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Biomechanical analysis of masticatory system configuration in Neandertals and Inuits.

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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:
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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
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k
k
1+->1+a
b
k
a
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->-
1 1
->a
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a
1>b
Therefore, if the length of the muscle force
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