How the Human Dentition Works: a thegotic analysis.

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In 1958 at a Dental Conference in Christchurch, New Zealand, Dr RG Every, a Christchurch dentist, presented an argument that tooth-grinding in human beings was, in fact, tooth-sharpening. The biological and clinical significance of this discovery was outlined in two papers in The Lancet: “The Significance of Extreme Mandibular Movements” [Every 1960] and “The Teeth as Weapons” [Every 1965]. In his first published account on tooth-sharpening Every said: “Most mammals sharpen their teeth. This ensures that they are effective tools and weapons.” [Every 1960].

Homer first recorded the phenomenon of a specific tooth-sharpening behaviour when he described specific dental weapon sharpening in the wild boar:

Just as when hounds, and young men in their bloom, press round a boar, and he comes forth from his deep lair whetting his white tusks between crooked jaws, and round him they rush: and in the midst of all this is heard the noise (from the whetting) of his teeth, and straightway they await his assault, dread as he is – even so then round Odysseys dear to Zeus, rushed the Trojans. – Homer the Iliad, 700 BC in Every 1972 (Fig1).

In 1967 at a symposium of vertebrate palaeontology and comparative anatomy in London Dr Every introduced the word thegosis to define the apparently instinctive tooth-sharpening behaviour which he had discovered [Every 1970]. It is appropriate, in view of Homer’s observation, that a term to describe the innate tooth-sharpening behaviour should be derived from “the Greek word thego – meaning to whet, sharpen; also, metaphorically, to excite, provoke”. [Every 1972]

During the 1970s, Every presented extensive evidence to support for the hypothesis outlined in The Lancet [Every 1970, 1971, 1972, 1973, 1975; Every and Kuhne 1970, 1971]. While early dental writers understood that our teeth cut with blades and crush and pierce with cusps, they were led to the mistaken interpretation that tooth-grinding and abrasion destroyed the tooth’s morphological, or primary, sharpness [Balkwill 1867]. In contrast, Every demonstrated that the usual abrasive wearing, due to incision, mastication, and other processes, was complemented by thegosis – behaviour which he argued must be innate and genetically-programmed. This innate behaviour refreshed the tooth surface, refined and redefined the abraded, morphologically sharp features of the teeth in human and many other mammals.

Thegosis-facets can be seen as the biological equivalents of the honed back of a chisel, adze, or sickle (Fig 2).

He was a meticulous photographer, and he gave many demonstrations that detailed examination of thegosis-facets reveals evidence for this behaviour, which he recorded both in metal casts, and in high quality colour slides. At that time there were serious obstacles to the presentation of this evidence in publications showing the numerous, high quality photographs required; those which Every did publish were often poor quality reproductions [e.g. Every 1970, 1973, 1975].

There is now evidence that thegosis is a multiphyletic phenomenon, found in many animals, both vertebrates and invertebrates. In each of them it affords the same biological advantage – the honing of teeth and tooth-like structures for efficient function as tools and/or weapons [e.g. Tunnicliffe 1973; Scally 1973; Scally et al 1974, Tunnicliffe et. al. 1974].

It can now be proposed that: “Whenever articulated opposable teeth or analogous structures evolve there is the possibility of the evolution of a thegotic mechanism to service them” [Scally 1987]. This view that tooth-grinding is a normal innate behaviour is completely contrary to the orthodox teaching that facets are evidence of bruxism and that this behaviour is pathological [e.g.Murray and Sanson.1998].

Since 1958, Every’s hypothesis has been exhaustively tested, and his original views have found considerable independent support [Craddock & Johnston 1961; Simpson 1972; Scally 1973; Tunnicliffe 1973; Scally 1979; Scally 1979; Scally 1980; Wells, et al 1982; Von Vierhaus 1983, Mills 1976, Krebs 1971, Thulborn 1974].

In spite of this there has been little or no acceptance of the phenomenon, or recognition of its clinical significance and general applicability. During the 60s and early 70s there was some independent discussion of thegosis in two disciplines – vertebrate palaeontology and physical anthropology – with cautious acknowledgment of Every’s discovery [Simpson 1972]. However, in recent literature, discussion or citation of thegosis and its significance has largely ceased in these disciplines, though some authors, while aware of the concepts, have ignored, or in some instances misinterpreted, Every’s ideas and/or misquoted his arguments [Teaford & Walker 1983, Teaford 1988]. Murray and Sanson [Murray and Sanson 1988] have argued that published evidence is insufficient to support the Every’s thegotic hypotheses, and while conceding that “There is evidence for the sharpening of anterior teeth as a social weapon, and this activity could appropriately be called ‘thegosis'”, they considered that “there is no evidence for the genetic programming of a mechanism to sharpen the teeth for weapons and the mastication of food”, and “no evidence that any animal sharpens its molar teeth independently of the masticatory process in order to improve the efficiency of that process”. They contend that “Thegotic facets are formed during the same mandibular movements that occur during mastication” and “Parallel striations on thegotic facets are not an integral part of the formation of those facets”.

It is argued here that the full understanding of thegotics involves a paradigm shift, and as a present-day consequence of the difficulties which have existed in adequately presenting the evidence of these three-dimensional phenomena within the constraints of black and white illustrations on paper in journals, almost the entire literature on tooth-wear, including studies of the human dentition and the dietary and behavioural conclusions derived from them, is still fundamentally flawed [Scally 1991] . While Every explored many hypotheses following his discovery of the evolutionary aspects of thegosis, he did not publish a thegotic interpretation of how human teeth function and are sharpened in a form accessible to the dental profession, both teachers and dental practitioners. It is towards this end that this present account is directed, since at present the phenomena of thegosis are better recognised and understood by some distinguished zoologists and paleontologists than by the dental profession.

I argue that to appreciate the present chaotic situation, the human dentition needs to be understood in terms of both an efficient cutting and crushing dental tool with that analysis in place it is then possible to present an argument for the evolution of a specialised segmentive biting dental weapon tool and dental weapon.

In mammals, teeth serviced by thegosis (thegotic teeth) first evolved as dental tools [Every1973]. The ability to sharpen a dentition opened up a new opportunity for dental evolution. Instead of a continuous replacement dentition of morphologically sharp teeth, as seen in reptiles and sharks, for example, there was the opportunity to evolve a permanent dentition. This was a dentition that could be re-faced by specific jaw actions, at a time other than during mastication, and genetically integrated into the tooth’s anatomy and function – thegosis [Every 1972].

Every understood, all to clearly, that terms to describe tooth morphology were so ambiguous and confusing – more often geological than functional, that the biological advantage of thegosis could not be understood without a radical reworking of current terminology for the morphological features of mammalian teeth [Every1973]. To this end he deconstructed teeth to their primary functions: cutting and crushing. From these basic concepts he reconstructed a functional terminology for tooth morphology, published, without illustrations [Every 1972]. In two subsequent papers (with illustrations) the application of this terminology was used to show its general applicability to mammalian especially primate evolution and to the dentition of prosimians [Every1973, 1975]

The evolution of thegotic mechanisms also introduced another opportunity for the evolving sharpenable tooth – the ability to change the tooth’s shape over time, which is the fourth dimension of tooth morphology.

Unlike other tissues, enamel is inert. Thegosis presented an additional biological advantage of sculpting the tooth to change its shape. This phenomenon has been extensively exploited in some mammals with continuously growing teeth, where the teeth are sculpted on eruption into specific shapes prior to being sharpened [Every 1972, Scally 1973]. This feature is also present in echinoderms, pre-empting mammalian sculpting by perhaps some 500 million years.

This remarkably early development of thegosis supports the hypothesis that whenever articulated opposable teeth or analogous structures evolve there is the possibility of the evolution of a thegotic mechanism to service them [Scally 1987].

The invertebrate evidence also supports the hypothesis that in the thegosing phyla there are specific and phylogenetically ancient areas in the neural tissue of these animals devoted to thegosis. In the case of echinoderms within the neural complex, in the case of arthropods, the cephalic ganglion respect, and in the case of chordates, it can be hypothesised, that there is a specific and phylogenetically ancient area in the brain of devoted to thegosis, which might be termed a “thegosis centre”.

HOW THE HUMAN TEETH WORK: A BRIEF THEGOTIC ANALYSIS.

For convenience, the human dentition can be divided into an anterior and a posterior dentition with a division between the canines and premolars.

The incisors and canines: how they work and how they are sharpened

The current teaching in dentistry is that any movement of the mandible beyond the canine edge-to-edge position is “parafunctional”. However to sharpen the anterior teeth, the mandible moves beyond the canine edge-to-edge position, in a direction diametrically opposite to mastication and oblique to incision to the limit of the thegotic stroke. This is 10 or more millimetres further than is currently accepted as a functional mandibular movement [Every 1970]. This action produces the bevelled, leading-blade edges of the incisors. (Fig 3).

Every hypothesised that in the evolving hominid there was a change in the anterior dentition and that this change allowed all the anterior teeth to be sharpened. This biological advantage was further exploited with the evolution of a segmentive bite with a retrusive-cutting stroke; that is to say, a condition where two horizontal sickle-shaped blades, after confining a chunk of material, cut and crush the interposed material [Every 1972 p19]. This scissor-like action was also alluded to by Balkwill a century earlier [Balkwill 1867].

Just as there is confusion in contemporary texts about the occlusal surfaces of premolar and molar teeth, the flat, faceted surfaces of the incisors are incorrectly regarded as the “incisal edge”, whereas, in the thegotic interpretation, the true functional edge is that edge which is defined by the thegosis-facet and the leading tooth-surface. In the case of the mandibular incisors this is the lingual enamel edge (Fig. 3&6). The large blades of the segmentive bite are made up of a series of sub-unit sickle-shaped blades of the incisors and canines (Fig. 4, 5&6).

Every hypothesised that this evolutionary change in the dental weapon system, – a change from a slashing canine system, as found in our pongid ancestors, to a system with a biologically advantageous “segmentive” bite [Every 1970](Figs. 7&8). He listed a number of biological advantages for this change [Every 1970, 1972]. His interpretation is consistent with evolutionary processes as currently understood, in that it involves selection for biological advantage [Every 1970, Every 1972]. This is in contrast to the implausible contemporary view, in which natural selection of the evolving hominids is apparently held to have been suspended for 2.5 million years while they continued to lose the efficiency of their dental weapon with the gradual reduction of the snout, and during this long period of time it is supposed that they waited, weaponless, for their wits to evolve so that eventually they could make weapons to defend themselves. This begs the question why evolutionary processes selected for such a disadsvantagous feature. One author, spotting this difficulty argued that the development of the brain was so important the genes for this had an inverse relationship with jaw and tooth size [see C.L. Brace’s comments in Wallace 1975].

Unfortunately, the hypothesis of the dentally-weaponless hominid is so ingrained in paleoanthropology that it colours every current theory of human evolution.

In contrast, Every’s model of human dental evolution makes sense of the palaeoanthropological record and is a more plausible and parsimonious account, for it presents a case for a developing hominid that was never weaponless. Snout reduction, and other changes from the Australopithecines through to Homo, progressively improved the biomechanics of the segmentive bite by reducing the length of the lever arm from the temporo-mandibular joint to the incisal edges, thus conferring a biological advantage. Evolutionary success requires present, not future, biological advantage [Every 1970, 1972, 1973, 1975].

A significant difference between the segmentive-biting hominid and other apes is the hominids ability to sharpen the anterior teeth by thegotic servicing. Apes and monkeys have a specialised, sexually dimorphic, canine weapon and their anterior dentition (with the exception of the male maxillary canines) relies on abrasion to keep the incisors sharp. The short-canined hominids, in contrast, with wide lateral jaw movements, which are oblique to incision and diametrically opposite to mastication, can sharpen all the teeth, especially the canine-incisor complex.

Premolars and molars: how they are sharpened and how they work, a simplified account.

The full thegotic stroke begins with the teeth in centric occlusion and ends at the stokes anatomical limit. For the first few millimeters or so the posterior teeth and the canines are in thegotic contact. As the mandible moves further obliquely the posterior teeth are lifted out of occlusion by the canines. As the mandible moves further obliquely the anterior teeth are sharpened. It is not hypothesised that every thegotic event involves a full thegotic stroke. From clinical observations it seems that there are discrete posterior and anterior events that flow into one another.

The sharpening of the premolars and molars also involves movements of the mandible that are diametrically opposite to mastication and oblique to incision [Every 1972]. With the exception of the historical analyses by Balkwill [1867] on how the human dentition works, all contemporary references to human premolar and molar teeth say that the occlusal surfaces become flat and blunt when worn. When viewing the surface of the worn tooth it does appear flat and blunt, but, in the thegotic interpretation, this is like arguing that the honed surface of a scythe, sickle, adze, or chisel is flat and blunt. The ground surface is not applied to the object being cut. In function, the leading edge is the cutting agent.

Here, it is argued that a pre-molar or molar tooth is a composite dental tool, which consists of a complex of both a crushing surface, and cutting bladest [Every 1975]. However, the cutting blade component is not readily apparent until the masticatory action and the relationships of the mandibular and maxillary molars and premolar teeth are considered. The ground, occlusal surfaces and the occlusal morphology define the crushing features and the blade edges. This is clear when considering the mandibular lingual cusp’s thegosis-facets and the lingual enamel wall. Where they meet defines a blade edge. Although this situation is obvious in a man-made tool, there appear to be difficulties in conveying the same point in relation to mammalian dentition (Fig 2). It is not the ground surface but, rather, the leading edge that is applied to cut the object during the cutting stroke.

One of the explanations for this difficulty is that odontologists often study teeth in isolation with little consideration of the precise masticatory action. The function of the biological tool can be understood by detailed observation and analysis of its action, which make obvious the cutting and crushing functions of the mammalian and human premolar and molar teeth. In profile, the movement of the mandible during simple masticatory movements, whether left or right, can be viewed as a teardrop with its upper end clipped off (Fig 9).

Each phase of mastication is worth analysis in it own right, since large variations on this schematic representation occur. This variation can be a function of food consistency, the position of the tooth within the row, and individual variation. However, for the purpose of this description it is useful to identify the primary function of each phase. After the material to be crushed and cut is placed on the occlusal surfaces of the mandibular teeth by the tongue and it is initially squeezed and crushed (positions 3 to 4). The buccal cusps of the maxillary premolars and molars and the lingual cusps of the mandibular premolars and molars engage the material and, as the mandible moves medially, the material is stretched over the occlusal surface. As the teeth approximate in the final medial stroke, it is crushed and cut by the sickle-shaped enamel blade row of the buccal cusps of the mandibular premolar and molar teeth and the palatal cusps of the maxillary premolar and molar teeth. (from positions 5 to 6). Final crushing and cutting occurs at position 6. This is not a simplistic crushing, or mortar and pestal action, but a precise series of actions with tooth anatomy to facilitate it. (Also see Fig. 10,11,12).

The morphology of the mandibular molar reflects this function. In profile, the lingual leading edge of the mandibular first molar is higher than the buccal cusps. The leading lingual blade is maintained by transverse thegosis. The lingual blades of the mandibular premolars and molars, and the buccal blades of the maxillary premolars and molars, between them, engage, confine and cut the interposed material. The mandibular buccal blades and the maxillary palatal blades crush and cut the interposed material as the mandible moves through positions 5 to 6. In centric relation the interposed material is finally crushed when the cusps and fossas interdigate.

This was understood by at least one of the early oral anatomists [Balkwill 1867] but for reasons that are not clear, the idea was lost and premolars and molars were subsequently likened to mortars and pestles. (Also see Every 1973 for a critical analysis of this misinterpretation).

The premolars and molars are part of an integrated unit, and form a precise cutting and crushing biological machine. With this understanding of premolar and molar function, it becomes clear how the leading and trailing blades are serviced by thegosis and how this behaviour maintains the basic functional occlusal morphology.

Figures

5-6 scissor-like action.
6 final simultaneous cusp-to-fossa crushing contact (incusion).

Phases 4-5-6 depend on food consistency and texture.

Each phase of mastication is worth analysis in it own right since there is large variation on this schematic. This variation is a function of food consistency, the position within the tooth row and individual variation. However, for the purpose of this exercise it is useful to identify the primary function of each phase.

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