Rak, 1986; Demes, 1987). This hypothesis is supported by features of the anterior dentition that suggest heavy use, such a s the robust morphology of the incisors, including the roots, and the intense wear common on these teeth (Brace, 1962,1964,1967; Brose and Wolpoff, 1971; Smith, 1976a,b,c; Brace et al., 1981; Puech, 1981; Smith, 1983; Trinkaus, 1983, 1987; Smith and Paquette, 1989). Many authors have argued that these features of the anterior dentition are evidence of their use in paramasticatory functions, the “teeth-as-tools’’ hypothesis (Brace, 1962). Conversely, it has been proposed that the posterior position of the zygomatic roots and the prognathism of Neandertals point to a reduced ability to produce high and/or repeated bite forces (Coon, 1962; Trinkaus and Howells, 1979; Trinkaus, 1982, 1983, 1987; Smith, 1983; Rak, 1986; Smith and Paquette, 1989).This idea is a major element of the “zygomatic retreat” model of Trinkaus (1987). It is counterintuitive that a form would show features related to a n increase in the ability to resist stresses in the anterior facial region in conjunction with features indicative of a reduction in the ability to produce bit forces a t the same location. The present study was undertaken to examine the adaptive significance of Neandertal masticatory system configuration using a biomechanical model and a comparative approach. Morphological changes that would allow more efficient force production’ on the incisors have been derived from a biomechanical model developed by Greaves ‘It is not possible to distinguish between skeletal adaptations for the production of either high forces, or repetitive forces on the incisors. The term “efficient” is therefore used in this paper to imply an increased mechanical advantage for the masticatory muscles. This increased mechanical advantage would, on theoretical grounds, allow the production of either higher magnitude forces without a n increase in muscular effort or the production of bite forces more repeatedly without an increase in total muscular effort. (1978). These hypotheses were tested by comparing Neandertal cranial specimens to those of less specialized and chronologically older forms, commonly referred to as early Homo sapiens. Additionally, a pairwise comparison of Inuits to other Native Americans was performed as a further test of the predictions of the biomechanical model. This comparison is important to the present study because precontact Inuits are known from ethnographic reports to have produced high magnitude and repeated forces on their anterior dentition (see summary in Hylander, 1977). While a direct comparison of Neandertals and Inuits would have little significance for developing adaptive scenarios, a dual comparison of these forms to less specialized but closely related populations allows a more complete examination of adaptations for increased incisor use. Biomechanical framework The biomechanical model to be used in this study was developed by Greaves (1978) and combines a n analysis of forces in both the sagittal and frontal planes. Comparative studies of masticatory function in primates are traditionally based on biomechanical models developed from a n analysis of forces in only the sagittal plane. These models may not, however, be adequate to examine the complex interaction of forces within the masticatory system (Gysi, 1921; Greaves, 1978; Smith, 1978; Walker, 1978; Wolff, 1984; Hylander, 1992). For example, differential loading of the temporomandibular joints cannot be evaluated in these models. Important constraints on masticatory system configurations are therefore neglected (see below). The temporalis, masseter, and medial pterygoid muscles exert an adducting (closing) moment on the mandible. During unilateral mastication, the forces produced by these muscles are resisted by reaction forces at the working side (biting side) temporomandibular joint (TMJ), the balancing side (nonbiting side) TMJ, and the bite point. When the vertical components of these forces are viewed superiorly (Fig. la), they form the corners of a triangle that Greaves (1978) termed the “triangle of support.” A critical constraint on muscle activity within NEANDERTAL AND INUIT MASTICATORY CONFIGURATION a Balancing Working Side TMJ Side TMJ Balancing Working Side TMJ Side TMJ Fig. 1. a: Superior view of mandible showing Greaves’s (1978) “triangle of support” for a P, bite point. The vertical component of the muscle resultant force (m) is seen end on and exerts an upward pull on the mandible that is resisted by forces at three points: the balancing side TMJ ( 0 ), the working side TMJ ( ), and the bite point ( 0 ) . b: Triangle of support produced during biting on the second molar. A midline muscle resultant force will not pass through this triangle and must be repositioned (arrow) to avoid producing tensile forces a t the working side TMJ. This repositioning of the muscle resultant toward the working side may be achieved through a reduction of balancing side muscle activity. + this model is that the resultant muscle force vector (that is, a single vector mathematically equivalent to all muscle force vectors combined) must pass through the triangle of support. If this constraint is violated, rotation of the mandible will occur, resulting in tensile forces within the working side TMJ. For example, when the masticatory muscles are maximally active, with the balancing side muscle force equaling that of the work- 3 ing side muscle force, the resultant force vector will lie in the midline. However, the triangles of support produced during isometric biting at points along the posterior end of the dental arcade (e.g., on M,) may not envelop this midline muscle resultant. The result will be a tendency for the mandible to rotate around an axis passing between the bite point and the balancing side condyle, producing tension within the working side TMJ (that is, the working side condyle will be pulled away from the articular eminence). That this does not regularly occur is suggested by experimental studies of joint reaction forces showing the working side TMJ of primates to be consistently loaded in compression during normal mastication (Hylander 1979a, 1985a; Hylander and Bays, 1978; Brehnan et al., 1981; Mongini et al., 1981; Boyd et al., 1982).Additionally, the constituents of the TMJ do not appear to be designed for resisting consistent and high magnitude tensile forces (Greaves, 1978, 1988; Bell, 1983, 1990). Hylander’s (1979a) observation of either compressive forces, no forces, or tensile forces in the working side TMJ during the special situation of powerful isometric biting on the third molar in the macaque was interpreted by him as indicating an inability to control the placement of the muscle resultant to a degree sufficient to consistently maintain it within the narrow region of the triangle of support produced during biting on the third molar. This interpretation is accepted here. Given that tensile forces are not experienced within the TMJ on a consistent basis, some alteration in muscle resultant position must occur if tensile forces are to be avoided during biting on more posterior teeth. Anteroposterior movement of the muscle resultant is limited by the fixed attachment sites of the masticatory muscles. However, significant mediolateral movements can be produced through differential activity of the masticatory muscles bilaterally (Greaves, 1978; Smith, 1978; Hylander, 1985b, 1992). Movement of the muscle resultant toward the working side, so that it passes through a small triangle of support, can be achieved through a reduction in balancing side muscle force (Fig. lb). Electromyographic (EMG) data in support of reduced balancing side M.A. SPENCER AND B. DEMES 4 -5 0 5 10 15 20 25 Distance from Temporomandibular Joint 30 Fig. 2. Plot of theoretical bite force values for bite points with various moment arm lengths, a s predicted from Greaves’s (1978)model. Notice that bite force values for bite points posterior to the muscle resultant are not plotted, since biting at these points produces tension in the working side temporomandibular joint. See text for explanation of regions I and 11. muscle activity during isometric biting and bite point is moved posteriorly along the mastication on molar bite points have been dental arcade, and are equivalent along the reported by several authors (Ahlgren, 1966; distal dentition (Greaves, 1978; Wolff, Mfiller, 1966; Luschei and Goodwin, 1974; 1984). Few bite force studies have reported Hylander, 197913, 1983; Hylander et al., data for maximum bite force potentials 1992). In contrast, essentially equal activity along the tooth row, but those that do of the ipsilateral and contralateral muscle (Worner and Anderson, 1944; Mansour and groups during isometric biting on the second Reynik, 1975; Pruim et al., 1980) are suppremolar in humans has been reported by portive of the segmented bite force distribuVan Eijden (1990) and Van Eijden et al. tion suggested by Greaves (1978) and Wolff (1990). Presumably, the large triangle of (1984). The length of the region in which high and support for this bite point encloses a midline muscle resultant, and therefore requires no equivalent bite forces can be produced is determined by two factors (Fig. 3). First, it is drop in balancing side activity. The lower activity of balancing side mus- impossible for the muscle resultant to be pocles during biting on posterior teeth results sitioned within the triangle of support for a in a reduction in the magnitude of the mus- bite point that is posterior t o the most postecle resultant, lowering the maximum bite rior muscle resultant position. It is therefore force potentials in this region. For all bite not expected that teeth will be located postepoints anterior to this region, maximum bite rior to a transverse line through the muscle forces increase rapidly as the bite force mo- resultant, since biting on these teeth would ment arm length decreases (Fig. 2) (i.e., as result in tensile forces in the working side the bite point is moved posteriorly). This TMJ. Second, the anterior end of this region curvilinear increase in bite force magnitude of even bite forces is marked by the point stops, however, at the transition into this where a line passing through the balancing region, and maximum forces remain equal side condyle and the midline muscle resulin magnitude along more posterior bite tant intersects the tooth row. Any bite point points. Thus, bite force potentials are lowest anterior to this intersection will have a trion the anterior dentition, increase as the angle of support in which a midline muscle NEANDERTAL AND INUIT MASTICATORY CONFIGURATION Balancing Side TMJ CONDYLAR AXIS Working Side TMJ Fig. 3. Four separate bite points ( 0 )and their corresponding triangles of support. Biting a t point A will produce a triangle of support in which the muscle resultant cannot be positioned since this triangle is posterior to a transverse line (horizontal dashed line) through the muscle resultant. While the muscle resultant can be moved toward the working side along this line, it cannot be moved posteriorly to a position within the triangle of support for point A. Biting at point A will therefore result in tensile forces within the working side TMJ ( 0 ). Biting at point B produces a triangle of support in which the muscle resultant can be positioned through a reduction in the balancing side muscle activity. Such a reduction will move the muscle resultant toward the working side and into the triangle of support, but will result in a decrease in the muscle resultant magnitude. Biting at point D produces a triangle of support in which a midline muscle resultant will lie. Thus, the muscle resultant need not move, and the balancing side and working side muscles can be maximally (and equally) active. The most posterior bite point along the tooth row which will have a triangle of support that envelops a midline muscle resultant is at point C. Any bite point anterior to the diagonal dashed line through this point will not require a reduction in balancing side activity; any bite point posterior to this line will require a reduction. Maximum bite forces on points posterior to this line are therefore reduced relative to those that could theoretically be produced by an undiminished muscle resultant. resultant will lie; any bite point posterior t o this intersection will not, and a reduction in balancing side muscle activity will be necessary. The region anterior t o this intersection point, in which bite forces increase distally, will be referred to as region I. The region posterior to this intersection (but anterior to the muscle resultant), in which maximum bite forces are equivalent, will be referred to as region 11. The presence of a discrete region in which uniformly high bite force po- 5 tentials exist led Greaves (1978) to suggest that the molar teeth, on which relatively high forces must be produced for efficient function, should be located within this region. A final important aspect of this model is that narrowing the relative dental arcade width results in an increase in the maximum bite force magnitudes within region 11. A medial movement of the molar tooth row has the effect of shortening the distance laterally that the muscle resultant must move to be positioned within triangles of support for molar bite points. Thus, less of a reduction in balancing side muscle activity is required. The result is higher maximum bite force magnitudes along the molar row (that is, within region 11). Expectations for intensified incisor use The above described model was used to derive hypotheses about expected configurational adaptations to intensified use of the anterior dentition. Although much of the model applies to the posterior dentition, the structural integration within the masticatory system leads to interactions among all masticatory components. It is therefore irnportant to understand constraints imposed by this integration when examining specializations for specific functions. There are essentially two ways to increase the efficiency of anterior force production through configurational changes within this model. First, the moment arm for anterior bite points can be shortened by a posterior migration of the anterior dentition. Relatively less muscular effort would have to be applied to produce an equivalent bite force on the incisors, or the same muscular effort would now produce a higher bite force. In the absence of a simultaneous repositioning of the postcanine dentition, however, this posterior migration would be checked by the presence of more distal teeth. Migration of the entire dental arcade posteriorly would reposition the molar teeth as well, resulting in a migration of the distalmost teeth posterior to the muscle resultant (Fig. 4a,b). As discussed above, biting on points that are posterior to the muscle resultant produces tensile forces within the TMJ. Thus, the function of the distal portion of the molar M.A. SPENCER AND B. DEMES 6 0 - L Fig. 4. a: Occlusal view of maxillary dental arcade showing a midline muscle resultant (w) and the temporomandibular joints ( 0 ), The muscle resultant may move mediolaterally through differential working side and balancing side muscle activity along the dashed horizontal line. b: Shortening the moment arm for anterior bite points through a posterior migration of the entire dental arcade (dashed figure represents original position). The third molar is forced posterior to the muscle resultant. I t is expected that this will compromise the efficiency of producing force on this tooth due to resulting tensile forces in the working side TMJ. c: Lengthening the moment arm for the muscle resultant through an anterior migration of the masticatory muscles. The third molar is similarly forced posterior to the muscle resultant. d: An anterior migration of both the muscle resultant and the dental arcade will result in an increase in the mechanical advantage of the muscles relative to the anterior dentition without compromising the function of the third molar. tooth row would be compromised, reducing the efficiency with which food is processed with the molars. Such a compromise may not be selected for if maintaining a minimal amount of masticatory efficiency is required. Limited posterior migration without compromising molar function is possible by flattening the anterior dental arcade against the postcanine dentition. A second configurational change that would increase the efficiency of force production on the anterior dentition is an anterior migration of the muscles of mastication, which would increase the length of the muscle force moment arms (Fig. 4a,c). However, maintenance of dental arcade position while moving the muscle resultant anteriorly will result in the distal end of the dental arcade being dropped behind the muscle resultant, again compromising the effectiveness of the grinding dentition (Fig. 4c). It is possible to avoid this sacrifice in molar efficiency through an anterior migration of the entire dental arcade, along with the muscle resultant (Fig. 4a,d). This would maintain the molar row within region 11, and would also result in an increase in the length of the muscle force moment arm relative to the moment arm for anterior teeth, increasing the efficiency of anterior bite force production. Essentially, moving the muscle resultant and the dental arcade anteriorly an equivalent amount increases the moment arms for both the muscle force and bite force by the same absolute value; adding a constant to the numerator and the denominator increases the ratio, thereby increasing the mechanical advantage of the muscle force (see Appendix A). Based on considerations described above, specific hypotheses regarding adaptations for intensified anterior tooth use can be formulated. First, it is expected that relative to their respective comparative groups, both Neandertals and Inuits will exhibit features related to increasing the mechanical efficiency with which forces are applied to the anterior dentition. Specifically, relative to their respective comparative groups, both forms should have either posteriorly positioned incisors, anteriorly positioned masticatory muscles, or both. Additionally, however, both forms should exhibit some modification of the molar dentition related to the above described constraints. There should either be evidence of reduced efficiency of third molar function or a repositioning of the molar tooth row so that it is retained within region 11. Such a repositioning may actually lead to an increase in the distance of the anterior dentition from the TMJ while still allowing for increased efficiency of incisal force production. MATERIALS AND METHODS To test the above hypotheses regarding morphological specializations for increased incisor use, the spatial distribution of relevant masticatory components was quantified. Four samples of hominids were examined: early Homo sapiens (n = 4) including NEANDERTAL AND INUIT MASTICATORY CONFIGURATION 7 Medial Pterygoid Masseter Temporalis Fig. 5. Occlusal view of cranium showing measurements of tooth and muscle position relative to the defined baseline axis (see text for explanation). Steinheim, Petralona, Kabwe (Broken Hill), and Bodo; Neandertals (n = 8) including Monte Circeo 1 (Guattari), La Ferrassie 1, Amud 1, La Chapelle-aux-Saints, Saccopastore 1, Gibraltar 1, Tabun C1, and Shanidar 1; Native Americans from Grand Gulch, Utah (7 females; 10 males); and precontact Inuits from Point Barrow, Alaska (8 females; 12 males). All specimens were measured at the American Museum of Natural History. The fossil specimens were casts. Published values (Morant, 1928; Conroy et al., 1978; Trinkaus, 1983, 1987) for various facial dimensions were compared to measurements taken from the casts and no significant differences were observed (average difference = 1.93%) for any of the specimens. Both male and female specimens were included in the modern samples since the sexes of all examined fossil specimens are not certain, and exclusion of one sex in the modern samples would artificially reduce the observed variance. Additionally, because the majority of the fossil specimens lack an associated mandible, all measurements were taken from crania alone. Not all measured features were present on each skull and thus sample size varies by measurement. Measurements were taken as projections onto the occlusal plane to make them most comparable to the above model, which is de- rived from an analysis of forces vertical to this plane. Video images of each skull oriented in the occlusal, frontal, and lateral views were recorded using a S-VHS camcorder. These images were digitized into a personal computer using a video framegrabber board. Linear distance data were then gathered from each image using a screen cursor within the JAVA video analysis software (Jandel Scientific, Corte Madera, CA). The accuracy of this technique was tested by collecting six measurements from each specimen using digital calipers. The resulting values were compared to those obtained from the computer system. The two sets of measurements differed on average by 2.5%,with a maximum difference of 4.8%. The measurements taken include: maximum dental arcade width, dental arcade length, and biarticular breadth (this was measured as the distance between the most inferiorly projecting point on the postglenoid processes to maintain consistency). Also measured were the positions of individual teeth. From an occlusal view, a line was drawn between the postglenoid processes and used as a baseline axis (Fig. 5). Tooth positions were then measured as the perpendicular distances from this baseline to the bite points. In many specimens the teeth were missing, making exact bite point position determination difficult. Therefore, 8 M.A. SPENCER AND B. DEMES prosthion was used as the landmark for determining central incisor position. While prosthion is damaged in the Gibraltar 1, La Ferrassie, and La Chapelle-aux-Saints specimens, it was felt that the damage was minimal enough to allow reliable estimates of the position of this point. (Removal of these estimated values from the data set actually results in a n increase in the observed differences between the Neandertal and early Homo sapiens samples.) Prosthion position could not be measured in the Steinheim cranium, which lacks a n anterior maxilla. The position of each molar was determined by a landmark in the center of the tooth, and was only measured if the tooth was present. For example, Monte Circeo 1, La Chapelle-auxSaints, and Gibraltar 1all lack a first molar and they were therefore excluded from measurements of M1 position and subsequent calculations of indices based on this measurement. Tooth position measurements were used a s estimates of the moment arm length for bite forces produced on the respective teeth. Although this estimate of moment arm length assumes that the bite force is vertical relative to the occlusal plane, this assumption is justified on the basis of experimental human bite force data of Van Eijden (1991). In these experiments it was found that maximum bite forces were most consistently produced in a direction perpendicular to the occlusal plane. In addition to the above measurements, estimates of muscle position were measured for each of the adducting masticatory muscles (masseter, temporalis, and medial pterygoid) in those specimens retaining the relevant morphology. Although estimates of moment arm length for muscular force vectors would be more valuable for a biomechanical analysis, such estimates are possible only on specimens that possess a mandible. It is therefore assumed that the anteroposterior position of the origin of each muscle reflects the length of the moment arm for that muscle (see e.g., Du Brul, 1974, 1977; Simons, 1976; Hylander, 1977, 1979b; Carlson and Van Gerven, 1977; Hinton and Carlson, 1979; Ward and Molnar, 1980; Rak, 1983). Muscles positioned more anteriorly relative to the baseline axis would, therefore, have longer moment arms. Muscle po- sition was measured a s the perpendicular distance from the baseline axis to the anteriormost point of origin of each muscle as follows (see Fig. 5): masseter-inferior border of zygomatic root at zygomaticomaxillary suture; temporalis-most posterior point on the lateral orbital margin a s seen from a lateral view (near zygomaticofrontal suture); medial pterygoid-pterygopalatine suture a t posterior edge of hard palate (near the apex of the medial and lateral pterygoid plates). The lateral pterygoid muscle was not included in this analysis because it has no adducting component (Stern, 1988). The values for individual muscle positions were treated in two ways. First, a n average muscle position was calculated for each specimen (this is simply the average of the muscle position values for all muscles examined) and was compared to the bite force moment arm for individual bite points by computing ratios of these values. These ratios express the combined mechanical advantage of all adducting muscles relative to specific teeth. Second, individual muscle positions were weighted by multiplying them by the percentage physiological cross-sectional area of the respective muscle as measured in modern Homo sapiens. For example, the masseter represents approximately 37% of the total cross-sectional area of the mandibular adductor musculature in modern humans (Weijs and Hillen, 1984), so the position of the masseter muscle was multiplied by a factor of 0.37. Similarly, the medial pterygoid and temporalis positions were multiplied by factors of 0.24, and 0.39, respectively, corresponding to their percentage cross-sectional area in modern humans. The weighted positions for all muscles were averaged for each specimen and compared to bite force moment arms for incisal and molar bite points, as above. The purpose of using a weighted muscle position value was to represent individual muscles in approximate proportion to the amount of force they could produce, thereby reducing a bias toward less powerful muscles. Due to small sample sizes, and because these data include ratios, the conservative nonparametric Mann-Whitney U-test was utilized to compare the means for all variables of interest in this study. NEANDERTAL AND INUIT MASTICATORY CONFIGURATION 9 RESULTS true for the medial pterygoid muscle. As a The results of this analysis are listed in result, the average muscle position value Tables 1 and 2 and shown graphically in was also greater in Neandertals, indicating Figures 6 and 7. Important differences ob- that relative to the TMJ, the muscles of masserved within each pairwise comparison tication are consistently (although not sig(Neandertals vs. early Homo sapiens, and nificantly) more anteriorly positioned in NeInuits vs. Native Americans) will be pre- andertals. This result does not differ when the weighted muscle position values are sented separately below. Examination of Table 1 reveals that there compared. As discussed above, it is unlikely are no significant differences between Ne- that size differences could account for more andertals and early Homo sapiens in the posteriorly positioned incisors as well as raw values for various masticatory dimen- more anteriorly positioned masticatory sions. While there are differences in the muscles in Neandertals compared to early mean values between groups, the pattern of Homo sapiens. When ratios of average muscle position difference is completely cross-cutting, with Neandertals exhibiting larger values for 6 of divided by incisal moment arm length are the 10 dimensions and early Homo sapiens compared, Neandertals are found to have a having larger values for the remaining 4 di- greater value than early Homo sapiens and mensions. In the comparison of Inuits to Na- this difference is significant at a probability tive Americans, while there are significant level of P < 0.05 (although this difference is differences observed in many of the mea- not significant when a n experimentwise sured dimensions, the pattern of difference probability level of P < 0.005 is used). This is cross-cutting as well; Inuits and Native indicates an increase in the mechanical adAmericans are each greater than the other vantage of the masticatory muscles relative group for 5 of the 10 dimensions. These ob- to the anterior dentition. In fact, there is no servations suggest that the masticatory sys- overlap of ranges between the two groups for tem is similar in size in the two groups in this value. Thus, although the differences in each of these comparisons, and that differ- incisor position and muscle position beences in size are not driving the observed tween Neandertals and early Homo sapiens are not independently significant, when a morphological differences. Because the dimensions of the mastica- biomechanicaIIy important ratio of these tory system that are examined in this study two values is computed, Neandertals appear are functionally interconnected, they consti- to have been significantly better designed tute a unique shape, the form of which de- for producing force on their anterior dentitermines the mechanical efficiency of the tion. This result is supported by comparisystem. The resulting mechanical efficiency sons of the weighted muscle position/ is dependent solely on the exact shape of the anterior dentition moment arm values, system and not on its size. It is changes in indicating that the outcome is not biased by this shape, and therefore in the mechanical overemphasizing relatively less powerful efficiency of the system, that are of interest muscles. The molar dentition is, on average, 5.2 in this project. mm more anteriorly positioned relative to Neandertals vs. early Homo sapiens the TMJ in the Neandertal specimens than The position of the anterior dentition rela- in the early Homo sapiens specimens, a s intive to the TMJ was found to be more poste- dicated by the distance of the first molar rior in Neandertals than in the early Homo from the baseline axis. This difference is not sapiens sample (Fig. 61, although this differ- significant. However, the Tabun specimen ence is not statistically significant. Mea- has a n unusually low value for this dimensurements of muscle position show that the sion, falling well within the range for the masseter, medial pterygoid, and temporalis modern Homo sapiens samples. When it is muscles were all more anteriorly positioned removed from the calculations of molar row relative to the TMJ in Neandertals than in position, Neandertals are significantly difearly Homo sapiens. This is particularly ferent from the early Homo sapiens speci- n 8 5 8 8 8 8 8 8 8 8 Prosthion M1 Masseter Medial pterygoid Temporalis Dental arcade length Dental arcade width Average muscle position Weighted muscle position Biarticular breadth 107 59 t 2 63 80 48 t 4 70 67 22 i 1 2 9 49 20 i- 2 54 49 59 5 2 18 54 63 +- 2 24 70 21 t 1 57 55 33 I 1 31 18 73 i- 0 40 113 31 i- 2 02 ~ 3 4 4 4 4 < > > < > , > -, > 3 3 4 4 4 (i- ~ ~ Sig _ n _ > 3 > 3 .. 3 >'I 3 >:,: 3 > 3 -> 3 >. 3 > 3 ; 3 ~ ' c ' 20 20 20 20 20 20 20 20 20 20 90.47 t 0.94 66.37 t 1.09 64.70 t 0.64 37.14 5 0 . 4 1 49.37 i- 0.68 47.38 i 0.79 60.33 i- 0.99 51.40 2 ,048 17.38 i 0.17 104.72 i- 1.01 20 20 20 20 20 20 20 20 20 20 ~~~ n ~~ Sig.. _ 17 17 17 17 17 17 17 17 17 17 n 93.43 t 0.67 66.84 t 0.78 62.01 t 0.52 38.43 i ,072 46.59 % ,085 51.51 * ,046 63.95 % 0.68 49.34 i 0.49 16.87 i 0.17 104.51 i 1.18 Native Americans (t standard error) 71.57 0.58 41.07 i 0.34 54.64 i 0.82 55.76 i 0.48 19.23 5 ,018 97.77 t 1.16 58.10 5 0.61 74.71 i- 1.45 76.20 i 0.99 26.95 i 0.58 * 17 17 17 17 17 17 17 17 17 17 66.40 5 0.56 42.19 i 0.64 49.88 5 0 . 9 1 52.82 i- 0.44 18.06 i ,016 92.88 i 0.83 58.96 * 0.66 69.78 ? 1.31 73.87 5 0.54 25.22 5 0.41 Native Americans Inuits n Sig. it standard error) i -t standard error) ~ - ~_ _ _ - ~ Inuits n it standard error) ~- -~ -~ 0 005 (Klocknrs and Sax, 1986; Rice, 1989). 57.39 i- 2.98 39.52 i 0.89 37.12 i- 2.61 44.67 i- 2.10 15.06 t 0.77 80.51 i 2.16 57.61 t 1.79 56.14 i- 6.38 64.75 i 3.38 22.07 -t 1.92 (istandard error) -~~ Early Homo sapiens TABLE 2. Ratios (mmlmini f0.05Ino. ofcomparisons) or P 62.62 t 1.32 45.61 f 1.48 46.12 rt 1.96 51.45 t 0.40 17.41 & 0.22 84.54 C 3.92 62.73 t 1.52 60.21 k 5.03 68.83 -t 2.39 23.53 -t 1.18 "_ ~~ 113.70 t 3.20 75.30 ? 1.35 64.43 i- 3.88 43.73 i 1.49 44.28 i 3.55 62.44 i 2.06 72.70 t 5.92 51.14 k 5.47 17.28 ? 0.97 113.03 t 6.71 ~~ Early Homo sapiens (istandard error) ( 0 OSIno. afcomparisons)or P c 0.005 (Klockars and Sax, 1986; Rice, 1989) Neandertals standard error) LO listed point Significant a t P 0.05 ' Significant at P 0.01. " Sipnilcant using Bonferroni's prolncted probability level o f P ' Linear distance from Imseline axis 8 8 8 8 Masseteriprosthion Medial pterygoidlprosthion Temporalisiprosthion Average muscle positiodprosthion Weighted muscle positiodprosthion MasseterMl Medial p t e r y g o i m l TemporaIisM 1 Average muscle p o s i t i o a l Weighted muscle Dosition/MI 8 5 5 5 5 5 n Variable * Significant a t P :0.01. * : Significant using Bonferrani's protected probability level o f P c. n - i 'Linear distance from baseline axis to listed point (unless otherwise indicated~. Significant a t P c: 0.05 ~~ Variable Neandertals (i- standard error) Sig. ~- TABLE 1. Linear dimensions (mmi' NEANDERTAL AND INUIT MASTICATORY CONFIGURATION 8 I Baseline Axis 11 8 . Fig. 6. Proportionate diagram of average Neandertal and early Homo supiens masticatory system configuration. Boxes with identification symbols at corners represent the average maximum width and length of the dental arcade in each group. The average positions of prosthion and the first maxillary molar are indicated. Dental arcades are shown in position relative to the baseline axis, the line connecting landmarks on the postglenoid processes. Muscle positions relative to the baseline axis are shown as vertical bars to the right in scale to the dental arcades. Neandertals are shown as open bars and early Homo supiens as filled bars. mens at a probability level of P < 0.05, with the first molar being positioned, on average, 9.5 mm more anteriorly in Neandertals. The observation of more anteriorly positioned molars, combined with more posteriorly positioned incisors in Neandertals, results in an average dental arcade length that is shorter (although not significantly) in Neandertals than in early Homo sapiens. Shortening of the dental arcade in Neandertals was suggested by Rak (1986), and is minimally supported by measurements from the mandible (Trinkaus, 1987). Ratios of individual muscle positions divided by the length of the moment arm for an M1 bite point show that the masseter, temporalis, and medial pterygoid muscles all have a greater mechanical advantage for M1bite points in Neandertals than in early Homo sapiens. Ratios of average muscle position and weighted muscle position divided by the moment arm length for M1 are also higher in Neandertals than in early Homo sapiens. None of these differences is significant. The maximum width of the dental arcade is not significantly different in Neandertals compared to early Homo sapiens. Inuits vs. Native Americans There are several similarities in the pattern of differences observed between Inuits and Native Americans and that described for the Neandertal-early Homo sapiens comparison. The length of the moment arm for anterior bite points is significantly shorter in the Inuit sample at P < 0.05 (see Table 1 and Fig. 71, as was also reported by Hylander (1972, 1977). The positions of the masseter and temporalis muscles were found to be significantly more anterior in Inuits relative to Native Americans (see also Hylander, 1972, 1977). Unlike in Neandertals, however, the medial pterygoid of Inuits was not found to be anteriorly positioned. Rather, the average position of this muscle is slightly (but not significantly)more posterior than in the Native American sample. The possible significance of this difference will be discussed below. As in the Ne- M.A. SPENCER AND B. DEMES 12 I Prosthion 0 I e Baseline Axis Fig, 7. Proportionate diagram of average Inuit and Native American masticatory system configuration. See legend for Figure 6 for explanation of symbols. andertallearly Homo sapiens comparison, it is unlikely that differences in size alone could result in a shorter distance from the TMJ to prosthion and a greater distance from the TMJ to the masticatory muscle origins in Inuits compared to Native Americans. Ratios of average muscle position divided by incisal bite force moment arm length are significantly greater in Inuits than in Native Americans even when a n experimentwise probability level of P < 0.005 is used. A similar result is obtained when weighted muscle position values are compared. Thus, like Neandertals, Inuits have masticatory muscles that are favorably positioned for producing either high magnitude or repeated forces on the anterior dentition. The molar teeth of Inuits are the same distance from the baseline axis as they are in the Native American sample. There is no significant difference in molar dentition moment arm lengths between these two groups. This maintenance of relative molar position in Inuits, along with the more posteriorly positioned anterior dentition, results in a significantly shorter dental arcade length a t P < 0.005. Ratios of individual muscle positions divided by the length of the moment arm for M1 bite points are significantly greater in Inuits than in Native Americans for the masseter and temporalis muscles a t P < 0.005 and P < 0.01, respectively, but less for the medial pterygoid muscle. Both the average and weighted muscle position values are greater in Inuits compared to the moment arm lengths for M1 bite points a t P < 0.01. Dental arcade width is significantly less (P < 0.01) in the Inuit sample than in Native American specimens. DISCUSSION The above results reveal a number of similarities in the morphological specializations exhibited by Neandertals and Inuits, In addition, however, there are important differences in the pattern of morphological specialization found in these groups. This section will first discuss the possible biomechanical and functional significance of alterations observed within each pairwise comparison, and will then contrast the unique features of Neandertals with those of Inuits. NEANDERTAL AND INUIT MASTICATORY CONFIGURATION 13 Neandertals vs. early Homo sapiens efficiency. The results of the present study In contrast to those authors who have sug- therefore suggest that it is unlikely that Negested that the masticatory system of Nean- andertals were comparatively poorly dedertals was poorly designed for producing signed for producing forces on their anterior high or repeated forces on the anterior den- dentition, and that they may have been tition (e.g., Coon, 1962; Trinkaus and How- quite proficient in this f ~ n c t i o n . ~ An examination of Table 1 reveals that ells, 1979; Smith, 1983; Trinkaus, 1983, the modern Homo sapiens samples (both In1987; Rak, 1986; Smith and Paquette, 1989; Anton, 1990),our results suggest that Nean- uits and the Grand Gulch sample) exhibit dertals were in fact well designed for effi- greater values for ratios of muscle position cient incisal use in comparison to closely re- over bite force moment arm length than the lated but less specialized forms.’ This Neandertal specimens (with the exception of increased efficiency arose by virtue of an an- the medial pterygoid muscle). As discussed terior migration of the masticatory muscles in the introduction, such a comparison is of and a dimunition of the distance between little significance for this study since Neanthe incisors and TMJ. The hypothesis that dertal and Inuit specializations must be unthe masticatory muscles of Neandertals had derstood within an evolutionary framework. a comparatively poor mechanical advantage The comparative groups used in this study, is based largely on the posterior position of while not necessarily directly ancestral to the zygoma relative to the maxillary molars Neandertals or Inuits, are used because in Neandertals (above M2-M3) compared to they are assumed to exhibit a relatively genearly and modern Homo sapiens (above MI- eralized form from which these groups are M2) and the increased frequency of a retro- thought to have become specialized. That molar space between the anterior border of modern Homo sapiens groups exhibit larger the ascending ramus of the mandible and values for the biomechanical ratios examthe mandibular third molar in Neandertals ined in this study is interpreted as being a (Trinkaus, 1987).These features are cited as result of the overall trend toward orthoindications of a posterior shift of the masti- gnathy that characterizes the evolution of catory muscles relative to the dental arcade. modern Homo sapiens and not a specific adHowever, it is the Iength of the moment aptation to intensive use of the anterior denarms for muscular force vectors relative to tition. This position is supported by the the moment arm for a particular bite force absence of any additional craniodental feathat is the most biomechanically relevant tures suggestive of intense anterior tooth variable when discussing the efficiency with use in the generalized modern Homo sapiens which bite forces are produced. These ratios skull. The anterior position of the molar teeth are higher in Neandertals than in early relative to the TMJ in Neandertals comHomo sapiens. Although muscular moment arms were not directly measured for reasons pared to early Homo sapiens indicated by discussed above, it is reasonable to assume the present data can be interpreted in terms that more anteriorly positioned masticatory of the biomechanical model used in this muscles would have had increased moment study. As outlined in the expectations for arm lengths. Indeed, a correlation between intensified anterior tooth use, an anterior muscle position and muscle moment arm migration of the muscle resultant should be length is also implicit in the “zygomatic re- accompanied by an anterior migration of the treat” model (Trinkaus, 19871, as well as dental arcade so that the molar row is mainmany other interpretations of masticatory tained within region 11,thereby allowing the retention of a fully functional molar region ’The phrase “welldesigned for”is used interchangeablyin this paper with “adaptedto”to reflect our inability to determineif the observed differences between Neandertals and early Homo sapiens were adaptive in the strictest sense (i.e.,they were heritable and conferred a selective advantage on the individuals possessing them). 3The absolute maximum magnitude of the bite force is ultimately determined by the positional parameters that are the subject of this paper and by the size of the masticatory muscles, This study does not examine the latter ofthese factors. 14 M.A. SPENCER AND B. DEMES while improving the mechanical efficiency of producing forces on the anterior dentition. The observation in Neandertals of: (1) an anterior migration of the masticatory muscles, ( 2 ) an anterior migration of the molar dentition, and (3) an improved mechanical advantage for the masticatory muscles relative to the anterior dentition is support for this hypothesis and is difficult to interpret within other theoretical frameworks. It is suggested that this unique combination of differences between Neandertals and early Homo sapiens may have allowed Neandertals to produce forces on their anterior dentition more efficiently while maintaining some critical molar occlusal area. Shortening of the dental arcade in Neandertals is interpreted as being the result of the anterior migration of the molar teeth and the posterior repositioning of the incisors indicated by the present data. A consequence of this configurational change is the apparent reduction in curvature of the anterior dentition so that the teeth are more coronally aligned than in early Homo sapiens. It is suggested that this alteration in dental arcade shape is the result of the differential demands placed on the separate functional regions of the dentition. The apparent separation of the masticatory muscles relative to the molar dentition suggested by zygoma position and the high frequency of a retromolar space in Neandertals is here interpreted as the result of a differential anterior migration of the molars and the masseter and temporalis muscles. That these morphological features do not imply a reduction in masticatory efficiency is shown by the greater values of Neandertals for ratios of muscle position divided by the moment arm length for M1 and incisal bite points. The greater absolute distance between the first molar and the masseter and temporalis muscle positions in Neandertals than in early Homo sapiens is the result of a more marked anterior migration of the molar teeth than of the masseter and temporalis muscles. The difference in the amount of anterior migration of these elements is, however, not great enough to diminish the mechanical advantage of these muscles (Fig. 8). The medial pterygoid muscle shows the a BF BF b Fig. 8. a: Lateral view of masticatory system showing TMJ ( 0 ) and mandibular molar dentition. A hypothetical muscle force vector (M) results in a joint reaction force (JRF) and a bite force (BF). The magnitude of the vectors J R F and BF are determined by the magnitude of M and by the ratio of the muscle force moment arm (b) divided by the bite force moment arm (a). Note that the muscle vector lies between M2 and M3. b: In this figure, the ratio d/c is equal to the ratio bia, indicating that the mechanical advantage of this muscle for a n M1 bite point has not been altered. However, both the muscle vector and the molar dentition have been moved anteriorly relative to a. The differential amount of absolute anterior migration of these elements, while not affecting the efficiency of molar force production, results in a more posterior position of the muscle vector relative to the molar teeth (the vector now lies over M3). greatest anterior migration of the muscles in Neandertals. The observed anterior migration of the molar dentition in Neandertals would have opened a space into which the origin of the medial pterygoid muscle could have migrated. This muscle has been frequently neglected in comparative studies of masticatory function, but is clearly an important component of the musculature, representing 24% of the total physiological cross-sectional area of the mandibular adductors in modern humans (Weijs and Hillen, 1984). The marked anterior migration of this muscle could have relaxed the need for a greater anterior migration of the masseter and temporalis muscles. That is, for a given amount of anterior migration of the combined muscle resultant (this would be limited by the requirement that the mus- NEANDERTAL AND INUIT MASTICATORY CONFIGURATION cle resultant not pass anterior to the distal dental arcade), a large anterior migration of the medial pterygoid would reduce the extent to which the masseter and temporalis muscles would have to migrate. Inuits vs. Native Americans The conclusions reached by Hylander (1972,1977) regarding the functional significance of Inuit craniofacial morphology are supported by the present study. The more anterior position of the masseter and temporalis muscles and the more posterior position of the incisors in Inuits indicate an increased efficiency for the application of either high magnitude or repeated bite forces on the anterior dentition. Despite the observation of more posteriorly positioned incisors and more anteriorly positioned masticatory muscles in Inuits, the molar teeth were observed to be positioned almost exactly the same distance from the TMJ in Inuits and Native Americans. As outlined above, an anterior migration of the muscle resultant will result in a compromise in the usefulness of the distal end of the tooth row in the absence of a simultaneous anterior shift of the molar teeth. Indeed, Inuits have been reported by numerous authors to exhibit a high rate of third molar agenesis compared t o other human populations (see summary in Hylander, 1977). It could be expected that functionally deficient teeth that aid little in the processing of foods would be selected against due to the susceptibility to dental pathologies (Mayhall, 1977). The high frequency of third molar agenesis noted in Inuits, therefore, serves as support for the predictions of the current biomechanical model. While third molar agenesis has been explained previously as the result of developmental perturbations, or as an associated characteristic of overall facial reduction (Bermudez de Castro, 19891, these explanations and that offered here are not mutually exclusive. The posterior migration of the incisors and positional maintenance of the molar teeth in Inuits result in a shortening of the dental arcade length in this group relative to Native Americans. Contrary to Neandertals, the medial pterygoid muscle does not contribute t o the increased value of the aver- 15 age muscle position in Inuits. The retention of the molar teeth in a similar position in Inuits and the structural proximity of the pterygoid fossa and the lateral pterygoid plate (from which the medial pterygoid muscle originates) to the maxillary third molar could constrain the degree to which the position of this muscle might change. Even those specimens that exhibit third molar agenesis retain an alveolus posterior to the second molar. Thus, it would not be possible for the medial pterygoid muscle to migrate anteriorly, due simply to the presence of the maxillary dental arcade. A final difference observed between Inuits and Native Americans is the decreased width of the dental arcade in Inuits. Narrowing of the dental arcade would allow greater bite forces to be produced within region 11. This is because the balancing side muscles may be more active during biting on region I1 bite points while still maintaining the muscle resultant vector within the triangle of support. Thus, narrowing of the dental arcade in Inuits is here interpreted as an alteration for producing higher magnitude forces along the molar dentition. This hypothesis is supported by several unique features of Inuit dental and skeletal morphology identified by Hylander (1977), including: increased root resorption, palatal tori, more vertically oriented tooth roots, and a high frequency of triple rooted molars. Also supportive of the hypothesis of high postcanine force production in Inuits are ethnographic reports of dietary behaviors requiring extreme bite forces, such as bone crunching and consumption of frozen meats (Hylander, 1977). Finally, maximum molar bite force measurements have been recorded for Inuits and these are roughly two to three times higher than has been recorded for any other modern population (Hylander, 1977). This powerful molar bite force is undoubtedly also the result of relatively more powerful masticatory muscles in Inuits but may in part be a function of decreased dental arcade width. Comparison of Neandertals and Inuits: adaptive scenarios The biomechanical parameters examined in this study suggest that Neandertals and 16 M.A. SPENCER AND B. DEMES Inuits share several specializations of the masticatory system related to an increased emphasis on anterior tooth use. It has been shown through ethnographic accounts that Inuits used their anterior dentition in a variety of paramasticatory functions that would require high magnitude and repeated force production (e.g., Birket-Smith, 1935; de Poncins, 1941; Vanstone, 1962). Examples of such functions include: holding a bit for a bow and bit drill, softening frozen seal skin, holding slats of wood while shaping, and pulling seals out of the water. It may therefore be inferred that the similarities between Neandertals and Inuits regarding masticatory morphology are an indication of intensified use of the anterior dentition in Neandertals. Strong support for these biomechanically derived hypotheses comes from the similarities in anterior tooth condition in these forms. Both are characterized by relatively large anterior tooth dimensions (Brace, 1962,1964,1967; Turner and Cadien, 1969; Smith, 1983; Trinkaus, 1982, 19871, by excessive amounts of attrition on their anterior dentition (Smith, 1976a,b,c; Smith, 1983; Trinkaus, 1983; Ryan and Johanson, 1989; Schour and Sarnat, 1942; Merbs, 1968; Turner and Cadien, 1969; Molnar, 1971, 1972; Brace et al., 1981), and by the presence of dental chipping and pitting on the incisors (Ryan, 1980; Brace et al., 1981; Wolpoff et al., 1981; Ryan and Johanson, 1989).These latter features have been associated with “the crushing of food with adhering grit, or the clamping of hard nondietary materials” (Ryan and Johanson, 1989: p. 249). While there is some debate over the precise significance of these similarities (Wallace, 1975; Ryan, 1980; Brace et al., 1981; Puech, 19811, in conjunction with the results of the present study, they strongly support the hypothesis of extensive anterior tooth use in Neandertals. As in Inuit populations, it is likely that this adaptation would have been useful in a wide spectrum of functions. While there are numerous similarities between Neandertals and Inuits that can be interpreted as biomechanical specializations for anterior tooth use, there are also many differences evident in the postcanine dentition. The most marked dissimilarity is the anterior migration of the molar tooth row in Neandertals (compared to early Homo sapiens) and the lack of such a migration in Inuits (compared to Native Americans). It is suggested that the anterior migration of the molar row in Neandertals allows the retention of the entire molar dentition within the zone of most efficient bite force production. This would allow the maintenance of a fully functional molar dentition without a sacrifice in occlusal area. That Inuits do not show this configurational modification and, as a consequence, sacrifice molar occlusal area (as evidenced by the high rate of third molar agenesis reported for this group) is an important factor in support of the predictions of the current model. Additionally, however, this difference suggests that the retention of an unreduced occlusal area in Inuits was of limited importance. It is possible that reduced postcanine occlusal area in Inuits was adaptive in allowing for greater occlusal pressures with the same muscle force (see Walker, 1981; Demes and Creel, 1988). The size of the occlusal surface of a tooth has been related to both the functional requirements needed to process foods with certain mechanical properties (Jolly, 1970; Kay, 1975; Kay and Hylander, 1978; Pirie, 1978; Lucas and Luke, 1984) and t o attrition rates resulting from specific diets (Molnar, 1972; Hylander, 1975; Smith, 197613; Ungar and Grine, 1990). Thus, a reduction in the total postcanine occlusal area in Inuits may be possible due to lower attrition rates. This hypothesis is supported by data summarized in Hylander (1977) that indicate a much lower level of molar dental attrition in Inuit populations than in Native American samples. As stated by Hylander (1977),“the lower rates of attrition and high frequencies of third molar agenesis among Eskimo populations suggest that selective pressures favoring large tooth size have been relaxed.” (p. 146) Hylander (1977)has suggested that it was the nature of the diet in Inuits that resulted in lower rates of attrition within this group. “The Eskimo diet is composed of high energy food. Therefore it is unnecessary to process excessively NEANDERTAL AND INUIT MASTICATORY CONFIGURATION large amounts of food per day. Although eating frozen meat and crunching bones might necessitate a powerful bite, attrition rates associated with this diet would be less than with a diet consisting of low energy food with relatively large amounts of abrasives.” (p. 146) 17 Inuits, would have had access to extensive plant resources, and this is reflected in both their postcanine dental morphology and in the configuration of their masticatory system. Conclusions The application of the biomechanical The tundra environment in which northern Alaskan Inuits lived offers few plant re- model used in this study has allowed the sources; their primary food sources con- identification of similar morphological spesisted of seal, walrus, caribou, and fish (Mo- cializations in Neandertals and Inuits reran, 1979). These foods are low in structural lated to a n increased efficiency of force procarbohydrates, and are therefore relatively duction on the anterior dentition. These nonabrasive. Conversely, the plant foods configurational similarities, along with evithat made up a large percentage of Native dence from tooth morphology and attrition, American diet are composed largely of struc- and modifications of the facial skeleton for tural carbohydrates and are comparatively increased resistance to masticatory forces, highly abrasive. The absence of plant food in support the hypothesis that Neandertal the Inuit diet would, therefore, provide the craniofacial morphology is in part specialopportunity for some reduction in postca- ized for intensive anterior tooth use. The hynine occlusal areas, as has apparently oc- pothesis that the masticatory muscles of Neandertals were relatively poorly positioned curred. It is suggested that morphological special- for the application of forces on the anterior izations that allow the retention of a n undi- dentition is rejected. Differences in Neandertal and Inuit masminished occlusal area in Neandertals are a n indication of a relatively abrasive diet. ticatory specializations are interpreted as This conclusion is supported by the observa- indicating different dietary preferences in tion of similar postcanine dental dimensions these groups. The facial configuration of Inin Neandertals and early Homo sapiens uits appears to have been optimized for the (Brace, 1967; Brose and Wolpoff, 1971; production of forces on the anterior dentiBrace et al., 1981; but see Smith, 1976a,b,c) tion and on the molar dentition, but at the and extremely high rates of attrition in sacrifice of postcanine occlusal area. This Neandertal postcanine dentition (Smith, can be related to: (1)various aspects of den1976a,b,c). It is possible that the taurodont tal morphology and attrition patterns, (2) form of Neandertal molars is related to this ethnographic reports of excessive force prohigh attrition environment as well. A duction on both the anterior and posterior greater depth of dentine would prolong the dentition, and (3) a high energy, low attrilife of a tooth being exposed to highly abra- tion diet that requires high masticatory sive foods. Exploitation of plant foods was forces to utilize but does not require large probably a n important component of the Ne- occlusal areas. In contrast to this pattern, the biomechanical factors identified in this andertal subsistence strategy. Climatic conditions during the chronologi- study suggest that Neandertal facial configcal range of Neandertals are characterized uration was optimized for the production of a s interglacial (128-118 Kyr) and early gla- high forces on the anterior dentition and the cial (118-32 Kyr). During the interglacial retention of some critical occlusal area on period, ample plant resources were avail- the postcanine tooth row. This can also be able in the European and Near Eastern ar- related to aspects of dental morphology and eas inhabited by Neandertals. While food attrition patterns and may be used to infer a sources would have been reduced during the diet consisting in part of some abrasive food early glacial stages, “these environments source such as plant materials. were certainly extremely productive and exACKNOWLEDGMENTS hibited a unique combination of floral and We thank the following for their imporfaunal elements.” (Gamble, 1986: p. 101) It is therefore likely that Neandertals, unlike tant comments on this paper. Jack T. Stern M.A. SPENCER AND B. DEMES 18 Jr., William Jungers, and Lillian Spencer. We also thank Jamie Brauer and Ian Tattersall for access to the collections at the American Museum of Natural History. Heinz Herwig also provided valuable advice on the mathematical proof. than the moment arm for an anterior bite point, moving both the muscles of mastication and dental arcade forward by an equal amount will result in an increased efficiency of anterior bite force production. APPENDIX A If the muscle force moment arm is defined as ‘a’ and the bite force moment arm is defined as ‘b’then the relative mechanical advantage of the muscle can be defined as: Ahlgren J (1966)Mechanism of mastication: A quantitative cinematographic and electromyographic study of masticatory movements in children, with special reference to occlusion of the teeth. Acta Odontol. Scand. 24t1-109. Anton SC (1990) Neandertals and the anterior dental loading hypothesis: A biomechanical evaluation of bite force production. Kroeber Anthropol. SOC. Papers. 71-72t67-76. Bell WE (1983)Understanding temporomandibular biomechanics. J. Craniomand. Pract. 1t27. Bell WE (1990) Temporomandibular Disorders. Chicago: Year Book Medical Publishers, Inc. Bermudez de Castro JM (1989) Third molar agenesis in human prehistoric populations of the Canary Islands. Am. J. Phys. Anthropol. 79t207-217. Birket-Smith K (1935) The Eskimos. New York: E.P. Dutton. Boyd RL, Gibbs CH, Richmond AF, Laskin JL, and Brehnan K (1982) Temporomandibular joint forces in monkey measured with piezoelectric foil. J. Dent. Res. Abstract: 351. Brace CL (1962) Refocusing on the Neandertal problem. Am. Anthropol. 64t729-741. Brace CL (1964) The fate of the classic Neandertals: A consideration of hominid catastrophism. Curr. Anthropol. 5:3-43. Brace CL (1967) Environment, tooth form, and size in the Pleistocene. J. Dent. Res. 46t809-816. Brace CL, Ryan AS, and Smith BH (1981)Tooth wear in La Ferrassie man: Comment. Curr. Anthropol. 22426430. Brehnan K, Boyd RL, Laskin J, Gibbs CH, and Mahan P (1981) Direct measurement of loads a t the temporomandibular joint in Mucaca urctoides. J. Dent. Res. 60:1820-1824. Brose DS, and Wolpoff MH (1971) Early upper paleolithic man and late middle paleolithic tools. Am. Anthropol. 73t1156-1194. Carlson DS, and Van Cerven DP (1977) Masticatory function and post-Pleistocene evolution in Nubia. Am. J. Phys. Anthropol. 46:495-506. Conroy G. Jolly C, Cramer D, and Kalb J (1978) Newly discovered fossil hominid skull from the Afar Depression Ethiopia. Nature 275t67-70. Coon CS (1962) The Origin of Races. New York: Alfred A. Knopf. de Poncins G (1941) Kablooma. New York: Reynal. Demes B (1987)Another look at an old face: Biomechanics of the Neandertal facial skeleton reconsidered. J. Hum. Evol. 16r297-303. Demes B, and Creel N (1988)Bite force, diet, and cranial morphology of fossil hominoids. J. Hum. Evol. 17t657670. LITERATURE CITED a b - Adding an equal distance ‘k to both the muscle force and bite force moment arms will increase this ratio as follows: a a+k b<b+k ‘(b 1 +;) For this last expression to be true it must be shown that: k k 1+->1+a b k a k b ->- 1 1 ->a b a 1>b Therefore, if the length of the muscle force moment arm is less than that of the bite force moment arm, then adding a constant value to both will increase the relative mechanical advantage of the muscles. Since the muscle resultant moment arm is shorter NEANDERTAL AND INUIT MASTICATORY CONFIGURATION Du Brul EL (1974)Origin and evolution of the oral apparatus. InY Kawamura (ed.): Frontiers of Oral Physiology. Basei: Karger, pp. 1-30. Du Brul EL (1977) Early hominid feeding mechanisms. Am. J . Phys. Anthropol. 47:305-320. Gamble C (1986) The Paleolithic Settlement of Europe. Cambridge: Cambridge University Press. Greaves WS (1978) The jaw lever system in ungulates: A new model. J. Zool. Lond. 1841271-285. Greaves WS (1988) The maximum average bite force for a given jaw length. J. Zool. Lond. 214t295-306. Gysi A (1921) Studies on the leverage problem of the mandible. Dent. Dig. 27t7P84, 144-150,203-208, Hinton RJ, and Carlson DS (1979) Temporal changes in human temporomandibular joint size and shape. A m J. Phys. Anthropol. 50:325-334. Howells WW (1974) Neandertal man: Facts and figures. In RH Tuttle (ed.): Paleoanthropology: Morphology and Paleoecology. The Hague: Mouton Publishers, pp. 389-407. Hylander WL (1972) The Adaptive Significance of Eskimo Craniofacial Morphology. Unpublished PhD Thesis. University of Chicago. Hylander WL (1975) Incisor size and diet in anthropoids with special reference to Cercopithecoidea. Science 189t1095-1098. Hylander WL (1977) The adaptive significance of Eskimo craniofacial morphology. In AA Dahlberg and TM Graber (eds.): Orofacial Growth and Development. The Hague: Mouton Publishers, pp. 129-169. Hylander WL (1979a) An experimental analysis of temporomandibular joint reaction force in macaques. Am. J . Phys. Anthropol. 51:433-456. Hylander WL (1979b)The functional significance of primate mandibular form. J. Morphol. 160:223-240. Hylander WL (1983) Mechanical properties of food and recruitment of masseter force. J . Dent. Res. 62:1150. Hylander WL (1985a) Mandibular function and temporomandibular joint loading. In DS Carlson, JA McNamara, and KA Ribbens ieds.): Developmental Aspects of Temporomandibular Joint Disorders. Monograph 16, Craniofacial Growth Series, Center for Human Growth and Development. Ann Arbor, MI: University of Michigan, pp, 19-35. Hylander WL (1985b) Mandibular function and biomechanical stress and scaling. Am. Zool. 25t315-330. Hylander WL (1992) Functional anatomy. In BG Sarnat and DM Laskin (eds.):The Temporomandibular Joint: A Biological Basis for Clinical Practice. Philadelphia: W.B. Saunders Co., pp. 60-92. Hylander WL, and Bays R (1978) Bone strain in the subcondylar region of the mandible in Macaca fascicularis and Macaca mulatta. Am. J. Phys. Anthropol. 48t408. Hylander WL, Johnson KR, and Crompton AW (1992) Muscle force recruitment and biomechanical modeling: An analysis of masseter muscle function during mastication in Macaca fascicularis. Am. J . Phys. Anthropol. 88:365-387. Jolly C J (1970)The seed-eaters: A new model of hominid differentiation based on a baboon analogy. Man 5.5-26. Kay RF (1975) The functional adaptations of primate molar teeth. Am. J . Phys. Anthropol. 43t195-216. 19 Kay RF, and Hylander WL (1978) The dental structure of mammalian folivores with special reference to primates and phalangeroids (Marsupiala). In GG Montgomery (ed.): The Ecology of Arboreal Folivores. Washington D.C.: Smithsonian Institution Press, pp. 173-192. Klockars AJ, and Sax G (1986) Multip1e.Comparisons. Quantitative Applications in the Social Sciences Series. Beverly Hills: Sage Publications. Lucas PW, and Luke DA (1984) Chewing it over: Basic principles of food breakdown. In DJ Chivers, BA Wood, and A Bilsborough (eds.):Food Acquisition and Processing in Primates. New York: Plenum Press, pp. 283-330. Luschei ES, and Goodwin GM (1974) Patterns of mandibular movement and jaw muscle activity during mastication in the monkey. J Neurphysiol. 37:954966. Mansour RM, and Reynik RJ (1975) In uiuo occlusal forces and moments: 1. Forces measured in terminal hinge position and associated moments. J. Dent. Res. 54t114-120. Mayhall J T (1977) Cultural and environmental influences on the Eskimo dentition. In AA Dahlberg and TM Graber (eds.): Orofacial Growth and Development. The Hague: Mouton, pp. 215-227. Merbs CF (1968) Anterior tooth loss in Arctic populations. Southwestern J. Anthropol. 24.20-32. M ~ l l eE r (1966)The chewing apparatus: An electromyographic study of the action of the muscles of mastication and its correlation to facial morphology. Acta Physiol. Scand. 69(Suppl. 280):1-129. Molnar S (1971) Human tooth wear, tooth function and cultural variability. Am. J. Phys. Anthropol. 34t175189. Molnar S (1972) Tooth wear and culture: A survey of tooth functions among some prehistoric populations. Curr. Anthropol. 13511-526. Mongini F, Preti G, Calderale PM, and Barberi G (1981) Experimental strain analysis on the mandibular condyle under various conditions. Med. Bio. E. C. 19t521-523. Moran EF (1979) Human Adaptability. Boulder, Colorado: Westview Press. Morant GM (1928) Studies of palaeolithic man 111. The Rhodesian skull and its relations to neanderthaloid and modern types. Ann. Eugenics 3r337-361. Pine PL (1978)Allometric scaling in the postcanine dentition with reference to primate diets. Primates l9:583-591. Pruim GJ, De Jongh HJ, and Ten Bosch JJ (1980)Forces acting on the mandible during bilateral static bite a t different bite force levels. J. Biomech. 13r755-763. Puech P F (1981) Tooth wear in La Ferrassie man. Curr. Anthropol. 22t424-425. Rak Y (1983) The Australopithecine Face. New York: Academic Press. RakY (1986)The neanderthal: A new look at an old face. J. Hum. Evol. 15:151-164. Rice WR (1989)Analyzing tables of statistical tests. Evolution 43t223-225. Ryan AS (1980) Anterior dental microwear in Neandertals (abstract). Am. J. Phys. Anthropol. 52274. 20 M.A. SPENCER AND B. DEMES Ryan AS, and Johanson DC (1989) Anterior dental microwear in Austrulopithecus ufurensis: Comparisons with human and nonhuman primates. J . Hum. Evol. 18t235-268. Schour I, and Sarnat BG (1942) Oral manifestations of occupational origin. J. Am. Med. A. 120:1197-1201. Simons E (1976) The nature of the transition in the dental mechanism from pongids to hominids. J. Hum. Evol. 5:511-528. Smith FH (1983)A behavioral interpretation of changes in craniofacial morphology across the archaidmodern Homo supiens transition. In E Trinkaus (ed.): The Mousterian Legacy: Human Biocultural Change in the Upper Pleistocene. Br. Arch. Rep. S164, pp. 141163. Smith FH, and Paquette SP (1989)The adaptive basis of Neandertal facial form, with some thoughts on the nature of modern human origins. In E Trinkaus (ed.): The Emergence of Modern Humans. Cambridge: Cambridge University Press, pp. 181-210. Smith P (1976a1 Dental Pathology in fossil hominids: What did Neandertals do with their teeth? Curr. Anthropol. 1 7: 149-15 1. Smith P (197610) Regional variation in tooth size and pathology in fossil hominids. Am. J . Phys. Anthropol. 47r459-466. Smith P (1976~)Selective pressures and dental evolution in hominids. Am. J. Phys. Anthropol. 47:453-458. Smith R J (1978) Mandibular biomechanics and temporomandibular joint function in primates. Am. J. Phys. Anthropol. 49t341-350. Stern JT (1988) Essentials of Gross Anatomy. Philadelphia: F.A. Davis Company. Trinkaus E (1982) Evolutionary trends in the Shanidar Neandertal sample (abstract). Am. J . Phys. Anthropol. 57t237. Trinkaus E (1983) the Shanidar Neandertals. New York: Academic Press. Trinkaus E (1987) The Neandertal face: Evolutionary and functional perspectives on a recent hominid face. J. Hum. Evol. 16t429-443. Trinkaus E, and Howells WW (1979) The Neandertals. Sci. Am. 241:94-105. Turner CG, and Cadien J P (1969) Dental chipping in Aleuts, Eskimos and Indians. Am. J. Phys. Anthropol. 31:303-310. Ungar PS, and Grine FE (1990) Incisor size and wear in Austrulopithecus ufricunus and Purunthropus robustus. J. Hum. Evol. 20:313-340. Van Eijden TMGJ (1990)Jaw muscle activity in relation to the direction and point of application of bite force. J. Dent. Res. 69:901-905. Van Eijden TMGJ (1991)Three-dimensional analyses of human bite-force magnitude and moment. Arch. Oral Biol. 36.535-539. Van Eijden TMGJ, Brugman P, Weijs WA, and Oosting J (1990) Coactivation of jaw muscles: Recruitment order and level as a function of bite force direction and magnitude. J. Biomech. 23t475-485. Vanstone JW (1962) Point Hope: An Eskimo Village in Transition. Seattle: University of Washington Press. Walker AC (1978) Functional anatomy of oral tissues: Mastication and deglutition. In J H Shaw, EM Sweeney, CC Cappuccino and SM Meller (eds.):Textbook of Oral Biology. Philadelphia: W.B. Saunders Co., pp. 277-296. Walker AC (1981) Diet and teeth: Dietary hypotheses and human evolution. Phil. Trans. SOC.Lond. B292: 57-64. Wallace JA (19751 Did La Ferrassie I use his teeth as a tool? Curr. Anthropol. 16t393-401. Ward SC, and Molnar S (1980) Experimental stress analysis of topographic diversity in early hominid gnathic morphology. Am. J. Phys. Anthropol. 53t383395. Weijs WA, and Hillen B (19841Relationship between the physiological cross-section of the human jaw muscles and their cross-sectional area in computer tomograms. Acta Anat. 118r129-138. Wolff JEA (1984) A theoretical approach to solve the chin problem. In DJ Chivers, BA Wood and A Bilsborough (eds.): Food Acquisition and Processing in Primates. New York and London: Plenum Press, pp. 391405. Wolpoff MH, Smith FH, Malez M, Radovcic J, and Rukavina D (1981) Upper Pleistocene hominid remains from Vindija Cave, Croatia, Yugoslavia. Am. J. Phys. Anthropol. 54r499-545. Worner HK, and Anderson MN (1944) Biting force measurement on children. Aust. J. Dent. 48tl.