CAPÍTULO 2: MARCO TEÓRICO
2.2 DESCRIPCIÓN GENERAL DEL PROYECTO “SERVICIO TÉCNICO
The hyoid bone measurements are grouped with a large number o f cranial measurements in the principal component analysis. Differences in the lengths and breadths o f the mandible, cranial base and face relate to the size and shape o f the hyoid bone in this analysis. This is not surprising. The hyoid bone is attached by muscles and ligaments to all these cranial regions. Given the high inter-relationship that exists between the different areas o f the cranium any change in one region is likely to have consequences for the shape and morphology o f other regions (Schultz, 1942; Gould, 1977; Dean, 1988; Ross & Ravosa, 1993; Spoor, 1997). During human evolution, there has been a reduction in facial prognathism and alterations in the length and width o f the cranial base. These changes are related to bipedal locomotion, brain expansion and dietary changes.
Bipedal locomotion is associated with an altered position and direction o f the foramen magnum (Bolk, 1909; Schultz, 1942; Dubrul, 1977; Dean & Wood, 1981). In humans, the foramen magnum is positioned in an anterior and horizontal position to allow the head to sit upright on the spine (Bolk, 1910; Moore et al. 1973; Adams & Moore,
1975). Changes in head position in both humans (Oh et a l 1996; Peng & Cook, 1999) and in rats (Riesenfeld, 1969; Reidenburg & Laitman, 1991; Dubrul & Laskin, 1961)
have been shown to lower the position o f the hyoid in the pharynx. Aiello (1996) has argued that the repositioning o f the foramen magnum to allow the spinal cord to enter from below rather than behind the skull constricts the space available for the larynx between the mouth and the spinal cord. Support for this hypothesis comes from Lieberman and McCarthy (1999) who have shown that the adult hyoid could not be accommodated in an adolescent pharynx, the dimensions are too small.
No studies have been conducted that show that an alteration in head posture changes the shape or size o f the hyoid bone. However, there is an association between the length o f the hyoid bone and the anterio-posterior dimensions o f the pharynx. Individuals with pharyngés that are deeper anterio-posteriorly have longer hyoid bones (Trenmouth & Timms, 1999). Chapter 1 o f this thesis provides evidence that smaller individuals have smaller vocal tracts. There is also an association between age, sex and hyoid bone size.
The length o f the cranial base has been associated with the increase in brain size in humans (Gould, 1977; Dean, 1988; Ross & Ravosa, 1993; Camperio Ciani &
Chiavelli, 1992; Strait, 1999). A variety o f researchers have shown that the length and flexion o f the human cranial base is related to the more globular crania o f modem humans (Gould, 1977; Dean, 1988; Ross & Ravosa, 1993; Camperio Ciani & Chiavelli, 1992; Strait, 1999). Alterations in the position o f muscles attached to the hyoid will have consequences in the type o f strain placed upon the bone. Mechanical strain has been shown to re-model other bones (Malina et al. 1986). Furthermore, Saber and Hofinann (1985) have shown that the shape o f the hyoid in ruminants is related to the shape o f the crania.
The differences between humans and African apes in the degree o f facial prognathism exhibited might be associated with the differences in diet in these species. During the course o f human evolution a higher quality diet and the introduction o f intense food preparation has meant that the food eaten by humans requires less preparation in the mouth. Consequently, the jaws and faces o f humans have reduced in size. Hilloowala et a l (1998) have proposed that the differences in the thickness o f the hyoid body are
related to the type o f diet eaten by the different primate species. They propose that species with a higher quality diet will have a hyoid bone that is flatter anterio-posteriorly. Species with the flattest hyoid bodies (anterior posterior thickness) were found among the
insectivorous species. This can certainly be seen in humans and the great apes. This is the only study that has ever looked at the consequence o f dietary changes in hyoid bone morphology.
Co n c l u s i o n s
Human male hyoids are larger than female hyoids and this is probably related to differential growth at puberty. Final fusion o f the hyoid bone in humans in this sample does not occurred until at least the age o f forty. In contrast to earlier studies, greater horn fusion was found in the chimpanzee. Final fusion o f the greater horns in the chimpanzee occurs very late in life. This fusion pattern was not found in the gorilla.
Hyoid bone fusion may provide an additional technique for determining age in adult humans. The late onset o f fusion o f the greater horns to the body may narrow the possible range o f ages that are assigned to adult archaeological or forensic specimens. A large sample o f known aged individuals would be needed to determine the rate and timing o f the onset o f final fusion.
There appear to be different factors influencing the growth o f the hyoid in humans and the African apes. This is particularly true for the growth o f the hyoid body. There is little difference between the immature hyoid bone in humans, chimpanzees and gorillas. The differences between the adult bones need to be further explored in the context o f an ontogenetic study to see at what age or stage o f development the differences first appear.
The use o f new techniques such as morphometric analysis would allow the three dimensional differences between the species and the changes throughout life in a single species to be investigated.