Examining cranial robusticity

Palaeoanthropologist Darren Curnoe (2009) gives the following biological definition of the term ‘robust’:

…a descriptive anatomical term referring to individuals, complexes, organs, structures or traits which are heavily built, rugged, well defined or corpulent.

Bones tend to more robust where muscles, tendons or ligaments insert into the periosteum. When these insertion sites are subjected to stress, blood flow increases. This in turn stimulates the production of osteoblasts, which lay down extra bone. With respect to the skull the term robust is generally used to refer to so-called superstructures, such as the supraorbital ridges, occipital crests or zygomaxillary tuberosities. Anthropologists often classify robusticity based on the relative expression of a particular trait, or indeed its absence. Given that robusticity is related to physical stress, traits tend to be more pronounced in males and in certain populations (e.g. Aboriginal Australians and Fuegians).

The retention of robust features in certain populations, particularly Aboriginal Australians, has been used to support the multiregional hypothesis of human origins (e.g. Wolpoff et al. 2001; Frayer et al., 1993). On the other hand, proponents of a replacement model see robust traits (e.g. in Australian Aboriginal populations) as retained plesiomorphies and argue that these traits cannot be used to show continuity (Lieberman 2000). In response, many multiregionalists have revised their position to suggest that the reduction of the browridge in later Neandertals, such as St Césaire and Vindija, represents a synapomorphy between Neandertals and modern humans, likely due to interbreeding. The underlying assumption here is that these robust traits have a strong genetic component. Furthermore, there is a notable decrease in cranial robusticity from the early Upper Palaeolithic to late Upper Palaeolithic. It has been suggested that this may reflect changes in diet. Transition from hunter-gather to agricultural lifestyle is associated with a reduction in cranial robusticity, although correlation does not necessarily prove causation. However, not all hunter-gather groups are universally more robust than argriculturalists, which might suggest some other factors at play.

A recent in press paper by Baab et al. sets out to examine the possible mechanisms behind robust cranial characters. The null hypothesis in their study is that neutral evolutionary processes (e.g. genetic drift) were responsible for the pattern of cranial robusticity in modern humans – the rejection of which would suggest selection acting on these traits. To test the null hypothesis of neutral evolution of cranial robusticity Mahalonobis D2 distances for robust characters were compared to Ddm distances derived from microsatellite data. Microsatellites are useful in reconstructing evolutionary relationships due to their unusually high mutation rates, which result in largely selectively neutral polymorphisms.

Of the variables examined, only cranial shape was significantly correlated with robusticity, while cranial size, climate and neutral genetic distances were not. This is at odds with an earlier study by Mirazón Lahr and Wright (1996) (1996) who found the strongest correlation between cranial robusticity and cranial size. This finding may be due to use of geometric morphometrics by Baab and colleagues, which is better at separating size and shape compared to the linear morphometrics used by Mirazón Lahr and Wright (1996).

Cranial robusticity was not correlated with neutral genetic distances, suggesting that neutral evolutionary processes (e.g. genetic drift) were not responsible for the pattern of cranial robusticity in the populations studied. As noted by the authors, this finding could also be explained by a non-perfect match of populations among some of the cranial and molecular samples. In studies such as this one, it is often difficult to find an exact match between the populations from which we derive our cranial and molecular data. In such cases, we are left with the choice of eliminating samples or using another genetically similar population. The authors choose the latter but neither option is ideal and both have their own disadvantages. Unfortunately, the reason for including Upper Palaeolithic and Neolithic samples in this study is never fully explained and the assumption that modern genetic populations are appropriate proxies for such populations is never justified. Setting this aside, the findings of this study caution the use of robust traits in constructing phylogenetic relationships in modern humans.

The strongest correlations were found between cranial robusticity and either cranial or masticatory shape. This lends support to the hypothesis that robusticity is in some part functionally determined. The study also found crania with more prognathic faces, longer skulls, expanded glabellar and occipital regions to be more robust. Mirazón Lahr and Wright (1996) noted a similar tendency of longer skulls to have superstructures, while further emphasising their tendency to be associated with narrow skulls and a large palatal region.

While most of the robust variables in this study were areas of muscle insertions, the supraorbital region has a distinct aetiology. While many have interpreted the supraorbital region as an area of stress reinforcement in the skull (the so-called beam model) which is strongly influenced by mastication (Endo 1966, 1970; Russell 1985), there is a strong evidence to suggest that this is not its primary purpose. Supraorbital development begins early in life, suggesting that the supraorbital ridge may be part of the overall craniofacial complex and is likely under genetic control. While the beam model is intuitive, it is unsupported by empirical data. Hylander and colleagues (Hylander et al. 1991a, 1991b, 1992; Hylander and Ravosa 1992) conducted in vivo strain gauge experiments in different primates to assess the amount of strain magnitudes generated during mastication. They found these levels to be low to induce bone deposition in all the species they studied, even when chewing hard food. Moreover, anthropoids do not show a correlation between the browridge and the moment arms of the masticatory muscles, as the beam model would predict (Ravosa 1991). These researchers adopt the model proposed by Moss and Young (1960), which views supraorbital development as the result of placement of the brain and eyes. They postulated that the reduction of the brow ridge in modern humans was related to the expansion of the frontal lobe in our species. In hominins with orbits positioned well in front of the frontal lobes, as in chimpanzees or the erectines, the space between the orbits and the brain case is bridged by a brow ridge. If the supraorbital region is under genetic control, as the research of Hylander and Ravosa suggests, it would be of interest to examine this region in isolation to assess if it correlates with neutral evolutionary processes, particularly in light of a recent paper by Von Cramen-Taubedal which found the shape of the frontal bone to be consistent with neutral genetic expectation.

References

Baab KL, SE Freidline, SL Wang, T Hanson. 2009. Relationship of cranial robusticity to cranial form, geography and climate in Homo sapiens (in press). Am. J. Phys. Anthropol.

Curnoe D. 2009. Possible causes and significance of cranial robusticity among Pleistocene-Early Holocene Australians. Journal of Archaeological Science (2009) vol. 36 (4): 980-990.

Endo B. 1966. Experimental studies on the mechanical significance of the form of the human facial skeleton. J Faculty Sci Univ Tokyo (Section V, Anthropol) 3:1–106.

Endo B. 1970. Analysis of stress around the orbit due to masseter and temporalis muscles respectively. J Anthropol Soc Nippon 78:251–266.

Frayer DW, MH Wolpoff, AG Thorne, FH Smith, GG Pope. Theories of modern human origins: the paleontological test. American Anthropologist (1993) vol. 95 (1): 14-50.

Hylander WL, Picq PG, Johnson KR. 1991a. Masticatory–stress hypotheses and the supraorbital region of primates. Am J Phys Anthropol 86:1–36.

Hylander WL, Picq PG, Johnson KR. 1991b. Function of the supraorbital region of primates. Arch Oral Biol 36:273– 281.

Hylander WL, Ravosa MJ. 1992. An analysis of the supraorbital region of primates: a morphometric and experimental approach. In: Smith P, Tchernov E,
editors. Structure, function and evolution of teeth. Tel Aviv: Freund Publishing. p 223–255.

Lieberman, DE. (2000) Ontogeny, homology, and phylogeny in the Hominid craniofacial skeleton: the problem of the browridge. In P. O’Higgins and M. Cohn (eds.) Development, Growth and Evolution: implications for the study of hominid skeletal evolution. London: Academic Press, pp. 85-122.

Moss ML, RW Young. 1960. A functional approach to craniology. Am. J. Phys. Anthropol. 18:281-292.

Mirazón Lahr M, RVS Wright. 1996. The question of robusticity and the relationship between cranial size and shape in Homo sapiens. Journal of Human Evolution.

Ravosa MJ. 1991. Interspecific perspective on mechanical and nonmechanical models of primate circumorbital morphology. Am J Phys Anthropol. 86(3):369-96.

Russell MD. 1985. The Supraorbital Torus:” A Most Remarkable Peculiarity”. Current Anthropology. vol. 26 (3) pp. 337

Wolpoff MH, J Hawks, DW Frayer, K Hunley. 2001. Modern Human Ancestry at the Peripheries: A Test of the Replacement Theory. Science. vol. 291 (5502):293-297.

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Above photo modified from original by Thomas Hawk under creative commons license.

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