Support for the Spandrel Hypothesis in Artificially Modified Skulls

Intentional cranial modification, alike other forms of body modification, has been practiced by many cultures around the globe. Anthropologists have studied the reasons that support such tradition, the methods used for it, and the changes it produces on the cranium among different groups. However, the biological mechanisms that allow for the intentional modification of the skull to take place have not been extensively considered. Using a functional perspective and considering signaling pathways between tissues, four fronto-occipitally modified skulls are analyzed. The change in shape of the cranial vault and the bulging of the sagittal suture caused by the modification are correlated with the spatial redirection of brain growth. The lack of frontal sinuses seen in all individuals is associated with the technique used for the modification. Finally, the potential of a functional approach in the study of human populations and evolution is emphasized. There has been a lot of debate surrounding the paranasal sinuses of vertebrates. In the center of this debate is their function or evolutionary advantages. Two theoretical avenues have approached this question, one considering possible physiological roles for these cavities and the other structural roles. Researchers that have taken the former approach have related the sinuses to the development of the olfactory complex, to the resonance of emitted sounds, to the conditioning of inhaled air, and other factors (e.g. Braune and Clasen, 1877; Leakey and Walker, 1997; and Gannon et al., 1997). Theories that have related the sinuses to structural functions have suggested that they are linked to the lightening of the skull, to an assistance in facial growth, to the dispersion of masticatory forces, and other structural necessities (e.g. Davis et al., 1996; Moor and Persaud, 1993; and Preuschoft et al., 2002). In recent years, however, with the advancement of non-intrusive imaging techniques (specifically three dimensional computer tomography), a new wave of studies from different disciplines has approached the paranasal sinuses. The new observations have led to the resurfacing of an often overlooked hypothesis that does not assume a specific function for these structures. This hypothesis proposes the paranasal sinuses as biological spandrels, phenotypic characteristics selected as a product of evolutionary forces acting upon a separate trait, an idea introduced by Gould and Lewontin (1979). Key to this idea is the allometric development between different cranial components, or functional matrices. The spandrel hypothesis proposes that cranial bones, which are part of different functional matrices, must accommodate this allometric growth, and in this manner allow for pneumatization to occur. This dynamic can be observed well on the frontal bone, since its inner table is part of the cerebral compartment while its outer table responds to the development of the face (Moss and Young, 1960). In the case of humans both tables of the frontal bone advance together until approximately the age of seven. This advancement is directed by the accelerated expansion of the frontal lobe in relation to the nasomaxillary complex. After this period the expansion of the frontal lobe is arrested, however there is still significant facial growth. As a consequence, the outer table of the frontal bone begins to drift forward significantly faster than the inner table (Shapiro and Schorr, 1980). This separation results on a spatial gap in the vertical portion of this bone, which is initially filled with trabecular bone. Just as is the case with long bones, there is here a transition from red to yellow marrow. However, this is then followed