AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 134:312 322 (2007)
Genetic, Geographic, and Environmental Correlates
of Human Temporal Bone Variation
Heather F. Smith,1*y Claire E. Terhune,1*y and Charles A. Lockwood2
1
School of Human Evolution and Social Change, Arizona State University, Tempe, AZ 85287-2402
2
Department of Anthropology, University College London, London WC1E 6BT, UK
KEY WORDS geometric morphometrics; molecular distance; cranial morphology
were relatively high. Comparisons of morphological dis-
ABSTRACT Temporal bone shape has been shown to
reflect molecular phylogenetic relationships among homi- tances to molecular distances based on short tandem
noids and offers significant morphological detail for distin- repeats (STRs) revealed a significant correlation between
guishing taxa. Although it is generally accepted that tem- temporal bone shape and neutral molecular distance
among Old World populations, but not when Native Amer-
poral bone shape, like other aspects of morphology, has an
icans were included. Further analyses suggested a similar
underlying genetic component, the relative influence of
pattern for morphological variation and geographic distri-
genetic and environmental factors is unclear. To determine
bution. No significant correlations were found between
the impact of genetic differentiation and environmental
variation on temporal bone morphology, we used three- temporal bone shape and environmental variables: tem-
dimensional geometric morphometric techniques to evalu- perature, annual rainfall, latitude, or altitude. Significant
ate temporal bone variation in 11 modern human popula- correlations were found between temporal bone size and
tions. Population differences were investigated by discrim- both temperature and latitude, presumably reflecting
inant function analysis, and the strength of the relation- Bergmann s rule. Thus, temporal bone morphology ap-
pears to partially follow an isolation by distance model of
ships between morphology, neutral molecular distance,
evolution among human populations, although levels of
geographic distribution, and environmental variables were
correlation show that a substantial component of variation
assessed by matrix correlation comparisons. Significant
is unexplained by factors considered here. Am J Phys
differences were found in temporal bone shape among all
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Anthropol 134:312 322, 2007. 2007 Wiley-Liss, Inc.
populations, and classification rates using cross-validation
Like other aspects of phenotype, cranial morphology ways in which cranial morphology covaries with molecu-
reflects a combination of genetic and environmental lar distances and environmental factors and a test of the
influences (Moss, 1962, 1972). Within this framework, hypothesis that cranial base elements have a strong
some authors have suggested that particular portions of genetic component.
the cranium may be less prone to variation due to envi- Several recent studies of variation in the temporal
ronmental variables, and therefore more phylogenetically bone have demonstrated this region s utility in distin-
informative (Olson, 1981; Strait et al., 1997; Lieberman guishing among species and subspecies of extant great
et al., 2000a; Harvati, 2001; Wood and Lieberman, 2001; apes, and for quantifying levels of variation within and
Harvati and Weaver, 2006a,b). For hominins, traits asso- between taxa (Harvati, 2001, 2003; Lockwood et al.,
ciated with heavy chewing have been argued to be homo- 2002, 2004, 2005; Terhune et al., 2007). In particular,
plastic and consequently unreliable indicators of phylog- Lockwood et al. (2004) demonstrated that, using shape
eny (Walker et al., 1986; Wood, 1988; Skelton and distributions of coordinate data from modern humans,
McHenry, 1992; Turner and Wood, 1993; McHenry, 1994, orangutans, gorillas, chimpanzees, and bonobos, the re-
1996; Lieberman et al., 1996; but see Strait et al., 1997; sultant phylogenetic tree of these taxa was identical to
Asfaw et al., 1999; Collard and Wood, 2001). The mor- the molecular phylogeny of these species. Similarly, sev-
phology of the cranial base has especially been regarded
as a reliable reflection of genetic relationships, as it
forms very early during ontogeny and ossifies endochon-
Grant sponsors: AMNH Collections Study Grant and ASU Depart-
drally (Moore and Lavelle, 1974; Olson, 1981; MacPhee
ment of Anthropology; Grant number: NSF BCS-9982022.
and Cartmill, 1986; Lieberman et al., 2000a,b). The cra-
nial base also mirrors the shape of the developing brain,
*Correspondence to: Heather F. Smith or Claire E. Terhune,
which is relatively constrained (Houghton, 1996). Basi-
School of Human Evolution and Social Change, Arizona State Uni-
cranial characters may therefore be less influenced by
versity, Box 872402, Tempe, AZ 85287-2402, USA.
epigenetic forces than are the externally sensitive intra-
E-mail: heather.f.smith@asu.edu or claire.terhune@asu.edu
membraneous bones of the facial skeleton.
y
The morphology of the temporal bone, as part of the
These authors contributed equally to this work.
cranial base, may also reflect neutral molecular distan-
ces within species and phylogenetic relationships among
Received 12 December 2006; accepted 8 May 2007
species. However, the temporal bone also serves a vari-
ety of functional roles, such as posture, hearing, balance,
DOI 10.1002/ajpa.20671
mastication, and formation of the braincase. Conse- Published online 13 July 2007 in Wiley InterScience
quently, this element can serve as a test case of the (www.interscience.wiley.com).
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TEMPORAL BONE VARIATION IN MODERN HUMANS 313
Fig. 1. Map of the world
showing the approximate lo-
cations of populations used
in the morphological analy-
sis (triangles), populations
used in the molecular analy-
sis (circles), and waypoints
(squares). Lines link the mor-
phological populations and
their genetic representatives.
eral recent studies (Harvati, 2001, 2003; Terhune et al., lution in human populations (Wright, 1943). More specif-
2007) have used the morphology of the temporal bone to ically, three research questions were investigated:
test hypothesized taxonomic divisions among fossil taxa.
Given this background, we sought to investigate the
Q1. Are modern human populations significantly differ-
association between temporal bone morphology and mo-
ent in temporal bone morphology?
lecular distance among human populations, together
Q2. What is the strength of the correlation between tem-
with geographic distance and external factors such as
poral bone morphology and molecular distance
environmental variables. Some recent studies have ex-
among populations of modern humans?
plicitly evaluated these influences on cranial anatomy
Q3. How do external variables such as environmental
(Relethford, 1994, 1998, 2001, 2002; Gonzales-Jose et al.,
differences or geographic distance covary with pat-
2004; Roseman, 2004). Linear dimensions of the skull
terns of temporal bone morphology in humans?
have been shown to reflect genetic relationships of
human populations, such that closely related populations
tend to be more similar in overall cranial form (Rele-
MATERIALS AND METHODS
thford, 2001, 2002; Gonzales-Jose et al., 2004; Roseman,
2004). However, selective pressures acting on the skull
Data collection
of certain human populations have also been identified
and can have a significant impact on cranial morphology A total of 243 individuals from 11 modern human pop-
of populations living in regions with extreme tempera- ulations were included in this study (Fig. 1, Table 1).
tures, such as Siberia (Roseman, 2004). Diversifying re- Specimens were housed at the American Museum of
gional selection due to climate also affects the cranial Natural History and Arizona State University. Individu-
morphology of several other human populations (Carey als with extensive antemortem tooth loss were generally
and Steegmann, 1981; Franciscus and Long, 1991; Rose- avoided to minimize the possibility of alveolar resorption
man, 2004). affecting the morphology of the temporomandibular joint
Harvati and Weaver (2006a,b) analyzed the correlation (TMJ). Following Lockwood et al. (2002), 22 landmarks
between human morphological variation in three cranial from the ectocranial surface of the temporal bone were
regions the temporal bone, cranial vault, and facial employed, which describe the morphology of the mandib-
skeleton with molecular distances and environmental ular fossa, tympanic, mastoid, and petrous portions of
variables. They concluded that the morphology of the the temporal bone (Fig. 2, Table 2). In comparison, Har-
temporal bone and cranial vault are correlated with mo- vati and Weaver (2006a) used 13 landmarks.
lecular distance in human populations, while facial mor- An Immersion Microscribe point digitizer was used to
phology covaries more reliably with environment. The record the three-dimensional coordinates of each land-
correlation between temporal bone shape and molecular mark. These three-dimensional data were then analyzed
distance was significant but low, suggesting that other using Morphologika (O Higgins and Jones, 1998). First,
factors also play a significant role in patterns of tempo- three-dimensional coordinate data were registered
ral bone morphology in humans. In addition, temporal through a generalized Procrustes analysis (GPA) (Gower,
bone centroid size was found to be correlated with tem- 1975; Goodall, 1991; Dryden and Mardia, 1998). Subse-
perature, a finding that is consistent with environmental quently, variation in shape was investigated through
variation in body size as first outlined by Bergmann (1847). principal components analysis (PCA). Output from these
Our goal is to use an independent dataset and an analyses (Procrustes residuals from the GPA and PC
expanded set of landmarks on the temporal bone to scores from the PCA) was recorded and copied into other
replicate part of the study of Harvati and Weaver statistical programs for further analysis. All three-
(2006a). We also include additional environmental varia- dimensional data were collected by the second author,
bles such as rainfall and altitude, and explore the rela- and intraobserver error for a subset of the data set used
tionship between morphology and geographic distance. here is reported by Terhune et al. (2007).
In general, we are testing the hypothesis that the tem- Data on 783 STRs in matched analogues of nine of the
poral bone follows an isolation by distance model of evo- human populations discussed earlier were used to obtain
American Journal of Physical Anthropology DOI 10.1002/ajpa
314 H.F. SMITH ET AL.
TABLE 1. Modern human populations used in the morphometric analysis
Populationa Total Genetic representative Centroid size Average geographic coordinates
Alaskan Natives 20 None 106.43 68.4N, 166.7W
Australian Aborigines 21 Australians 94.69 34.8S, 138.5E
Hungarians (Medieval) 21 French 98.69 46.6N, 18.4E
Khoisan 19 San 98.21 20.5S, 19.5E
Malaysians 21 Cambodians 100.23 4N, 109.5E
Mongolians 18 Mongolians 103.43 46.9N, 103.8E
Native American (Grand Gulch, Utah) 20 Pima 102.91 37.6N, 109.8W
New Guineans 20 Papua New Guineans 97.52 6.4S, 150.2E
Nubians (Semna South, Sudanese Nubia) 43 Mozabite 98.63 20.0N, 30.1E
Pare (Tanzania) 19 Kenyan Bantu 98.35 4.3S, 38.1E
Southern Indians 21 None 94.64 13N, 77.56E
Total 243
a
Specimens were housed at Arizona State University (Nubians) or the American Museum of Natural History (all others).
neutral molecular distances. STRs have been shown to
be particularly useful and appropriate for determining
genetic relationships of populations of Homo sapiens.
These loci are autosomal and evolve neutrally such that
shared mutations are accepted as evidence of common
ancestry. The dataset used here was originally used by
Ramachandran et al. (2005) and Rosenberg et al. (2005)
and consists of the largest and most inclusive STR data-
set published to date. Several of the populations meas-
ured in the craniometric study have not been typed for
STRs, particularly the archaeological samples (the
Nubians and Medieval Hungarians). In these cases, it
was necessary to substitute a representative population
from the same geographic region and/or linguistic group
(Table 1). This practice has been employed in previous
studies of the relationship between morphological and
molecular distances in modern humans (Relethford,
1994; Roseman, 2004; Harvati and Weaver, 2006a,b).
The Alaskan natives and southern India sample had to
be omitted from the molecular analysis as neither they nor
any other comparable population has been typed for a
sufficient number of STR loci. However, these populations
were still included in all other analyses in this study.
Approximate geographic coordinates of population ori-
gins were estimated using an atlas and published infor-
mation for the samples. In the case that a range of coor-
dinates was obtained, an average location was used.
Data were also compiled on environmental variables in
regions from which the populations originated, using
data from nearby weather stations (New et al., 1999,
2000) and almanacs. These included rainfall, tempera-
ture, altitude, and latitude. The link between these envi-
ronmental variables and temporal bone morphology
could stem directly from local adaptations of cranial
shape or indirectly from behaviors mediated by the envi-
ronment, such as diet or activity levels.
Analytical methods
The first research question examined the degree to
which the morphology of the temporal bone can discrimi-
nate among populations of Homo sapiens, and was eval-
uated in two ways. First, Procrustes distances between
groups were calculated, and the significance of these
values was assessed via a permutation test (Good, 1993).
This form of significance testing compares the observed
Fig. 2. Twenty-two temporal bone landmarks digitized in
distance (i.e., test statistic) with a distribution of per-
the present study (following Lockwood et al., 2002). Refer to
muted distances, where individuals are randomly allo-
Table 2 for landmark definitions. Open circles show the relative
cated to each group and a mean distance is calculated.
positions of landmarks 1 and 18 when these landmarks are not
A test statistic is considered statistically significant directly visible.
American Journal of Physical Anthropology DOI 10.1002/ajpa
TEMPORAL BONE VARIATION IN MODERN HUMANS 315
TABLE 2. Definitions of landmarks used in the present study
No. Definition
1 Intersection of the infratemporal crest and sphenosquamosal suture
2 Most lateral point on the margin of foramen ovale
3 Most anterior point on the articular surface of the articular eminence
4 Most inferior point on entoglenoid process
5 Most inferior point on the medial margin of the articular surface of the articular eminence
6 Midpoint of the lateral margin of the articular surface of the articular eminence
7 Center of the articular eminence
8 Deepest point within the mandibular fossa
9 Most inferior point on the postglenoid process
10 Anteromedial apex of the petrous part of the temporal bone
11 Most posterolateral point on the margin of the carotid canal entrance
12 Most lateral point on the vagina of the styloid process (whether process is present or absent)
13 Most lateral point on the margin of the stylomastoid foramen
14 Most lateral point on the jugular fossa
15 Center of the inferior tip of the mastoid process
16 Most inferior point on the external acoustic porus
17 Most inferolateral point on the tympanic element of the temporal bone
18 Point of inflection where the braincase curves laterally into the supraglenoid gutter, in coronal plane of the mandibular fossa
19 Point on lateral margin of the zygomatic process of the temporal bone in the coronal plane of the postglenoid process
20 Auriculare
21 Porion
22 Asterion
After Lockwood et al. (2002).
(P-value 0.05) if it is reached or exceeded in less than the values by a pooled within-group covariance matrix,
5% of the random permutations. Second, a discriminant which assumes that all groups in the analysis have
function analysis (DFA) was conducted using the first 40 similar covariance structures (Ackermann, 2002, 2005;
PC scores from the PCA of Procrustes coordinates (which Klingenberg and Monteiro, 2005). This assumption is
accounted for [95% of variation). The differentiation tenuous given the sample sizes used here. In contrast,
among populations was then assessed using discriminant since Procrustes distances are not scaled by the pooled
analyses with jackknife cross-validation, where prior within-group covariance matrix, differences in covari-
probabilities were set equal to group size. Since the Nu- ance structure between populations should not affect
bian sample was significantly larger than all other sam- these distances as drastically as they would affect
ples used here (n 5 43), a reduced sample of 20 ran- Mahalanobis distances. Also, Mahalanobis distances are
domly chosen individuals was used for this analysis. affected by uneven sample sizes, while no similar bias
DFAs were conducted using SPSS (version 11.0.1). has been noted for Procrustes distances.
Although Procrustes superimposition scales all speci- The second research question addressed the degree of
mens to the same unit centroid size, size related shape concordance between temporal bone shape and genetic
changes (i.e., allometry) are not removed. Therefore, to relationships among human populations. This relation-
assess the role of allometry, a size matrix (i.e., a matrix ship was tested by examining the correlation between
of the absolute differences in centroid size between matrices of temporal bone morphology (i.e., size and
groups) was calculated and compared with the Pro- shape matrices) and molecular distances. Analogous
crustes distance (or shape) matrix using a Mantel test studies above the species level have compared phyloge-
(Mantel, 1976; Smouse et al., 1986) in PopTools, an add- netic trees based on morphology with those based on
on for Microsoft Excel. Additionally, correlations between molecular data (Lockwood et al., 2004; see also Collard
centroid size and shape were evaluated by regressing and Wood, 2001; Strait and Grine, 2004; Lycett and Col-
the principal component axes on centroid size using lard, 2005). However, within humans, a tree-like struc-
Morphologika. ture does not apply to population relationships for mor-
For each analysis, morphological distances (i.e., size or phological or molecular information (summarized by
shape distance matrices) were compared to the variable Sherry and Batzer, 1997; Athreya and Glantz, 2003).
of interest (e.g., molecular or environmental distances). The current analysis is therefore restricted to matrix
Both Procrustes and Mahalanobis distances were calcu- correlation comparisons.
lated for all populations used here, and these two dis- STR data were analyzed using Arlequin 3.0 (Excoffier
tance measures were found to be significantly correlated et al., 2005). Data on 783 STRs have been typed for
(r 5 0.662; P \ 0.001). Analyses using both of these dis- eight representative populations (Ramachandran et al.,
tance measures were found to lead to the same general 2005; Rosenberg et al., 2005), and a subset of 404 of the
pattern of results. However, while a number of authors same STRs has been typed in Native Australians. A ma-
(Ackermann, 2002; Strand Vi餫rsd贸ttir et al., 2002; Har- trix of STR population distances was constructed using
vati, 2003; Harvati et al., 2004; McNulty, 2005; Harvati Slatkin s genetic distance, a distance measure analogous
and Weaver, 2006a,b) have previously used Mahalanobis to FST but specifically designed for microsatellite loci in
distances in analyses such as this, only Procrustes dis- assuming a stepwise mutation model (Slatkin, 1995).
tances are reported here, as Mahalanobis distances The degree and significance of the correlation between
attempt to account for within group variation by scaling the distance matrices from the molecular and morpho-
American Journal of Physical Anthropology DOI 10.1002/ajpa
316 H.F. SMITH ET AL.
TABLE 3. Structure matrix for the discriminant function analysis (first 20 PCs only) showing the correlations
between each of the PC axes and discriminant functions
Function
123456789 10
PC1 0.131 0.439 20.088 20.105 0.145 20.127 0.230 0.034 0.113 20.052
PC2 0.140 0.057 0.182 0.246 20.033 0.312 20.200 20.196 0.051 0.056
PC3 0.076 20.211 20.061 0.063 0.327 20.118 0.071 0.122 20.124 20.169
PC4 20.022 20.068 0.070 0.070 0.043 0.164 0.316 20.005 0.203 20.133
PC5 20.037 20.010 0.124 0.149 20.028 20.381 0.102 0.037 20.052 0.309
PC6 0.189 0.018 20.238 0.379 20.051 20.153 0.031 0.094 20.113 0.144
PC7 20.069 0.228 20.073 0.186 0.142 0.152 20.062 0.165 20.357 20.068
PC8 0.029 0.022 0.141 20.136 0.193 0.037 20.049 20.069 20.033 20.093
PC9 0.138 20.055 0.013 0.060 0.172 0.014 0.175 20.137 0.023 0.009
PC10 20.173 0.079 0.029 0.287 0.133 20.005 0.149 0.038 0.223 0.018
PC11 0.053 0.035 0.149 20.091 0.043 0.060 0.130 0.165 20.094 0.286
PC12 0.012 0.037 0.037 0.006 0.059 20.080 20.049 0.141 0.087 0.164
PC13 0.061 20.028 20.039 20.140 20.066 0.156 20.135 0.445 20.081 20.062
PC14 0.102 20.094 0.208 0.224 20.125 20.022 20.050 0.064 20.056 20.244
PC15 20.105 0.015 0.192 0.121 0.023 20.035 20.044 0.042 0.071 0.053
PC16 0.020 20.030 0.084 20.046 0.010 0.211 0.206 0.012 20.065 0.242
PC17 20.019 0.085 0.032 0.023 0.127 0.072 20.143 0.225 0.390 20.093
PC18 20.011 20.098 0.015 20.037 0.182 0.123 20.142 0.025 0.029 0.250
PC19 0.010 20.033 0.196 20.001 0.110 20.092 20.070 0.210 20.034 20.096
PC20 20.049 20.073 20.018 0.037 0.150 0.015 0.086 20.276 20.129 0.064
TABLE 4. Eigenvalues, distribution of variance, and canonical
logical analyses was assessed using a Mantel test, again
correlations for the discriminant function analysis
in PopTools.
Finally, environmental variables and geographic dis-
%of Canonical
tances for populations were evaluated to determine how
Function Eigenvalue variance Cumulative % correlation
they covary with temporal bone morphology. Environ-
1 6.39 40.81 40.81 0.93
mental distance matrices were generated for each envi-
2 2.78 17.75 58.56 0.86
ronmental variable: temperature, rainfall, latitude, and
3 1.44 9.18 67.74 0.77
altitude. A single overall environmental distance matrix
4 1.29 8.23 75.97 0.75
5 1.21 7.74 83.71 0.74
(Euclidean distance, incorporating data from all four
6 0.91 5.82 89.53 0.69
environmental variables) was also calculated in Pop-
7 0.58 3.69 93.22 0.61
Tools. To address the possibility that environmental fac-
8 0.45 2.87 96.09 0.56
tors influenced morphological difference, the morphologi-
9 0.33 2.11 98.20 0.50
cal distance matrices were compared to each environ-
10 0.28 1.80 100.00 0.47
mental matrix using a Mantel test.
To test the association between geography and mor-
phology, geographic great circle distances among popula- continents that migrational distances are affected by
tions were calculated. Great circle distances use latitude geographical barriers and are not simply great circle dis-
and longitude and take into account the fact that these tances; this factor is considered later in discussing the
coordinates are on the circumference of a sphere to cal- results. The hypothesis that temporal bone morphology
culate distances between two locations. A geographic ma- covaries with geographic distance was then assessed by
trix was generated using great circle distances and comparing the geographic matrix with the morphological
including five waypoints (Fig. 1), geographic locations matrix using a Mantel test.
through which populations would have had to travel For all analyses, alpha was set at 0.05. All correlations
when migrating between two continents (Relethford, are reported as Pearson product moment correlation
2004; Ramachandran et al., 2005). This practice takes coefficients (r).
into account the conclusion that most human migrations,
until recently, did not usually traverse large bodies of
RESULTS
water (Ramachandran et al., 2005). The inclusion of
waypoints, therefore, permits a more accurate estimate In the DFA, the first function is influenced by a vari-
of the migrational distance among populations, rather ety of principal components and accounts for just over
than a line of minimal geographic distance that could 40% of variance among populations (Tables 3 and 4). As
run across an ocean. The pairwise distance between any expected, contributions of subsequent functions diminish
two populations was calculated as the sum of the dis- rapidly (Table 4).
tance between Population 1 and the waypoint, and Permutation tests of the Procrustes distances among
between the waypoint and Population 2, plus any dis- populations were all statistically significant with P-val-
tances between waypoints if more than one waypoint fell ues of less than 0.001 (Table 5). The DFA with crossvali-
between the populations. Following Ramachandran et al. dation demonstrates that the populations can be distin-
(2005), waypoints included were Anadyr, Russia; Cairo, guished relatively well, with classification rates between
Egypt; Istanbul, Turkey; Phnom Penh, Cambodia; and 56 and 85% (mean 73%) (Table 6). For 11 populations of
Prince Rupert, Canada. Geographic distances among roughly equal sample size, the expected proportion of
populations on the same continent were calculated as correct random classifications is 9%, so these results
normal great circle distances. It is probable even within indicate high success rates.
American Journal of Physical Anthropology DOI 10.1002/ajpa
TABLE 5. Procrustes distances between groups
Nubians Native Americans Australians Alaskans Hungarians Pare Malaysians Khoisan New Guineans Mongolians Indians
Nubians
Native Americans 0.0669
Australians 0.0681 0.0574
Alaskans 0.0704 0.0633 0.0476
Hungarians 0.0525 0.0546 0.0689 0.0721
Pare 0.0656 0.0715 0.0763 0.0799 0.0719
Malaysians 0.0793 0.0562 0.0551 0.0603 0.0634 0.074
Khoisan 0.0788 0.0904 0.0958 0.0953 0.0947 0.0727 0.1100
New Guineans 0.075 0.0667 0.0559 0.0699 0.0798 0.0745 0.0744 0.0835
Mongolians 0.0792 0.0643 0.0740 0.0707 0.0709 0.0804 0.0707 0.0853 0.059
Indians 0.0783 0.0828 0.0664 0.0677 0.0797 0.0848 0.0821 0.089 0.0628 0.0703
TABLE 6. Classification results of the discriminant function analysis using jackknife cross-validation
% Correct Nubians Native Americans Australians Alaskans Hungarians Pare Malaysians Khoisan Mongolians New Guineans Indians
Nubians 80 16 1 0 0 2 1 0 0 0 0 0
Native Americans 80 2 16 0 0 1 0 1 0 0 0 0
Australians 76 0 1 16 0 1 1 1 0 0 1 0
Alaskans 85 0 1 1 17 0 0 1 0 0 0 0
Hungarians 71 1 2 2 0 15 0 0 1 0 0 0
Africans 68 1 1 1 0 0 13 1 2 0 0 0
Malaysians 71 0 0 2 2 2 0 15 0 0 0 0
Khoisan 74 0 0 0 1 0 0 0 14 1 1 2
Mongolians 56 0 0 1 0 0 0 1 0 10 2 4
New Guineans 65 0 0 4 0 0 0 0 0 0 13 3
Indians 81 0 0 0 0 0 0 0 0 1 3 17
Jackknife cross-validation is the leave-one-out method as implemented in SPSS, with a priori probabilities based on group sample sizes. Each horizontal row summarizes the
number of correct classifications for each group as well as misclassifications; e.g., 1 Nubian was misclassified as a Native American.
TEMPORAL BONE VARIATION IN MODERN HUMANS
American Journal of Physical Anthropology
DOI 10.1002/ajpa
317
318 H.F. SMITH ET AL.
TABLE 8. Results of the Mantel tests performed between
morphological matrices (shape and size) and the molecular,
geographic, and environmental matrices
Shape Size
r P r P
Molecular distance 0.205 0.175 0.298 0.15
Molecular without 0.629a 0.003 20.032 0.469
Utah Native Americans
Geography 0.221 0.095 0.233 0.11
Geography without 0.338a 0.029 0.179 0.157
Utah Native Americans
Temperature 20.144 0.208 0.713a 0.001
Rainfall 20.045 0.516 20.114 0.415
Latitude 20.129 0.195 0.420a 0.021
Altitude 20.05 0.419 20.028 0.499
Combined environment 20.106 0.293 20.103 0.327
r 5 Pearson correlation coefficients.
a
Correlations significant at P \ 0.05.
Allometric affects within the sample were assessed
using a Mantel test of the correlation between the Pro-
crustes distance shape matrix (Table 5) and the size ma-
trix (Table 7). Results of this analysis indicate that the
size and shape matrices are uncorrelated (r 5 20.123,
P 5 0.28). Additionally, regression of the first 30 princi-
pal components (which account for 90% of the sample
variance) on centroid size indicated that, although a
number of these PCs are significantly correlated with
size, the R2 values for these correlation are very low
(i.e., R2 \ 0.04), with the exception of PC 4, where R2 5
0.172 and the P-value was highly statistically significant
(P \ 0.00001). These results suggest that while there
may be some allometric affects within the sample as a
whole, morphological differentiation between populations
is not primarily a result of allometry.
Mantel tests for morphological, molecular, geographic,
and environmental differences are summarized in Table
8. Results for the comparison of morphological and mo-
lecular distance are substantially different depending on
whether the Utah Native American sample is included.
When it is included along with all other populations, the
correlation between molecular distances (Table 9) and
temporal bone morphology was not statistically signifi-
cant (molecular distance vs. shape: r 5 0.205, P 5 0.175;
molecular distance vs. size: r 5 0.298, P 5 0.15). Exclud-
ing the Native American sample, the correlation between
the Procrustes distance and molecular distance was
strongly significant (r 5 0.629, P 5 0.003), although the
centroid size and molecular distance matrices remained
uncorrelated (r 520.032, P 5 0.469).
In explaining this result, we note that the molecular
distances between the Native Americans and all other
populations were extremely high (Table 9); in some
cases, the molecular distance between the Native Ameri-
can group and others was an order of magnitude greater
than distances observed between other populations. At
least according to the STR data, neutral genetic distan-
ces are not distributed in a way that facilitates compari-
son to morphological distances in this group. Therefore,
the analysis excluding Native Americans is probably
more representative of the true pattern of relationships.
No significant correlation was found between the tem-
poral bone shape matrix and any of the environmental
matrices. There was also no significant correlation
between the size matrix and the environmental variables
of altitude, rainfall, or the combined environmental
American Journal of Physical Anthropology DOI 10.1002/ajpa
8.784
2.872
5.912
5.541
6.199
1.537
1.875
0.043
11.782
4.046
3.709
5.584
3.566
4.004
7.736
3.666
8.074
0.337
3.523
8.216
0.480
0.143
2.018
2.829
8.911
1.175
0.837
2.712
0.694
8.741
2.998
4.738
5.075
3.200
5.218
11.740
8.218
3.521
4.215
4.552
2.677
4.695
5.389
0.523
8.261
TABLE 7. Pairwise differences in centroid size among all populations used in the morphometric analysis
1.593
3.990
3.948
7.792
0.056
0.282
0.425
4.794
Size matrix
Nubians
Native Americans
Australians
Alaskans
Hungarians
Pare
Malaysians
Khoisan
New Guineans
Mongolians
Indians
Nubians
Native Americans
4.271
Australians
Alaskans
Hungarians
Pare
Malaysians
Khoisan
New Guineans
1.119
Mongolians
Indians
Calculated as the absolute difference in centroid size. See Table 1 for the mean centroid sizes for each population.
TEMPORAL BONE VARIATION IN MODERN HUMANS 319
TABLE 9. Molecular distance matrix
New
Mozabite Pima Australians French Kenyan Bantu Cambodians San Guineans Mongolians
Mozabite
Pima 0.13097
Australians 0.05873 0.15705
French 0.01643 0.11735 0.05430
Kenyan Bantu 0.03332 0.15853 0.07665 0.04588
Cambodians 0.04064 0.10778 0.05266 0.03697 0.06628
San 0.07455 0.20845 0.11348 0.08725 0.05328 0.09976
New Guineans 0.07951 0.15405 0.06320 0.07234 0.08941 0.07179 0.12706
Mongolians 0.04371 0.09838 0.05739 0.03417 0.06230 0.00487 0.09888 0.07021
These values were calculated using Slatkin s genetic distance for microsatellites (Slatkin, 1995). Note the high values of molecular
distances between the Native American population (Pima) and all other populations, as indicated in bold.
matrix. However, a significant positive correlation was analyzed on its own. This result is consistent with simi-
found between size and temperature (r 5 0.713, P 5 lar studies on humans and other taxa (Harvati, 2003;
0.001), and size and latitude (r 5 0.420, P 5 0.021). Lockwood et al., 2002; Lockwood et al., 2004), and it pro-
Since Harvati and Weaver (2006b) found that the corre- vides an important comparison for previous analyses
lation between size and climate was only obtained if that have used the temporal bone to discriminate
their specifically cold-adapted population was included between species and subspecies of great apes and fossil
in the analysis, the Alaskan population was removed hominins (Harvati, 2003; Harvati et al., 2004; Lockwood
from the comparisons of size to temperature and lati- et al., 2004; Terhune et al., 2007).
tude. The rationale for removing this population is to Although it initially appeared that the correlation
determine whether there is a general pattern of correla- between molecular distance and morphological distance
tion among all populations, or whether it is primarily a based on the temporal bone was not significant, removal
single cold-adapted population driving the correlation.
of the Utah Native American population increased the
For temperature, although the correlation with centroid
correlation substantially. This finding may indicate that
size dropped to r 5 0.569, it remained significant (P 5
the modern genetic analogue, the Pima, was not repre-
0.01). For latitude, the correlation with size dropped to a
sentative of the older morphological sample from Grand
nonsignificant correlation of r 5 0.07.
Gulch, Utah. Alternatively, the marked genetic differen-
The correlation between geographic distance (Table
tiation of the Pima sample may be the result of the
10) and morphological distance for all 11 populations
extreme bottle-necking hypothesized to have occurred
was not significant (geography vs. shape: r 5 0.221, P 5
during the migration of early Americans to the New
0.095; geography vs. size: r 5 0.233, P 5 0.11). However
World (Szathmary, 1993; Santos et al., 1995; Monsalve et
removal of the Utah Native American group from the
al., 1999; Bortolini et al., 2002; Battilana et al., 2006).
analysis resulted in a significant correlation between
While neutral molecular markers may drift unchecked,
geographic distance and morphology (r 5 0.338, P 5
the cranium is likely to be under some degree of stabiliz-
0.029). The STRs used in this study were found to show
ing selection. A bottle-neck event may explain why the
a significant correlation with geographic distances (r 5
molecular distance of the Native Americans is high rela-
0.779, P \ 0.001).
tive to other populations and perhaps exaggerated, while
their morphology is broadly similar to other groups. In
DISCUSSION
any case, our results without Native American samples
are similar to those of Harvati and Weaver (2006a,b),
Although the shape of the temporal bone has long
who also did not include a native North American sam-
been used in analyses of population affinities and species
ple in their genetic analysis.
relationships, the degree to which it reflects neutral
Overall, the correlation between molecular and mor-
genetic evolution has not been fully addressed, and the
phological distance of the temporal bone was relatively
nature of the environmental influence on this element is
good. The finding that the morphology of the temporal
unclear. Our goal was therefore to explore the relation-
bone reflects genetic relationships among human popula-
ship between temporal bone morphology and genetic,
tions is consistent with studies that have identified an
environmental, and geographic variation. Three hypothe-
association between other aspects of cranial morphology
ses were tested, and the results suggest that: 1) there
and genetic relationships in humans (Relethford, 2001,
are significant differences in temporal bone morphology
2002; Gonzales-Jose et al., 2004; Roseman, 2004). These
among modern human populations; 2) shape (but not
results are also consistent with those of Harvati and
size) differences partially reflect neutral evolution; 3) ge-
Weaver (2006a,b), who found a significant correlation
ographic distance is a significant factor but plays a
between molecular and morphological distances using
smaller role in shape variation; 4) shape of the temporal
different populations and different temporal bone land-
bone is not significantly associated with climate, alti-
marks from this study. The temporal bone contains infor-
tude, or temperature, and 5) size of the temporal bone is
mation about genetic relationships within humans, as it
significantly correlated with temperature and latitude.
does among hominoid species, and it may therefore serve
as a reliable means of assessing relationships when mo-
Temporal bone morphology, group affiliation,
lecular data are unavailable. However, in addition to the
and genetic differentiation
difficulty in explaining low morphological distances
Our analysis shows that the temporal bone has high between Native Americans and other groups, the molec-
discriminatory power for human populations even when ular distance matrix among Old World populations
American Journal of Physical Anthropology DOI 10.1002/ajpa
320 H.F. SMITH ET AL.
explains only 39% of morphological variation in the
temporal bone. Clearly, other factors play a substantial
role in temporal bone morphology in humans.
Geographic distance
There is also a general association between morpholog-
ical and geographic distances. Together with the genetic
correlation, this finding indicates that the temporal bone
is evolving to some degree under an isolation by dis-
tance model (Wright, 1943; Morton et al., 1971; Cavalli-
Sforza et al., 1994), which predicts that variation in-
creases with geographic distance among populations.
The relationship of geographic distance, neutral genetic
distance, and temporal bone morphology points to the
neutral component of temporal bone variation.
As with the molecular distance analysis, the correla-
tion between morphological and geographic distance was
only significant if the Utah Native American population
was removed from the analysis. This group had the
highest average geographic distance from all other popu-
lations, but its morphological distances to other groups
were not particularly high. This pattern may reflect the
recent arrival of humans into the Americas. Also, there
may be a threshold beyond which additional geographic
distance does not translate into additional morphological
distance, especially if stabilizing selection restricts the
potential variation in temporal bone morphology. Along
similar lines, the Utah Native American group may
share morphology with distant populations due to aspects
of ecology not studied here.
Environment
None of the environmental variables included in this
study (altitude, latitude, rainfall, and temperature)
showed a significant correlation with temporal bone
shape. These findings are consistent with those of Har-
vati and Weaver (2006a,b), who found that temporal
bone shape was not significantly associated with humid-
ity, latitude, or temperature (they did not look at rain-
fall). Temporal bone size, however, was found to covary
with temperature and latitude, largely because of the
inclusion of a sample from Alaska. These environmental
variables are not entirely separate entities, as the tem-
perature and latitude matrices were found to be highly
correlated with each other (r 5 0.855, P \ 0.001). Thus,
it seems likely that temperature is the predominant
environmental influence over human temporal bone size,
as would be predicted by Bergmann s Rule (Bergmann,
1847), and that the correlation with latitude is simply a
by-product of that effect. Harvati and Weaver (2006a,b)
also found temporal bone size to be correlated with tem-
perature. As one might expect, the size of the temporal
bone is probably less informative than temporal bone
shape for inferring genetic affinities between populations.
Although temporal bone shape correlates with genetic
and geographic distance between populations, a rela-
tively large proportion of human temporal bone variation
remains unexplained by the factors investigated here.
Some of this variation may be related to variation in the
shape of the cranial component of the TMJ, the morphol-
ogy of which is described by the landmarks included in
this study. Within primates, some aspects of TMJ shape
have been linked to variation in masticatory function,
and specifically to food material properties and dental
function (Bouvier, 1986a,b; Wall, 1999; Vinyard et al.,
American Journal of Physical Anthropology DOI 10.1002/ajpa
4,496
5,276
9,922
9,952
12,629
3,384
1,4432
14,462
17,139
4,662
18,984
9,121
10,050
6,673
9,038
3,837
10,883
8,347
15,206
15,236
9,871
17,913
13,935
6,118
19,758
15,780
7,963
2,699
14,474
10,226
5,558
6,830
10,281
4,942
12,126
9,452
TABLE 10. Geographic distances between populations (in kilometers)
4,244
9,612
19,260
13,925
17,989
13,976
19,834
18,486
14,104
2,815
2,838
9,326
4,650
6,978
5,735
14,610
10,632
Nubians
Native Americans
Australians
Alaskans
Hungarians
Pare
Malaysians
Khoisan
New Guineans
Mongolians
Indians
Nubians
Native Americans
14,686
Australians
Alaskans
Hungarians
Pare
Malaysians
Khoisan
New Guineans
13,836
Mongolians
Indians
Distances were calculated using great circle distances including five waypoints through which populations would travel during migrations. See Figure 1 and Table 1 for approxi-
mate locations of populations.
TEMPORAL BONE VARIATION IN MODERN HUMANS 321
Athreya S, Glantz MM. 2003. The impact of character correla-
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tion and variable groupings on modern human population
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nificant factor in the observed morphological variation.
Battilana J, Fagundes NJ, Heller AH, Goldani A, Freitas LB,
Further analysis should focus directly on diet in an
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Temporal bone shape does not correlate strongly with and breadth. Am J Phys Anthropol 85:419 427.
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ACKNOWLEDGMENTS
Harvati K. 2003. Quantitative analysis of Neanderthal temporal
bone morphology using three-dimensional geometric morpho-
Special thanks go to Katerina Harvati and Timothy
metrics. Am J Phys Anthropol 120:323 338.
Weaver for sharing their book chapter with us while it
Harvati K, Frost SR, McNulty KP. 2004. Neanderthal taxonomy
was still in press. We are grateful to Ian Tattersall and
reconsidered: implications of 3D primate models of intra- and
Gary Sawyer of the American Museum of Natural History
interspecific differences. Proc Natl Acad Sci USA 101:1147
and Diane Hawkey of Arizona State University for per-
1152.
mission to study collections in their care. This manuscript
Harvati K, Weaver TD. 2006a. Reliability of cranial morphology
was greatly improved by comments from Mark Spencer,
in reconstructing Neandertal phylogeny. In: Harrison T, Har-
Katerina Harvati, the editor Clark Larsen, and one anony- vati K, editors. Neandertals revisited: new approaches and
mous reviewer. perspectives. Dordrecht: Springer. p 239 254.
Harvati K, Weaver TD. 2006b. Human cranial anatomy and the
differential preservation of population history and climate sig-
natures. Anat Rec A 288:1225 1233.
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