Modelos de Distribución Exponencial

The Neanderthals are an extinct group of archaic Homo which occupied Europe and parts of the middle East for over 150,000 years, from ca 200,000 - 30,000 BP ( Stringer, 1995; Lahr and Foley, 1998). They had a unique facial morphology, unlike that of other archaic or indeed modern humans (Trinkaus, 1983, 1984, 1987; Rak; 1986, 1993; Stringer and Gamble, 1993; Klein, 1989). They are classically described as having a large and platycephalic cranial vault, large and often convex supraorbital tori, an unusually pronounced nasal and midfacial prognathism, a large nasal cavity, large orbits, a sagittally oriented zygomatico-maxillary complex, a large and robust maxilla with a large anterior dentition and a retromolar space at the end of the posterior mandibular tooth row. Of those features quite a number are autapomorphic i.e. unique to Neanderthal facial morphologies. These features are those relating to the midfacial prognathism, the size of the nasal cavity, the size of the anterior dentition and the shape of the zygomatics.

The functional implication of these characteristics has been heavily debated. The main ideas postulated can be divided into two broad groups: those who see them as being the result of adaptations to life in a cold climate, and those who view them as an adaptation to masticatory functions.

The cold adaptive hypothesis was first postulated by Coon (1962). He considered the major aspects of the Neanderthal midfacial morphology to be the result of an extension of the pyriform aperture away from the base of the

skull, thus elongating the nasal cavity, making it more efficient at warming and humidifying the air as it passed through. This projection of the nasal opening had consequently pulled the rest of the face along with it, stretching the areas lateral to the nasal cavity to form a more prognathic face. A recent study by Schwartz and Tattersall (1996), found that this external difference in nasal morphology was accompanied by a suite of internal nasal features, unlike those seen in any other hominid, lending further support to the theories of a unique specialisation of the nasal region in Neanderthals. The results of their study have however recently been questioned (Franciscus, 1999), due, amongst other things, to their reliance on fragmentary fossil material, and failure to explore intraspecific variation in internal nasal morphology.

Franciscus and Long (1991), have shown that in modern humans, narrow and high nasal cavities are associated with cold dry climates, similar to those in which Neanderthals lived, while low wide noses are associated with warm and humid environments. This theory would predict a high, but not wide nasal cavity in Neanderthals, if their adaptation to cold air intake followed similar patterns to those observed in modern humans.

The masticatory hypothesis was first postulated by Brace (1979). He considered the main difference in midfacial morphology between Neanderthals and anatomically modern humans to be due to the reduction in dental size brought on by more advanced tool technology of the moderns, rendering the large teeth of the Neanderthals unnecessary. Since then several workers have postulated ways in which differences in dental function and loading could have brought on the distinctly different midfacial morphologies in these two hominid groups. The Neanderthal dentition, and in

particular the dental wear pattern is distinct from that of the generalised modern human. The midfacial anatomy and the dental characteristics could thus both be linked to stress moments from masticatory functions distinct from those of modern humans. Smith (1983), considered the midfacial prognathism and the steep nasoalveolar clivus of Neanderthals to be related to resisting vertical forces generated by heavy occlusal loads on the anterior dentition. An increased vertical dimension of the facial skeleton would reduce bending moments acting on the face during mastication heavily reliant on the anterior teeth. His hypothesis is supported by the heavy wear pattern often seen on Neanderthal front teeth, as well as the large size of the anterior dentition, relative to the posterior dentition.

Rak (1986), takes this hypothesis further. He sees Neanderthal morphology as being designed to oppose the rotation of the snout in the sagittal plane about a bilateral axis, due to heavy anterior occlusal loads. The anterior end of the palate is pushed upwards and forwards as the nasal process of the maxilla is pushed inwards and downwards towards the lacrimal region (Rak, 1986; Demes, 1987). From this perspective it is concluded that the Neanderthal midfacial morphology resists this rotation by shifting a large part of the infraorbital bony plate towards the sagittal plane from the typical (modern), coronal orientation. This is done by extending the relevant parts of the face in a lateral manner and by a medial migration of the lateral part of the face.

The retromolar space (first described by Coon in 1962), is often interpreted as being necessary for the mandible to maintain the length necessary for the lower teeth to remain in occlusion with those of the maxilla, as the midface

changed shape (Howells, 1975). However Trinkaus (1983; 1987), has claimed that the prognathism of the Neanderthal face is nothing but a figment of anthropological imagination. Neanderthals were no more prognathic than their predecessors (Franciscus and Trinkaus, 1995), and the retromolar space is a by-product of a shortened dental arcade, a posterior retreat of the zygomatic and a reduction in the breadth of the mandibular ramus.

This shortening of the mandibular dental arcade is now generally agreed upon, although how it took place is more highly debated. While Howells (1975), believed that the whole dental arcade had migrated forwards, Rak (1986), postulated that the posterior dentition had been reduced on par with that of modern humans, while the anterior dentition remained comparatively large (in keeping with his hypothesis of anterior loading). Spencer and Demes (1993), saw an anterior migration of the molars simultaneous with a posterior migration of the anterior dentition as being the cause for the reduction in the length of the dental arcade. It thus seems that the retromolar space developed as a secondary feature in response to changes in the position and length of the dental arcade in relation to the mandibular corpus and ramus, although the exact mechanisms involved are still unclear.

1.5.2 Th e e v o l u t io n a r y r e l a t io n s h ip b e t w e e n Ne a n d e r t h a l s a n d