Population affinity can also be studied using craniometric techniques. Craniometric studies use the basic principal that there are certain aspects of cranial morphology which are genetically dictated and thus differ between populations. Initial studies of populations using craniometric techniques were conducted with a Eurocentric slant, and this resulted in interpretations of cranial shape differences being used to perpetuate myths about the supremacy of European man (e.g. Hooton, 1931). Modern craniometric studies have, fortunately, moved away from this application and towards using craniofacial diversity to establish population histories. In order for this to occur many studies have been conducted to evaluate the how heritable cranial shape is, and therefore how representative of population affinity it may be.
Establishing which features of cranial shape are most related to population is complex as the skull is an integrated structure. Most regions are linked to others, and the shape of them is limited by the forces acting on, or functional constraints on the structure (c.f. Cheverud, 1988; Lieberman et al., 2004). Post-natal development and response to stress will change the shape of the cranium, no matter what the genetic background of the individual.
The cranial base is considered by most to be the oldest structure in the human skull, and therefore more strongly under genetic control (Martinez-Abadias et al., 2009). Conversely
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the shape of the face, which develops post-natally and responds to environmental and masticatory stresses, is likely to be most sensitive to non-genetic factors (Strand Viðarsdóttir et al., 2002). This response of certain cranial regions results to the environment results in a fundamental difference between craniometric and geneticdata. Craniometric differences between populations are perceived to be quite substantial (e.g. Hennessy &
Stringer, 2002), contrasting with genetic information obtained using DNA analysis, which shows the majority of genetic variation occurs at a local level (Relethford, 2002).
There are a number of studies highlighting that cranial shape is the result of both genetic pre-determination and response to stress. As well as Strand Viðarsdóttir et al.‟s (2002) study which showed the plasticity of facial growth in humans, Roseman & Weaver (2004) have also used craniometric data in combination with geographic and climatic data to show that measurements on the face (particularly the nasal and zygomatic heights) are closely related to climatic variation. Harvati and Weaver (2006) found that the shape of the temporal bone and basicranium could be most successfully linked to neutral genetic data, whilst facial shape was only weakly linked to genetic groups and more strongly affected by climatic variables. Hubbe et al. (2009) have added to this work through their analysis of over 7000 modern human crania. Their results highlight the fact that morphological variation in the vault can be explainedby geographic factors, while the face is more linked to climate. They also highlight links between certain climatic variables and craniometric variation e.g. level of precipitation correlates strongly with changes to nasal breadth, cranial breadth appears to be linked to humidity.
Studies of samples of known pedigree, however, have shown strong degrees of heritability in both cranial vault and facial measurements. A study of the Hallstadt collection, which
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represents a single closely related population, by Martinez-Abadias et al. (2009) used geometric morphometric techniques to examine which areas of the cranium are most heritable. The linear distances found to be most related to genetics were established using maximum likelihood methods (c.f. Koningsberg, 2000). They include the distance between the orbits, length of the nasal cavity, breadth of the skull, and length of the foramen magnum.Though this study focused on a single population of known pedigree, its results are relevant to craniometrics worldwide. They highlight that there is no significant difference in the heritability of dimensions between facial, basicranial and vault areas of the cranium, instead all of these regions may be useful in establishing population affinities.
Figure 2.5: Showing the anterior, inferior and lateral aspects of the skull with cranial measurements of highest heritability indicated with lines (red – facial features, blue – basicranial features, green – neurocranial features, black – interregional features). Taken
from Martinez – Abadias et al (2009).
Initial research into craniofacial morphology also suggested that regions which respond to the mechanical stress of mastication are unsuitable for use in population affinity studies
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(Herring, 1993; Lieberman, 1995). This has been addressed by a geometric morphometric study by von Cramon-Taubadel (2009), which showed that though areas associated with masticatory muscles are indeed more variable in form, their measurements are still moderately to strongly heritable.
It is important to recognise the contribution of geometric morphometric analysis to the field of craniometrics. Looking at single measurements across the cranium cannot fully answer the question of how much cranial shape is genetically inherited and how much is associated with plasticity in response to the environment, as it does not assess individual regions as separate entities (instead they are linked through measurement). Traditional craniometric techniques also cannot differentiate between the relative contribution of shape and size (Hubbe et al., 2009). Many of the studies cited here use geometric morphometric techniques to resolve this, which allow the consideration of whole shape and portioning of shape and size data (e.g. Strand Viðarsdóttir et al., 2002; Harvati & Weaver, 2006; Smith, 2009; Martinez – Abadias et al., 2009).These techniques are also used in this study, and are covered in detail in section 11.