Craniofacial orientation and parietal bone morphology in adult modern humans

Abstract

In adult humans, the orbits vary mostly in their orientation in relation to the frontal bone profile, while the orientation of the cranial base and face are associated with the anteroposterior dimensions of the parietal bone. Here we investigate the effect of parietal bone length on the orientation of the orbits, addressing craniofacial integration and head orientation. We applied shape analysis to a sample of computed tomography scans from 30 adult modern humans, capturing the outlines of the parietal and frontal bones, the orbits, and the lateral and midline cranial base, to investigate shape variation, covariation, and modularity. Results show that the orientation of the orbits varies in accordance with the anterior cranial base, and in association with changes in parietal bone longitudinal extension. Flatter, elongated parietal bones are associated with downwardly oriented orbits and cranial bases. Modularity analysis points to a significant integration among the orbits, anterior cranial base, and the frontal profile. While the orbits are morphologically integrated with the adjacent structures in terms of shape, the association with parietal bone size depends on the spatial relationship between the two blocks. Complementary changes in orbit and parietal bone might play a role in accommodating craniofacial variability and may contribute to maintain the functional axis of the head. To better understand how skull morphology and head posture relate, future studies should account for the spatial relationship between the head and the neck.

We investigate the structural relationship among the parietal bone, the orbits, and the anterior cranial base in adult modern humans. The image shows the main pattern of shape variation. We show that the orientation of the orbits changes in coordination with that of the anterior cranial base and in association with variation in parietal bone length. Morphological integration between these skull components depends on their relative positions, suggesting a role in craniofacial organization.

1. INTRODUCTION

Modern humans evolved a specialized skull architecture, with a globular neurocranium, a flexed cranial base, and a reduced and retracted face (Lieberman et al., 2000). All these changes are integrated to allow the skull to maintain structural and functional balance while accommodating an increasing brain size (Lieberman, 2011; Richtsmeier & Flaherty, 2013). Accordingly, the adult phenotype results from multiple ontogenetic and evolutionary interactions between adjacent soft and hard tissues (Bruner, 2015; Moss & Young, 1960; Richtsmeier et al., 2006).

Most of the cranial vault is composed of the parietal bones, which size and curvature has increased with increasing brain size in Homo sapiens (Bruner et al., 2011). Both paleoneurology and comparative neuroanatomy research point to the expansion of the parietal region in modern humans (Bruner, 2004; Bruner et al., 2003, 2011, 2017a; Bzdok et al., 2016; Catani et al., 2017; Grefkes & Fink, 2005; Pereira‐Pedro et al., 2020). The bulging of the parietal region occurs early in ontogeny, and contributes, together with basicranial flexion, to the unique globularization of the modern human neurocranium (Neubauer et al., 2009). It has been suggested that such dilation of the parietal region in modern humans may influence the functional axis of the head, inducing a global rotation of the brain in order to maintain a proper horizontal alignment of the skull (Bruner, 2003). In fact, within adult modern humans, longer parietal bones are associated with ventral rotation of the anterior cranial base, and consequently of the face (Bruner et al., 2017b).

Across primates, the horizontal axis of the orbits is roughly perpendicular to the posterior maxillary plane, and thus the anterior cranial base and the upper and midface rotate as a block (McCarthy & Lieberman, 2001; Ross & Ravosa, 1993). Hence, ventral rotation of the base and face affects not only the orientation of the orbits, but also the morphological interactions between the facial elements and the brain. These interactions between the face and the brain occur through the lateral cranial base, in particular the anterior and middle cranial fossae (Bastir & Rosas, 2006; Bastir et al., 2006; Bruner & Ripani, 2008; Lieberman et al., 2000). In modern humans, these two cranial fossae are in closer proximity with the orbits, as a result of facial reduction and encephalization (Lieberman et al., 2000, 2002). This likely results in constraints to the expansion of the brain, which became wider in modern humans compared to early Homo (Bastir et al., 2008; Beaudet & Bruner, 2017; Bruner et al., 2013; Bruner & Holloway, 2010). In turn, the enlargement of the brain might also impose constraints to the morphology of the orbit (Bruner et al., 2014; Masters, 2012). In fact, the floor of the anterior cranial base also constitutes the roof of the orbits, thus accommodating both the eyes and the frontal lobes (Moss & Young, 1960). The middle cranial fossa is longer and more forwardly projected in modern humans, with its poles extending anteriorly beyond the optic chiasm (Bastir et al., 2008, 2011). This forward projection might influence the position and orientation of the posterior maxillary plane, also affecting the orbits (Bastir et al., 2008). Compared to chimpanzees and extinct humans, modern humans have indeed the temporal poles closer to the orbits, which are smaller and anteroposteriorly shorter (Pereira‐Pedro et al., 2017). At the same time, within adult modern humans, an important source of individual variation regards, precisely, the vertical orientation of the orbits relative to the frontal profile (Pereira‐Pedro et al., 2017).

Given the structural interdependence of the anterior cranial base and the underlying orbits, the pattens of cranial base and orbit orientation observed in previous works (Bruner et al., 2017b; Pereira‐Pedro et al., 2017) likely reflect the same process. The present study aims to investigate this relationship, and to further explore the association between the parietal bone dimensions and the orientation of the orbits, in order to discuss craniofacial integration within the context of functional head axis and gaze orientation.

2. MATERIALS AND METHODS

The sample employed in this study comprises 30 computed tomography (CT) scans of adult modern humans from the NESPOS database (Neanderthal Museum, Mettmann, Germany), including both sexes (15 females, 15 males), and different geographical origins (Europe, N = 11; Africa, N = 8; Asia and Inuit, N = 6; Central and South America, N = 5; see subject codes in the supporting information). Voxel size ranges from 0.21 to 0.70 mm. Scout views of each subject were produced by lateral projection of the CT grayscale values, and the resulting craniofacial morphology was analyzed through geometric morphometrics (Bookstein, 1991; Zelditch et al., 2004). We combined the landmark sets from previous studies (Bruner et al., 2017b; Pereira‐Pedro et al., 2017) into a single comprehensive dataset (Figure 1a). The midline cranial base is composed of the posteriormost point of the anterior cranial fossa (AF), the sella (SE), and the basion (BA); the frontal bone curvature is represented by three equally distant semi‐landmarks between the foramen caecum (FC) and the endobregma (BR), and the parietal bone by three semi‐landmarks between endobregma and endolambda (LA); the orbits are defined anteriorly by the superior (SO) and inferior (IO) orbital margins, and posteriorly by the aperture of the optic nerve canal (PO), with an additional semilandmark midway between the superior and posterior edges. Finally, the lateral cranial base is represented by the anteriormost point of the temporal poles (TP). Right and left sides were averaged, providing mean coordinates for the temporal tips and orbits position.

FIGURE 1.

Landmark configuration (a) and principal components analysis (b–e). The scree plot shows that the first three PCs are above the threshold for random variation (red, dotted line), and the PCA plot shows the distribution of the specimens along the first two PCs (b). Variation in shape is shown toward the negative (left) and positive (right) extremes of each principal component (c–e). PC1 (46%) describes variation in vault extension and base and orbit orientation (c); PC2 (15.4%) depicts the dolicocephalic‐brachicephalic patterns (d); PC3 (10.6%) illustrates variation in parietal vs. frontal profile associated with base flexion and orbit orientation (e). LA: endolambda; BR: endobregma; FC: foramen caecum; AF: anterior cranial fossa, posterior point; SE: sella, center; BA: basion, anteriormost point; TP: temporal poles; PO: posterior orbital limit, on the aperture of the optic nerve canal; SO: superior orbital margin; IO: inferior orbital margin [Colour figure can be viewed at wileyonlinelibrary.com]

First, we obtained the main patterns of shape variation through principal component analysis (PCA), and correlated these shape vectors with centroid size and parietal bone length. Second, we investigated the covariation between distinct groups of landmarks through partial least squares (PLS). This method finds the maximal amount of covariation between two blocks of variables (Klingenberg et al., 2003; Rohlf & Corti, 2000). We considered two different partitions of landmarks: (A) the cranial vault (parietal and frontal bones) versus the base and orbits, and (B) the parietal bone versus the remaining landmarks (frontal bone, orbits, and base). The covariation between each pair of blocks was tested after both joint and independent Procrustes registration (Klingenberg, 2009). The first approach correlates the shape variation of two blocks within the overall normalized space, and therefore it considers information on both their shape (geometry) and their reciprocal position (including distance and orientation). The second approach computes shape variation in the two blocks separately, and thus it reveals local changes in shape independently of the reciprocal position of the anatomical elements. Finally, we tested the modularity of the overall configuration (Klingenberg, 2009). In particular, we considered whether the fronto‐parietal profile is relatively independent from the facial block. Geometric morphometrics were performed with MorphoJ 1.06d (Klingenberg, 2011) and additional statistics were computed with PAST 4.0 (Hammer et al., 2001).

3. RESULTS

3.1. Shape variation

The morphospace is characterized by three significant principal components (PCs), explaining 72% of the variation in shape, with the subsequent PCs below the threshold of random variation (Figure 1b). Figure 1c‐e shows the main patterns of shape variation. PC1 (46%) is associated with the extension of the cranial base and the fronto‐parietal vault. Individuals distributed toward the lower values of PC1 display anteroposteriorly shorter vaults, vertically extended and dorsally inclined cranial base, dorsally oriented orbits, and inferiorly positioned temporal poles. Individuals toward the higher values of PC1 show the opposite morphology, with a more extended parietal outline, vertically shorter and ventrally inclined cranial base, ventrally oriented orbits, and superiorly positioned temporal poles, closer to the posterior limit of the orbit (optic nerve canal). PC2 (15.4%) describes variation in longitudinal and vertical proportions, from taller, longitudinally shorter, rounder crania to vertically shorter, longitudinally elongated, flattened crania. Rounder, taller vaults are associated with taller, slightly flexed midline bases, more inferiorly positioned temporal poles, and longitudinally elongated but vertically shorter orbits. In contrast, elongated, flattened vaults are associated with shallower, slightly extended bases, more superiorly positioned temporal poles, and taller but longitudinally shorter orbits. PC3 (10.6%) describes variation in the curvatures of the frontal versus parietal bones associated with the angle of the cranial base and the orbits. That is, more bulging frontal bones are associated with flatter parietal bones, shorter and flexed bases, ventrally oriented orbits, and forwardly positioned temporal poles.

Centroid size explains about 7% of the whole variation in shape, and about 9% of the shape variation described by PC1, though both correlations do not reach statistical significance. In contrast, parietal bone measurements account for a larger and significant portion of shape variation. Parietal chord explains about 28% of the whole shape variation and 59% of variation along PC1, and the parietal arc accounts for 31% of the whole shape variation and 65% of PC1 shape changes (Table 1). PC2 and PC3 are unrelated to centroid size or parietal bone measurements.

TABLE 1.

Correlation between principal components with size measurements

	Centroid size		Parietal cord		Parietal arc
	% Predicted	P‐value	% Predicted	P‐value	% Predicted	P‐value
Whole shape	7.2	0.07	27.9	<0.001	30.8	<0.001
PC1	10.0	0.09	58.6	<0.001	64.7	<0.001
PC2	3.5	0.32	0.8	0.64	0.1	0.88
PC3	6.9	0.17	0.9	0.63	4.1	0.27

3.2. Integration and modularity

The PLS analyses based on the joint Procrustes fit of the two landmark blocks indicate a significant covariation between the cranial vault and the base and orbits (A: RV = 0.62, p < 0.001) and between the parietal outline and the non‐parietal landmarks (B: RV = 0.65, p < 0.001). For the two partitions, PLS1 accounted for the largest amount of covariation (A: 88%; B: 89%; Figure S1), describing shape changes similar to those shown by PC1. In contrast, PLS analyses suggests no covariation between the landmark sets of the two subdivisions when these undergo separate Procrustes fits (A: RV = 0.14, p = 0.403; B: RV = 0.08, p = 0.787; Figure S2). These results suggest that the two blocks have a reciprocal influence in terms of position and orientation (joint registration) but no reciprocal influence in terms of local shape (separate registrations).

According to the modularity analysis, the RV = 0.62 calculated for partition B (Figure 2a) is close to the lower extreme of the distribution of possible RV coefficients, with only 30 out of 502 alternative partitions (6%) resulting in a lower RV value, which is consistent with the cranial vault and the cranial base plus orbits belonging to partially independent modules. The partition displaying the lowest covariation (RV = 0.56) groups the anterior portion of the frontal profile with the anterior cranial base and orbits as a module separated from the rest of the vault and the posterior base landmarks, namely separating the upper face and the posterior braincase (Figure 2b).

FIGURE 2.

Modularity analysis: the hypothesized modules (a) were the cranial base and orbits (blue, light circles) and the parietal and frontal outline (red, dark circles); the minimum between‐blocks covariation (b) was obtained for the partition separating the upper face (blue) and posterior braincase (red). LA: endolambda; BR: endobregma; FC: foramen caecum; AF: anterior cranial fossa, posterior point; SE: sella, center; BA: basion, anteriormost point; TP: temporal poles; PO: posterior orbital limit, on the aperture of the optic nerve canal; SO: superior orbital margin; IO: inferior orbital margin [Colour figure can be viewed at wileyonlinelibrary.com]

4. DISCUSSION

4.1. General patterns of cranial variation

Previous surveys have showed that the size of the parietal bone, in adult humans, is associated with the orientation of the cranial base (Bruner et al., 2017b), and that the orientation of the orbits is a major source of individual variation when dealing with the spatial relationships between face and braincase (Pereira‐Pedro et al., 2017). In this study, we integrate the samples from the two previous analyses into a single dataset, in order to consider whether and to what extent these two morphological patterns are associated, within the general framework of cranial integration.

Our results show that the orientation of the cranial base and the orbits varies synchronously, and in association with variation in the extension of the vault. More specifically, individuals with relative ventral orientation of the cranial bases and orbits, tend to have flatter, anteroposteriorly elongated vaults. These changes correlate with parietal bone dimensions, such that individuals with anteroposteriorly larger parietal bones tend to have vertically shorter cranial bases and ventrally oriented cranial bases and orbits. An association between ventrally oriented cranial bases and vertically shorter faces in modern humans has been previously reported (Cole, 1988; Kuroe et al., 2004). In this context, and since the relative position of the temporal poles varies in coordination with the cranial base, it seems that the spatial relationship between the temporal poles and the orbits might be influenced by the vertical proportions of the basicranium. That is, individuals with shorter cranial bases, and flatter cranial vaults, tend to have their temporal poles closer to the optic nerve canal. It is worth noting that the temporal poles are located on the anteriormost point of the greater wings of the sphenoid bone, which also contributes to the bony orbit and to the midline base. The sphenoid bone is, in fact, central in the integration between the craniofacial modules (Esteve‐Altava et al., 2013), and its proportions are associated with facial projection and the orientation of the posterior maxillary plane (Bastir & Rosas, 2016). Hence, variation in the morphology or relative position of the sphenoid bone likely influences the spatial relationship between the neurocranium and the facial skeleton, and consequently between the brain and the eyes. It is thus interesting that the spatial relationship between brain and eyes in modern humans depends on the position of the eyeball relative to the temporal poles (Pereira‐Pedro et al., 2017), which are precisely separated by the sphenoid bone.

Subsequent variation observed in our results seems to account for the general brachycephalic‐ dolichocephalic proportions. The former refers to rounded neurocrania, with flexed bases and broad, short faces, while the latter to elongated, narrow braincases, with weakly flexed bases and narrow, tall faces (Bastir, 2008). In our sample, rounder, taller crania are indeed associated with more flexed cranial bases, but also with a vertical extension of the base and anteroposteriorly elongated orbits, which seem to suggest taller, anteroposteriorly elongated faces. Nonetheless, these shape variations are unrelated to size (i.e., parietal bone length and skull centroid size), and the vertical proportions of the face usually correlate with those of the nasal cavity (Bastir & Rosas, 2013, 2016). The typical brachycephalic‐ dolichocephalic classification is traditionally used to classify cranial diversity, but the differences could be generated by distinct and independent factors associated with spatial relationships among different cranial elements (Zollikofer & Ponce de León, 2002). In fact, it seems to provide no direct information on the genetic basis of the morphological integration of the skull (Martínez‐Abadías et al., 2009).

Finally, a reduced proportion of the shape variation describes the flatter versus rounder profile of the frontal and parietal bones, with the flattening of one associated with the bulging of the other. This pattern only involves relative proportions of the skull, as it is unrelated to centroid size or parietal bone length. A flatter parietal profile has been shown to covary with bulging and posteriorly projected occipital profile (Gunz & Harvati, 2007). However, further studies are needed to determine whether the flattening of the parietal bone associates with bulging of the frontal and occipital bones alike.

4.2. Integration and modularity

Covariation between the parietal bone outline and the remaining craniofacial structures, or between the cranial vault and the cranial base and orbits, is only observed when the relative position of the two blocks is accounted for, pointing to the important role of spatial and topological relationships (Klingenberg, 2009). Furthermore, there seems to be greater integration among the orbits, anterior cranial base, and the antero‐inferior portion of the frontal profile, on the one hand, and between the posterior vault and cranial base, on the other hand. An anterior‐to‐posterior partition has been previously hypothesized by using network models of the human skull and brain, when considering spatial contiguity and physical contact (Bruner, 2021; Bruner et al., 2019; Esteve‐Altava et al., 2013). In the skull, the anterior module includes the frontal bone and the face, and the posterior module includes the cranial vault and the cranial base (Esteve‐Altava et al., 2013). In the cortex, the frontal lobe forms an anterior topological module and the remaining cortex a posterior one (Bruner et al., 2019). Overall, these results evidence, once more, the role of local features in the anatomical integration of the whole human head. This anterior‐posterior partition seems to match differences in embryonic origins, as the frontal and facial portions of the head are neural crest‐derived while the posterior portions are mesoderm‐derived, with the interfaces between the two types of tissue in the juxtaposition between the frontal and parietal bones, at the coronal and sagittal sutures (Jiang et al., 2002; Lieberman, 2011; Morriss‐Kay & Wilkie, 2005).

4.3. Orbit orientation, head posture, and the parietal bone

During anthropoid evolution the orbits became relatively smaller, and more convergent and frontated, increasing the area of binocular visual field overlap (Heesy, 2004; Ross, 2000). The eyes evolved smaller corneas and pupillary apertures, enhancing visual acuity and resolution (Kirk, 2006; Ross, 2000). The primary orientation of the eyes consists in a straight gaze, and their position is held by the extraocular muscles, suspensory ligaments, and orbital fat (Wright, 2003). Across primates, the orientation of the orbits, computed as the angle between the orbital axis and the clivus, correlates with head and neck posture (Strait & Ross, 1999). Most primates seem to hold their heads with the orbits facing forward and slightly inferiorly, directing their gaze to an inferiorly placed substrate, suggesting the orbital plane might be related to locomotor behavior (Lieberman et al., 2000; Strait & Ross, 1999). In general, the cervical portion of the vertebral column displays a vertical orientation in mammals and birds, pointing to a common organizational principle for eye and head movements (Vidal et al., 1986). But humans and monkeys seem to have a limited range of motion of the atlanto‐occipital articulation, which might have constrained evolutionary adaptations to head posture (Graf et al., 1995a,b). Further modifications due to bipedal posture and locomotion in humans, such as a more vertical orientation of the neck, led to a reorientation of the orbits relative to the foramen magnum in order to maintain the forward orientation of the eyes and gaze (Lieberman, 2011). It has been hypothesized that several aspects of human head architecture, as well as neck and upper body morphology, might have evolved as adaptations for head stabilization during bipedal locomotion, and particularly endurance running (Bramble & Lieberman, 2004).

The abovementioned studies point to the orientation of the gaze and orbits as a critical factor that has influenced the evolution of head architecture and posture. In cephalometric studies, natural head position is usually determined based on gaze direction, that is, with the subjects looking into the image of their eyes in a mirror (Leitão & Nanda, 2000; Madsen et al., 2008; Moorrees & Kean, 1958). Cole (1988) measured natural head position as the inclination of the anterior cranial base (angle of nasion‐sella line with the true vertical), to investigate maxillary and mandiblular prognathism in extreme skeletal patterns, that is, morphologies diverging from the one considered normal. He observed that natural head position varied across subjects. Since the subjects were asked to look into their eyes in a mirror (Cole, 1988), such variation reflects differences in craniofacial morphology, rather than gaze direction. This means that individuals with opposite skeletal patterns will display a distinct natural position of the head to maintain a horizontal gaze, which involves a variation in the angle of the cranial base. The present survey shows that the variation in the orientation of the cranial base is indeed associated with changes in orbit orientation, and with variation in the longitudinal extension of the parietal bone. It can be hypothesized that these features are part of the same morphogenetic process, establishing a proper cranial balance.

The morphogenesis of the parietal bone mainly depends on the brain, which growth and development directly mold the vault of the skull (Moss & Young, 1960; Richtsmeier & Flaherty, 2013). However, the spatial correspondence between the boundaries of the parietal bone and of the parietal lobe varies, indicating that the position of the cranial sutures and of the cortical convolutions are influenced by multiple and partially independent factors (Bruner et al., 2015; Ribas et al., 2006). Still, there is a modest but significant correlation between the lengths of the parietal bone and lobe, within adult modern humans (Bruner et al., 2015). It has been hypothesized that the expansion of the parietal regions described for Homo sapiens might have influenced the orientation of the head, by tilting the fronto‐occipital axis downward and changing the orientation of the brain (Bruner, 2003, 2017; Figure 3). The association between the variation in orbit‐cranial base orientation and in parietal bone length described in the present study might reflect this global cranial adjustment, also operating at the intra‐specific level. It should be further investigated if the same intra‐specific pattern of covariation exists also in extant great apes and extinct Homo, and whether it is influenced by brain size or cortical (parietal) proportions. In addition, craniofacial morphology, natural head position, and gaze orientation ultimately depend on the structural relationship between the head and the body (Bastir, 2008). Hence, in order to better understand the mechanisms influencing skull architecture, postural information must also be taken into account, including the spatial relationship between the head and the neck, as well as the influence of the morphology of the upper body in this relationship.

FIGURE 3.

The morphology and spatial relationship of the skull and endocranial cavity in Homo sapiens are compared with extinct humans and apes, as to show the general effect of the longitudinal parietal dimensions on the head orientation. In chimpanzees (a), as in most nonhuman primates, the brain is generally oriented along a fronto‐occipital axis (thin arrow). In Neanderthals (b), the enlargement of the parietal regions does induce a minor rotation of the fronto‐occipital axis. This rotation is particularly stressed in modern humans (c) in which, because of the large parietal regions (thick arrow), the brain undergoes a pronounced rotation, to maintain the functional head axis [Colour figure can be viewed at wileyonlinelibrary.com]

4.4. Limitations and further considerations

This study comprises a two‐dimensional analysis on the sagittal view to investigate the effect of parietal length and curvature on the orientation of the head. The parietal bone displays noticeable changes and variability in its longitudinal extension at both intra‐specific and inter‐specific levels, as well as on an evolutionary and phylogenetic scale (Bruner, 2018). Therefore, the variations on the sagittal plane are particularly useful to evidence the possible effect of the parietal extension/reduction on the anteroposterior head axis. Nonetheless, future analyses should be dedicated to evaluate whether a third dimension can add to this issue. Latero‐lateral integration has been observed in previous studies both across primates (Singh et al., 2012) and human species (Bastir & Rosas, 2016), and covariation between the widths of the cranial base, neurocranium, and face have been shown to dominate the patterns of integration within the skull, in mice (Hallgrimsson et al., 2007) and humans (Martínez‐Abadías et al., 2009). Another aspect that is difficult to assess in two dimensions is the anteroposterior inclination of the temporal bone, which has been shown to vary with the dimensions and flexion of the cranial base, the position of the mandible and maxilla, and with posterior midfacial height (Costa et al., 2012). The temporal bones articulate with the parietal bones, the sphenoid bone and with the mandible, and have been suggested to play an important role in craniofacial dynamics (Costa et al., 2012; Sato, 2002). Understanding how the adjacent cranial bones interact in order to maintain functional head posture would certainly benefit from a three‐dimensional study of the skull. Finally, the influence of soft tissues (brain, eyes, muscles, and connectives) in the generation of a balanced craniofacial integration system should be also considered in future and more comprehensive approaches.

5. CONCLUSIONS

By combining data from previous studies, we were able to provide further insights into the structural organization of the human skull regarding the relationship between the parietal bone morphology and the anterior cranial base and orbits. The current analysis shows a coordinated orientation of the anterior cranial base and orbits in adult modern humans, reflecting the tight structural and functional integration between these regions due to spatial contiguity. The results also evidence a relationship between orbit orientation and parietal bone length. Morphological integration between these skull components are apparently not a matter of shape, but it depends on their relative positions and orientation, suggesting they play a key role in craniofacial spatial organization. That is, the seemingly reciprocal variation in orbit‐base orientation and parietal bone size might contribute to accommodating brain and craniofacial variability while maintaining functional gaze direction and head balance, at least within modern humans. In order to shed more light on this specific matter, future studies should apply a more comprehensive, three‐dimensional approach, including information on the lateral dimensions of the skull and on the relative position and morphology of the neck and upper body, as well as considering the local influence of soft tissues.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interest.

AUTHOR CONTRIBUTIONS

ASPP: study design, sampling and data processing, data analysis and interpretation, manuscript preparation. EB: study design, data analysis and interpretation, manuscript preparation.

Supporting information

Fig S1‐S2

Pereira‐Pedro, A.S. & Bruner, E. (2022) Craniofacial orientation and parietal bone morphology in adult modern humans. Journal of Anatomy, 240, 330–338. 10.1111/joa.13543

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available online database at www.nespos.org.