The basicranium has been described as phylogenetically informative, developmentally stable, and minimally affected by external factors and consequently plays an important role in cranial size and shape in subadult humans. Here basicranial variation of subadults from several modern human populations was investigated and the impact of genetic relatedness on basicranial morphological similarities was investigated. Three-dimensional landmark data were digitized from subadult basicrania from seven populations. Published molecular data on short tandem repeats were statistically compared to morphological data from three ontogenetic stages. Basicranial and temporal bone morphology both reflect genetic distances in childhood and adolescence (5–18 years), but not in infancy (<5 years). The occipital bone reflects genetic distances only in adolescence (13–18 years). The sphenoid bone does not reflect genetic distances at any ontogenetic stage but was the most diagnostic region evaluated, resulting in high rates of correct classification among populations. These results suggest that the ontogenetic processes driving basicranial development are complex and cannot be succinctly summarized across populations or basicranial regions. However, the fact that certain regions reflect genetic distances suggests that the morphology of these regions may be useful in reconstructing population history in specimens for which direct DNA evidence is unavailable, such as archaeological sites.
Cranial morphology is frequently studied with the purpose of identifying and interpreting the extensive range of variation that exists among modern human populations and in the hominin fossil record (e.g., [
Previous studies have demonstrated that the basicranium is a phylogenetically informative region among human and nonhuman primate adults [
Nonetheless, since the cranium starts to develop before birth [
Previous studies have compared the shape of the developing facial skeleton among groups from different geographic locations and genetic backgrounds to determine whether similar ontogenetic processes characterize divergent groups [
To date, there have been extensive studies on the morphology of the temporal bone and its applications to phylogeny [
Modern human populations can be readily distinguished from one another based on one particular component of the basicranium and temporal bone shape [
Several researchers have attempted to determine whether the basicranium is a more reliable region for reconstructing genetic distances than other regions of the cranium [
In light of the previous work and unresolved research questions outlined above, the aims of this study are to determine at what point during the ontogenetic process population-specific basicranial morphologies emerge and to compare basicranial ontogeny among human populations. Specifically, we test the following hypotheses: Human populations differ significantly in the shape of the basicranium and its various components irrespective of ontogenetic stage. Differences among human populations in basicranial morphology are significantly correlated with their genetic distances throughout ontogeny.
Three-dimensional data on basicranial morphology were collected by one of us (DHD). The data were collected from skulls of seven modern human populations at various ontogenetic stages (Table
Human population samples, their adult and subadult sample sizes, and molecular representative populations.
Population |
AC1 ( |
AC2 ( |
AC3 ( |
Total ( |
Molecular representative |
---|---|---|---|---|---|
Alaskan | 10 | 6 | 14 | 30 | Yakut |
Austrian | 5 | 10 | 13 | 28 | French |
Egyptian | 9 | 7 | 11 | 27 | Mozabite |
Mexican | 8 | 7 | 15 | 30 | Maya |
Peruvian | 3 | 8 | 19 | 30 | Colombians |
Polynesian | 2 | 6 | 9 | 17 | Solomon Islanders |
Utah Native American | 11 | 7 | 4 | 22 | Pima |
Total |
|
|
|
|
Prior to traveling to the museums, an intraobserver error study was conducted to ensure the accuracy of the data collection. Two adult crania were digitized ten times each. To minimize the effect of investigator fatigue, the data were collected on two separate days. A paired samples
Forty-four landmarks were collected from the basicranium and subcategorized into temporal, occipital, and sphenoidal regions (Tables
(a) Definitions of occipital bone landmarks used in the present study. Refer also to Figure
Occipital | |
---|---|
1 | Most anterior point on the basioccipital in the midline (sphenobasion, on the occipital if not connected) |
2 | Most lateral point on the basioccipital |
3 | Basion (anterior most point on the foramen magnum) |
4 | Most anterior point on the occipital condyle along the margin of the foramen magnum |
5 | Most anterior point on occipital condyle |
6 | Most lateral point on the occipital condyle (point on the middle of the lateral edge of the condyle) |
7 | Most posterolateral point on the occipital condyle |
8 | Most posterior point on the occipital condyle along the margin of the foramen magnum |
9 | Mid-point of the occipital condyle (inferior aspect) |
10 | Opisthion (posterior most point on the foramen magnum) |
11 | Mid-point on the median nuchal line between the external occipital protuberance and foramen magnum |
12 | Anteromedial point on the hypoglossal canal |
13 | Asterion (temporal, occipital, and parietal meet) |
Temporal | |
---|---|
13 | Asterion (where temporal, occipital, and parietal meet) |
14 | Parietal notch (not depicted) |
15 | Mastoidale (center of the inferior point on the mastoid process) |
16 | Most lateral point on the margin of the stylomastoid foramen |
17 | Most lateral point on the vagina of the styloid process (whether process is present or absent) |
18 | Most posterolateral point on the jugular fossa |
19 | Most posterolateral point on the margin of the carotid canal entrance |
20 | Point on anterior margin of tympanic element that is closest to carotid canal |
21 | Most posterolateral point on the external acoustic meatus |
22 | Most inferior point on the external acoustic meatus |
23 | Point on lateral margin of zygomatic process of the temporal bone at the position of the postglenoid process |
24 | Point of inflection where the braincase curves laterally into the supraglenoid gutter, in coronal plane of mandibular fossa (not depicted) |
25 | Point on the anterior of the lateral margin of the articular surface of the articular eminence |
26 | Most inferior point on the postglenoid process |
27 | Deepest point within the mandibular fossa (instrumentally determined) |
28 | Mid-point of the articular eminence |
29 | Most anterior point on the articular surface of the articular eminence |
30 | Auriculare (most lateral point on the temporal) |
31 | Suture between temporal and zygomatic bones on inferior aspect of zygomatic process |
32 | Most inferior point at the sphenotemporal suture closer to the midline (on the sphenoid if disconnected) |
33 | Most lateral point on the greater wing of the sphenoid (intersection between sphenoid, temporal, and parietal bone) |
34 | Most frontolateral point on the greater wing of the sphenoid (intersection between sphenoid, temporal, and frontal bone) |
36 | Most posterior, inferior point on the sphenotemporal suture |
37 | Apex of the petrous part of the temporal bone |
Sphenoid | |
---|---|
33 | Most lateral point on the greater wing of the sphenoid (intersection between sphenoid, temporal, and parietal bone) |
35 | Most anterior inferior point on the sphenozygomatic suture (sphenozygomatic) (not depicted) |
36 | Most posterior, inferior point on the sphenotemporal suture |
38 | Most lateral point of the foramen spinosum |
39 | Most lateral point on the margin of foramen ovale |
40 | Most anterolateral point of the lateral pterygoid plate |
41 | Most inferior part of the pterygoid hamulus (not depicted) |
42 | The most anteromedial point of the sphenoidal region on the sphenovomer suture |
43 | Most posterior point where the vomer meets the medial pterygoid plate |
44 | Point on the sphenoid in the midline in contact with the vomer (vomer notch) |
1 | Most anterior point on the basioccipital in the midline (sphenobasion, on the occipital if not connected) |
2 | Most lateral point on the basioccipital |
Forty-four landmarks of the basicranium digitized in the present study. Please refer to Tables
Data on individual genotypes for 783 short tandem repeats (STRs) were compiled from Ramachandran et al. [
STRs are composed of back-to-back repeating segments of two to six nucleotides and are found at many locations within the genome [
The morphological data were analyzed using a Generalized Procrustes Analysis (GPA), in which the digitized points were rotated and translated and specimens were scaled to the same size, such that the only remaining differences among them were directly attributable to shape. The GPA was followed by a Principal Components Analysis (PCA) using MorphoJ [
In order to assess the degree to which basicranial morphology can be utilized to correctly classify individuals of various ages into the population from which they derived, a discriminant function analysis (DFA) was conducted using the Principal Component (PC) scores from the PCA. The DFA was used to determine whether groups could be classified reliably or if there was excessive morphological overlap. This analysis was conducted for the morphology of the basicranium and then for each of its major components, the temporal, occipital, and sphenoidal regions. These tests were conducted with cross-validation using SPSS version 11.0.1 (SPSS, Chicago, IL). This analysis indicated how well basicranial shape discriminates among populations at the three subadult stages of ontogeny (AC1, AC2, AC3).
A matrix of Slatkin’s molecular distances among the molecular representative of these populations was calculated from the published molecular data using Arlequin 3.11 [
Population molecular distance matrix (
Alaska | Austria | Egypt | Mexico | Peru | Polynesia | Utah Native American | |
---|---|---|---|---|---|---|---|
Alaska | — | ||||||
Austria | 0.0455 | — | |||||
Egypt | 0.0530 | 0.1068 | — | ||||
Mexico | 0.0580 | 0.0606 | 0.0731 | — | |||
Peru | 0.0951 | 0.0984 | 0.1110 | 0.0443 | — | ||
Polynesia | 0.0759 | 0.0759 | 0.0808 | 0.1028 | 0.1443 | — | |
Utah Native American | 0.0984 | 0.1076 | 0.1190 | 0.0600 | 0.0973 | 0.1461 | — |
A series of descriptive analyses were conducted to evaluate the ontogenetic trajectory in the sample. Principal Component scores were regressed against centroid size and biological age (as determined using dental eruption of each specimen, following Ubelaker [
Procrustes distances based on basicranial morphology of populations sampled in this study were found to be statistically significant for all combined-age subadult samples (Table
Procrustes distance matrix among populations based on subadult basicranial morphology. All pairwise population distances are significantly different at the
Alaska | Austria | Egypt | Mexico | Peru | Polynesia | Utah | |
---|---|---|---|---|---|---|---|
Alaska | — | ||||||
Austria | 0.0361 | — | |||||
Egypt | 0.0779 | 0.0703 | — | ||||
Mexico | 0.0398 | 0.0444 | 0.0745 | — | |||
Peru | 0.0493 | 0.0513 | 0.0846 | 0.0411 | — | ||
Polynesia | 0.0618 | 0.0532 | 0.0809 | 0.0631 | 0.0740 | — | |
Utah | 0.0694 | 0.0684 | 0.0998 | 0.0638 | 0.0548 | 0.0805 | — |
Morphological Procrustes distance matrix among populations based on basicranial morphology in the AC1 age category. Significantly different pairwise population distances (
Alaska | Austria | Egypt | Mexico | Peru | Polynesia | Utah | |
---|---|---|---|---|---|---|---|
Alaska | — | ||||||
Austria |
|
— | |||||
Egypt | 0.1110 | 0.1611 | — | ||||
Mexico |
|
0.1518 | 0.0816 | — | |||
Peru | 0.1127 | 0.1867 |
|
0.1571 | — | ||
Polynesia | 0.1045 | 0.1735 | 0.1066 | 0.1102 | 0.1519 | — | |
Utah |
|
0.1485 | 0.1040 | 0.0908 |
|
0.1019 | — |
Morphological Procrustes distance matrix among populations based on basicranial morphology in the AC2 age category. Significantly different pairwise population distances (
Alaska | Austria | Egypt | Mexico | Peru | Polynesia | Utah | |
---|---|---|---|---|---|---|---|
Alaska | — | ||||||
Austria |
|
— | |||||
Egypt |
|
|
— | ||||
Mexico |
|
|
0.0948 | — | |||
Peru |
|
|
|
0.0799 | — | ||
Polynesia | 0.0799 |
|
|
|
|
— | |
Utah |
|
|
|
0.0722 |
|
0.0715 | — |
Morphological Procrustes distance matrix among populations based on basicranial morphology in the AC3 age category. Significantly different pairwise population distances (
Alaska | Austria | Egypt | Mexico | Peru | Polynesia | Utah | |
---|---|---|---|---|---|---|---|
Alaska | — | ||||||
Austria |
|
— | |||||
Egypt |
|
|
— | ||||
Mexico |
|
|
|
— | |||
Peru |
|
|
|
|
— | ||
Polynesia |
|
0.0620 |
|
|
|
— | |
Utah |
|
|
0.1382 |
|
0.0795 |
|
— |
DFAs were performed on all regions of the basicranium: temporal, occipital, and sphenoidal. The DFA for the entire basicranium resulted in cross-validated classification rates ranging from 13.3–34.8% (Table
Classification results from discriminant function analysis (DFA) with cross-validation for the entire basicranium.
% Correct | Alaska | Austria | Egypt | Mexico | Peru | Polynesia | Utah | Total | |
---|---|---|---|---|---|---|---|---|---|
Alaska | 26.3 | 5 | 2 | 0 | 2 | 3 | 4 | 3 | 19 |
Austria | 34.8 | 3 | 8 | 1 | 6 | 4 | 1 | 0 | 23 |
Egypt | 33.3 | 0 | 0 | 3 | 1 | 1 | 3 | 1 | 9 |
Mexico | 13.3 | 3 | 3 | 1 | 2 | 3 | 1 | 2 | 15 |
Peru | 20.8 | 5 | 4 | 1 | 5 | 5 | 1 | 3 | 24 |
Polynesia | 20.0 | 2 | 0 | 4 | 0 | 1 | 2 | 1 | 10 |
Utah | 28.6 | 1 | 0 | 0 | 2 | 2 | 0 | 2 | 7 |
25.2% of cross-validated grouped cases correctly classified.
The Egyptian population was one of the most correctly classified in the basicranial (33.3%), temporal bone (55%), and occipital bone (35.7%) data sets (Tables
Classification results from discriminant function analysis (DFA) with cross-validation for the temporal bone.
% Correct | Alaska | Austria | Egypt | Mexico | Peru | Polynesia | Utah | Total | |
---|---|---|---|---|---|---|---|---|---|
Alaska | 65.4 | 17 | 2 | 0 | 3 | 0 | 0 | 4 | 26 |
Austria | 37.0 | 7 | 10 | 1 | 4 | 4 | 0 | 1 | 27 |
Egypt | 55.0 | 1 | 4 | 11 | 2 | 1 | 1 | 0 | 20 |
Mexico | 23.1 | 2 | 5 | 2 | 6 | 6 | 1 | 4 | 26 |
Peru | 48.3 | 2 | 5 | 1 | 3 | 14 | 0 | 4 | 29 |
Polynesia | 42.9 | 2 | 2 | 2 | 1 | 1 | 6 | 0 | 14 |
Utah | 11.1 | 5 | 1 | 1 | 4 | 4 | 1 | 2 | 18 |
41.3% of cross-validated grouped cases correctly classified.
Classification results from discriminant function analysis (DFA) with cross-validation for the occipital bone.
% Correct | Alaska | Austria | Egypt | Mexico | Peru | Polynesia | Utah | Total | |
---|---|---|---|---|---|---|---|---|---|
Alaska | 30 | 6 | 7 | 0 | 3 | 3 | 1 | NA | 20 |
Austria | 33.3 | 7 | 8 | 5 | 2 | 0 | 2 | NA | 24 |
Egypt | 35.7 | 1 | 5 | 5 | 1 | 1 | 1 | NA | 14 |
Mexico | 31.3 | 3 | 2 | 2 | 5 | 4 | 0 | NA | 16 |
Peru | 75 | 2 | 0 | 1 | 3 | 18 | 0 | NA | 24 |
Polynesia | 40 | 1 | 2 | 2 | 1 | 0 | 4 | NA | 10 |
Utah | NA | NA | NA | NA | NA | NA | NA | NA | NA |
42.6% of cross-validated grouped cases correctly classified.
Classification results from discriminant function analysis (DFA) with cross-validation for sphenoid morphology.
% Correct | Alaska | Austria | Egypt | Mexico | Peru | Polynesia | Utah | Total | |
---|---|---|---|---|---|---|---|---|---|
Alaska | 41.2 | 7 | 5 | 0 | 0 | 3 | 0 | 2 | 17 |
Austria | 55.0 | 3 | 11 | 2 | 2 | 1 | 0 | 1 | 20 |
Egypt | 30.0 | 0 | 3 | 3 | 1 | 2 | 0 | 1 | 10 |
Mexico | 57.1 | 2 | 0 | 0 | 8 | 4 | 0 | 0 | 14 |
Peru | 63.6 | 2 | 2 | 1 | 2 | 14 | 0 | 1 | 22 |
Polynesia | 42.9 | 0 | 0 | 1 | 1 | 2 | 3 | 0 | 7 |
Utah | 37.5 | 2 | 2 | 0 | 1 | 0 | 0 | 3 | 8 |
50.0% of cross-validated grouped cases correctly classified.
The morphological Procrustes distances based on the different age categories and regions of the basicranium were statistically compared to the molecular
Procrustes analyses comparing molecular distances with morphological distances based on each of the cranial data sets.
Basicranium | Temporal | Occipital | Sphenoid | |||||
---|---|---|---|---|---|---|---|---|
|
|
|
|
|
|
|
|
|
All subadults |
|
|
0.18 | 0.30 |
|
|
0.33 | 0.14 |
AC1 | − |
|
−0.38 | 0.09 | NA | NA | NA | NA |
AC2 | 0.36 | 0.06 | −0.18 | 0.28 |
|
|
0.09 | 0.38 |
AC3 | 0.33 | 0.18 | 0.14 | 0.34 |
|
|
−0.06 | 0.56 |
Procrustes analyses comparing molecular distances with morphological distances based on each of the cranial data sets excluding the Egyptian population.
Basicranium | Temporal | Occipital | Sphenoid | |||||
---|---|---|---|---|---|---|---|---|
|
|
|
|
|
|
|
|
|
All subadults |
|
|
|
|
|
|
0.55 | 0.05 |
AC1 | −0.45 | 0.17 | −0.15 | 0.36 | NA | NA | NA | NA |
AC2 |
|
|
|
|
−0.01 | 0.50 | 0.09 | 0.38 |
AC3 |
|
|
|
|
|
|
−0.26 | 0.22 |
When the subadult samples for each cranial region were divided into separate age categories, more distinct patterns emerged (Tables
Overall, the above patterns show that the shape of the basicranium, temporal bone, and occipital bone, for all populations excluding the Egyptian population, reflects genetic distances for the combined subadult sample and AC3 (13–18 years of age), thus supporting (H2) (basicranial differences are correlated with genetic distances) for these data sets. The temporal bone and basicranium also reflect genetic distances during AC2 (5–12 years of age) when the Egyptian population is excluded. The morphology of the sphenoid bone does not reflect genetic distances for subadults of any age category; however, its morphology can be used to discriminate among populations with slightly higher rates than the other two cranial regions. Therefore, (H2) is not supported for the sphenoid, AC1 (for any morphological region), or the occipital bone for AC2.
The regression of biological age with Principal Components scores (Tables
Principal Components of the basicranium of all populations significantly correlated with age and/or centroid size in the regression analysis.
Age | Centroid size | |
---|---|---|
PC1 |
|
|
PC2 |
|
|
PC3 |
|
|
PC4 |
|
|
Principal Components of the basicranium and its regions significantly correlated with age in the regression analysis.
Basicranium | Temporal | Occipital | Sphenoid | |
---|---|---|---|---|
All populations | PC1, PC2, PC4 | PC1, PC2, PC4 | PC3, PC5, PC6 | PC2, PC4 |
Alaska | PC1, PC2, PC4 | PC1, PC4 | PC2, PC3, PC4, PC6 | PC1, PC2, PC4 |
Austria | PC1, PC3, PC4 | PC1, PC2, PC3, PC6 | PC1, PC2, PC6 | PC4 |
Egypt | PC1 | PC1 | PC6 | PC4 |
Mexico | PC1 | PC1, PC2 | PC3, PC4, PC6 | PC4 |
Peru | PC1, PC2, PC3 | PC1, PC6 | PC1, PC2, PC5 | PC4 |
Polynesia | PC1 | PC1 | PC3, PC5, PC6 | |
Utah | PC1, PC2 | PC1, PC2 | PC5, PC6 | PC2, PC4 |
Regression plot of PC1 scores versus log centroid size for the basicranium. The individual population regression lines are indicated and their
Plot of the first four principal components of the basicranial landmark configuration that were significantly correlated with developmental age and centroid size: (a) PC1
The regression analysis of centroid size with PC scores for the entire basicranium revealed significant correlations between size and PC1 (14.83% of the variance), PC2 (9.31% of the variance), PC3 (7.57% of the variance), and PC4 (6.53% of the variance) (Tables
Principal Components of the basicranium and its regions significantly correlated with centroid size in the regression analysis.
Basicranium | Temporal | Occipital | Sphenoid | |
---|---|---|---|---|
All populations | PC1, PC2, PC3, PC4 | PC1, PC4, PC6 | PC1, PC2, PC3, PC4, PC6 | PC1, PC2, PC4 |
Alaska | PC1, PC2 | PC1, PC4 | PC2, PC3, PC4, PC6 | PC1, PC4 |
Austria | PC1, PC3, PC4 | PC1, PC2, PC4, PC6 | PC3 | PC2, PC4, PC5 |
Egypt | PC1, PC4, PC5 | PC1, PC4 | PC1, PC3 | PC1, PC3, PC4, PC5 |
Mexico | PC1, PC2, PC4 | PC1, PC3, PC4 | PC1, PC2, PC3, PC4, PC5, PC6 | PC4 |
Peru | PC1, PC2 | PC1, PC4 | PC1, PC3 | PC4 |
Polynesia | PC1, PC2 | PC2, PC3 | PC2, PC4 | |
Utah | PC1, PC2 | PC1 | PC1, PC3 | PC1, PC4 |
Mean Principal Components scores for the major PCs for each age group in each population for the entire basicranium. Note: sample size of complete basicrania of AC1 for the Polynesian population was insufficient to obtain a reasonable estimation of the mean.
Population | Age Cat. | PC1 | PC2 | PC3 | PC4 | PC5 |
---|---|---|---|---|---|---|
Alaskans | AC1 | 0.05323 | 0.00302 | 0.00175 | −0.00882 | 0.00855 |
Alaskans | AC2 | 0.04618 | −0.01399 | 0.01446 | −0.01085 | −0.00150 |
Alaskans | AC3 | 0.02569 | 0.00815 | −0.00014 | −0.01146 | 0.00114 |
Alaskans | Adults | 0.00969 | 0.00543 | 0.00499 | −0.00409 | 0.00546 |
Austrians | AC1 | 0.07867 | −0.02117 | −0.03936 | −0.03364 | 0.00580 |
Austrians | AC2 | 0.02695 | 0.00316 | 0.00068 | 0.02413 | 0.01203 |
Austrians | AC3 | 0.00584 | 0.02062 | 0.00347 | 0.02083 | −0.00779 |
Austrians | Adults | −0.02377 | −0.01075 | 0.00784 | 0.02859 | 0.00468 |
Egyptians | AC1 | 0.11390 | 0.02214 | 0.00609 | −0.01172 | 0.00766 |
Egyptians | AC2 | 0.04530 | −0.00072 | −0.00789 | 0.00435 | −0.02901 |
Egyptians | AC3 | 0.00920 | 0.01193 | −0.00507 | 0.03377 | −0.05306 |
Egyptians | Adults | 0.00662 | 0.03986 | −0.00195 | 0.01716 | −0.01972 |
Mexicans | AC1 | 0.08308 | 0.02326 | −0.02515 | −0.01663 | −0.01222 |
Mexicans | AC2 | 0.02838 | −0.02994 | 0.01315 | 0.01064 | 0.00299 |
Mexicans | AC3 | 0.01160 | −0.02266 | −0.01056 | 0.00150 | 0.00459 |
Mexicans | Adults | −0.03151 | −0.00378 | 0.00344 | −0.00451 | 0.00588 |
Peruvians | AC1 | 0.00004 | −0.10173 | 0.02027 | 0.01840 | 0.00450 |
Peruvians | AC2 | −0.01429 | −0.04447 | 0.00779 | −0.00991 | 0.01923 |
Peruvians | AC3 | −0.01735 | −0.03330 | 0.01130 | −0.01073 | −0.00269 |
Peruvians | Adults | −0.03927 | 0.00426 | −0.00411 | −0.00933 | 0.00495 |
Polynesians | AC1 | — | — | — | — | — |
Polynesians | AC2 | 0.06510 | 0.02825 | −0.00041 | −0.00531 | −0.00052 |
Polynesians | AC3 | 0.01269 | 0.04162 | 0.00118 | 0.00674 | 0.00423 |
Polynesians | Adults | −0.05657 | 0.04434 | −0.01331 | 0.01068 | 0.00484 |
Utah | AC1 | 0.05701 | −0.05851 | −0.00487 | −0.03191 | 0.01686 |
Utah | AC2 | 0.00952 | −0.01100 | −0.00275 | −0.03556 | −0.00031 |
Utah | AC3 | −0.06937 | −0.00900 | −0.00478 | −0.02770 | 0.00412 |
Utah | Adults | −0.04365 | 0.00478 | −0.00146 | −0.02958 | −0.00178 |
Mean Principal Components scores for the major PCs for each age group in each population for the temporal bone.
Population | Age Cat. | PC1 | PC2 | PC3 | PC4 | PC5 | PC6 |
---|---|---|---|---|---|---|---|
Alaskans | AC1 | 0.06370 | 0.03778 | −0.00467 | 0.003486 | −0.01318 | 0.002793 |
Alaskans | AC2 | 0.05955 | 0.01832 | −0.01152 | 0.012063 | −0.00951 | 0.006615 |
Alaskans | AC3 | 0.01364 | 0.01492 | −0.01702 | −0.00626 | 0.01089 | −0.00095 |
Alaskans | Adults | −0.04098 | 0.00911 | −0.03013 | −0.01553 | 0.003458 | −0.00611 |
Austrians | AC1 | 0.06432 | 0.04918 | 0.014509 | 0.0147 | 0.004334 | 0.041771 |
Austrians | AC2 | 0.01920 | −0.01637 | −0.01942 | −0.00831 | −0.02196 | 0.000631 |
Austrians | AC3 | −0.00658 | −0.00682 | −0.00317 | −0.0133 | −0.00137 | −0.00585 |
Austrians | Adults | −0.04079 | −0.01804 | 0.036348 | −0.00234 | −0.01313 | −0.01497 |
Egyptians | AC1 | 0.09614 | −0.01266 | −0.00793 | −0.00211 | 0.011929 | 0.021407 |
Egyptians | AC2 | 0.07456 | −0.07440 | 0.023618 | 0.005589 | 0.039431 | 0.00116 |
Egyptians | AC3 | 0.03027 | −0.01573 | −0.01638 | −0.02419 | −0.01115 | 0.008317 |
Egyptians | Adults | −0.00847 | −0.01948 | −0.0062 | −0.01665 | 0.009855 | 0.022317 |
Mexicans | AC1 | 0.07394 | 0.03163 | −0.00879 | 0.015592 | 0.024777 | 0.01061 |
Mexicans | AC2 | 0.04101 | 0.00014 | −0.0111 | 0.009107 | 0.003618 | −0.02052 |
Mexicans | AC3 | −0.00346 | 0.00340 | −0.01835 | 0.013748 | 0.007935 | −0.00232 |
Mexicans | Adults | −0.02252 | −0.03734 | 0.005864 | −0.02859 | 0.023767 | 0.001946 |
Peruvians | AC1 | 0.04345 | −0.01427 | 0.00188 | 0.00293 | 0.031773 | −0.00416 |
Peruvians | AC2 | 0.01024 | 0.00996 | 0.006532 | 0.025246 | −0.00551 | −0.01473 |
Peruvians | AC3 | −0.01153 | 0.01216 | 0.011737 | 0.022836 | −0.00735 | −0.0178 |
Peruvians | Adults | −0.04831 | −0.00314 | 0.015741 | 0.009016 | −0.00291 | 0.010455 |
Polynesians | AC1 | 0.09089 | 0.03686 | 0.01485 | −0.01808 | −0.08961 | −0.02056 |
Polynesians | AC2 | 0.03138 | 0.04798 | −0.01105 | −0.00197 | 0.01271 | 0.000566 |
Polynesians | AC3 | −0.01459 | −0.01922 | 0.003433 | −0.01135 | −0.00779 | 0.009155 |
Polynesians | Adults | −0.07627 | −0.00224 | 0.001625 | −0.01349 | −0.00344 | 0.022897 |
Utah | AC1 | 0.10426 | −0.00679 | 0.01661 | 0.030121 | −0.0139 | −0.01324 |
Utah | AC2 | 0.04445 | −0.00898 | 0.018823 | 0.00818 | −0.00028 | 0.003031 |
Utah | AC3 | −0.05003 | 0.02901 | 0.014978 | −0.01203 | 0.021723 | −0.00859 |
Utah | Adults | −0.04109 | 0.00599 | 0.030287 | 0.003773 | 0.000573 | −0.01027 |
In order to further explore how populations and age categories differed with regard to basicranial morphology, we conducted a shape space exploration in which wireframes of each cranial data set were morphed along the major PC axes to visualize how the shape varied along each PC.
In basicranial shape, subadults were commonly located on the +PC1 axis (Figure
Associated with a positive PC2 score (Figure
To date, extensive research has been conducted on the temporal bone and how it reflects genetic relationships in nonhuman primates (e.g., [
The hypotheses that formed the basis for the study were found to be only partially supported. In actuality, the patterns of morphological variation turned out to be more complex than the relatively simply stated hypotheses. The patterns of the individual basicranial bones differed from each other and across the various age categories. Thus, the ontogenetic processes driving basicranial development cannot be succinctly summarized across all regions of the basicranium. Human populations differ in the shape of the basicranium and its various components irrespective of ontogenetic stage.
This hypothesis was not supported by our results. All populations were found to differ significantly in basicranial morphology in the combined subadult samples. However, in the separate age cateories there was a trend toward increasing differentiation with age. In AC1, several population were not significantly different, by AC2 most populations differed, and by AC3 all populations were significantly different. Interestingly, of all the individual cranial bones, the sphenoid was revealed to be the most distinct among populations and therefore the most reliable region for population classification. Differences among human populations in basicranial morphology are significantly correlated with their genetic distances throughout ontogeny.
This hypothesis was partially supported for several subsets of our data. Our results show that morphology of the basicranium, occipital, and temporal bones each significantly reflects genetic distances in the combined subadult sample. The sphenoid bone, however, is not a good indicator of genetic distances in subadults in the combined subadult sample or at any ontogenetic stage. In the separate age categories for the basicranium, temporal, and occipital regions, the older the individuals, the more congruent the patterns between the genetic and morphological datasets, such that AC3 was the most highly correlated, followed by AC2 and then AC1.
The discriminant function analyses revealed that the Egyptian population was the most correctly classified population for the basicranium, occipital, and temporal bone in subadults. The Egyptian population was most likely classified correctly because it is the most geographically and genetically distant of the populations in this study. This finding is consistent with the previous description of Egyptian cranial morphology as unique and distinctive [
Unsurprisingly, the closely related Native American groups of Utah and Mexico were often classified as each other in basicranial, temporal, and occipital bone for all subadults. This trend has been supported by previous studies [
Based on the findings of this research, the basicranium, temporal, and occipital bone reflect genetic distances in childhood and adolescence, but this study suggests that these differences are not seen in infancy. AC1 was not correlated with molecular distances among populations for any of the cranial regions. Two possible scenarios may explain these results in infancy. First, infant basicranial morphology may be similar among populations and the observable differences in later ontogenetic stages have not yet developed. However, an alternative explanation is that as a result of the fact that many basicranial bones are not fully fused in infants, our sample sizes in the AC1 age category may have been insufficient to reveal subtle differences among populations in infancy.
These findings contrast with some of the results found previously for the temporal bone [
Interestingly, while morphology of the sphenoid was not found to reflect the molecular distance matrix at any ontogenetic stage evaluated (Table
Overall patterns revealed by this study show that the shape of the basicranium, the temporal bone, and occipital bone, for all populations excluding the Egyptian population, reflects genetic distances in subadults. Removing the Egyptian population from the analyses yielded significant results in the temporal bone, occipital bone, and age categories AC2 and AC3. It cannot be definitively ascertained why the Egyptian population deviated from the patterns of the rest of the included samples, but one possibility is the relatively high degree of mismatch between the morphological Egyptian sample and its molecular population representative, the Mozabite. The Mozabite people live in Algeria and speak a Berber language. As one of the few northern Saharan populations to have been extensively studied for neutral molecular loci, they are the closest well-typed molecular representative for the Egyptians, but certainly not a perfect match.
Patterns on the basicranial PC wireframe showed that the Egyptian crania appeared on the +PC2 scale, indicating that the basicranium in the Egyptian population is relatively shorter anteroposteriorly compared to the other populations. Most of the Egyptian population clustered towards an increasingly long occipital bone (–PC2, +PC3, and –PC4) and increasing large temporal bone. It appears that the Egyptian cranium might have a relatively smaller overall basicranium but that the temporal and occipital bone increased in length, with the sphenoid decreasing in length to compromise.
A study by A. C. Berry and R. J. Berry [
One potentially limiting factor of this study is the degree of mismatch between the molecular and morphological population representatives. The morphological samples were chosen based on the availability of subadult cranial material in museum collections. Thus, we were not able to include a wide geographically distribution of populations. In North American natural history collections, there is a natural bias towards Native American specimens. Consequently, the present study included a few Native American samples. Similarly, the molecular representative for each population was not a perfect match with these morphological populations. However, there is precedent for this approach. As noted by Roseman (2004), such mismatch is not necessarily problematic, but the correlations obtained from this type of analysis should be interpreted as minimal approximations of their actual value. Thus, the significant correlations obtained here should be interpreted as minimum estimates of a real biological relationship between morphology of the cranial region and genetic relatedness.
The landmark dataset employed in the present research expands the smaller landmark sets used by previous studies. Harvati and Weaver [
Occipital bone morphology is valuable for assessing relatedness in adolescents starting around the age of 13 years old. The sphenoid bone, on the other hand, does not reflect genetic relatedness at any ontogenetic stage; however, despite this, the morphology is distinct enough among populations to allow unidentified specimens to be classified by population with rather high accuracy. This is likely because the sphenoid body of basicranium reaches adult size and shape more rapidly than other portions, presumably because vital cranial nerves (II–VI) run through the cranial base in the sphenoid region [
Population relatedness can be inferred using the basicranium, temporal, and occipital bone of subadults, especially those of 13–18 years of age. The findings of this study have implications for future studies of archaeological specimens for which genetic material is not well-preserved. Given that the morphology of the temporal bone and basicranium reflect genetic distances in young subadults, the morphology of these cranial subsets can be used to sort human populations with a reasonable degree of precision (25% and 41% mean classification, resp.). This could be useful for child or adolescent cranial specimens of unknown affinity found at archaeological sites.
If future studies examine other hominoid (ape) species in a similar manner, the findings combined with hose of the present study would have implications for the development of hominoid brain size, posture, and evolution. First, the angle of the midline cranial base is hypothesized to correlate with the volume of the brain relative to its basicranial length [
In this study, the basicranium, occipital, and temporal regions were found to reflect genetic distances among populations in childhood and adolescence. The sphenoid bone, however, is not a good indicator of genetic distances in subadults, but its morphology may still serve as a reasonably accurate means of classifying individuals by population affinity. Unsurprisingly, the Native American populations, and especially the Utah sample, were commonly classified as another Native American population, as would be predicted if basicranial shape reflects genetic relatedness. These findings reveal valuable information on the population differences in basicranial morphology at various ontogenetic stages.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Funding for this study was provided by the Biomedical Sciences Department at Midwestern University (to Deepal H. Dalal) and by Midwestern University faculty start-up funds (Heather F. Smith). The authors wish to thank the museum curators, Dr. David Hunt (NMNH) and Gisselle Garcia (AMNH), for facilitating access to the collections in their care. The authors also thank Dr. Mark N. Coleman and Dr. Justin A. Georgi for providing helpful comments and guidance on this study and Brent Adrian for providing valuable insight into an earlier draft of this paper.