The Human Canine: Its Evolution and Adaptive Significance

DOI: 10.1051/odfen/2010103 J Dentofacial Anom Orthod 2010;13:4-10 © RODF / EDP Sciences

The human canine: Its evolution and adaptive significance

Pascal PICQ Collège de France

ABSTRACT The canine is a tooth with special characteristics and adaptive significance that varies considerably between mammalian lines and the primates. No matter what the line, canine teeth are never involved in mastication and do not interfere with masticatory dynamics. Mastication, which is one of the most complex functions that monkeys and apes display, appears well before the large canines in both phylogeny and ontogeny. In apes, their size and shape have nothing to do with diet, but are linked to sexual selection. The human line of hominids possesses smaller canines that have become incisiform and have lost their sexual and social function. They are now used exclusively to tear apart meat and other types of solid foods. The development of a wide, short buccal surface that increased medial-lateral movement during the masticatory cycle, may explain this partic- ular development in recent hominids including humans. From an evolutionary point of view this means that the human canine has been subjected to stresses imposed by the biomechanical environment of the masticatory apparatus. In other words, the human canine, as well as those of the other anthropoids, does not guide mastication but has acquired a morphology and position restricted by masticatory functions. In evolutionary terms, it is therefore referred to as an “exaptation”; it has acquired, not a function, but a passive characteristic which makes it a marker for rehabilitation – on condition that its eruption is related to normal masticatory functions in individual histories, but it never serves as a mediator of mastication.

KEYWORDS Canine Mastication Address for correspondence: P. PICQ Evolution. Collège de France, 3, rue d’Ulm, 75005 Paris. [email protected]

Article available at http://www.jdao-journal.org or http://dx.doi.org/10.1051/odfen/2010103 THE HUMAN CANINE: ITS EVOLUTION AND ADAPTIVE SIGNIFICANCE

1 - INTRODUCTION

In 2009 we celebrated the bicente- tion (see Picq P., Les Mondes dar- nary of the birth of Charles Darwin and winiens. Syllepse, 2009) of finalised the one hundred and fiftieth anniver- morphogenesis, internal forces guid- sary of the publication of: On the ed by the divinely-inspired sphenoid, Origin of Species. But even after this the sacred basicranial flexion, the long passage of time, the theory of apocalypse according to the third evolution is still poorly understood, molar and the fairy neoteny. Oh poor especially when we consider the pop- epistemology! ular conception that mankind is a special case because of the lingering All science is based on observa- persuasive power of obsolete pre- tion, comparison, experimentation mises that attempt to detach humanity (when possible), modelling and, from the mammalian lineage, claiming above all, on the possibility of refuting that our species, with its genius and its the dominant paradigms, not on the cultural and technical innovations, has basis of arguments which consist of gained freedom from evolutionary con- refuting observed facts or revelation straints. However, this is not the case by experimentation in the name of and what we call co-evolution or inter- dogmatic ideas, but properly by taking action between our biological evolution these facts into account, as new and our techno-cultural evolution is in paradigms are developed. As Claude constant operation, particularly with Lévi-Strauss has reminded us, studies respect to masticatory functions, mas- devoted to examination of humans, as tication and canine teeth. well as sciences or those whose Although evolutionary theory per- scope encompasses humans, cannot meates all the life sciences, including claim scientific status if they limit medicine, outmoded, non-scientific their range of study to humanity beliefs about mankind‘s position in exclusively. If they ignore the species nature and its intimate participation most closely related to us, whether in in hominid evolution, still persist. It is the modern world or the past, they not just the creationists, defending a will produce blunders as ludicrous as narrow, unreasoning literalism, not those of Bouvard and Pécuchet and based on a coherent exegesis (see this applies not only to paleoanthro- Picq P., Lucie et l’Obscurantisme. pology but to dental science as well. Odile Jacob, 2007), who refuse to The adaptive significance and evolu- understand and argue against evolution of the canine and its functions is, tion. Many representatives of health as an English investigator has disciplines, who claim to base their remarked, “a case in point”. proposals on scientific procedures In this article, we shall begin with but are actually using concepts a short review of canine anatomy derived from theology and, especial- and comparative ethology and we ly certain schools of philosophy to shall observe that the canine has take an astonishing stand opposing diverse types of adaptive significance, evolution. Paleoanthropology is not as seen in the mammalian line, spared, with its ideas of hominisa- with regard to natural and/or sexual

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selection. We shall then see what the dentistry today. This taste of evolu- situation is for primates and monkeys. tionary science may not risk tearing Next, we shall engage the main the flesh of Platonian idealists but themes of the evolution of the canine could certainly tweak the sensitive in the human line over the past 7 mil- souls of these devotees of an intelli- lion years. Finally, we shall address gent design dictated by the great certain problems by canine teeth in heavenly occlusal architect.

2 - A GENERAL OVERVIEW OF THE COMPARATIVE ANATOMY AND THE FUNCTIONING OF MAMMALIAN CANINES A trend in the evolution of the den- contrary to what you may read or tition of mammals – which is not a law hear, the canines are not used to tear but an empirical a posteriori observa- flesh, but to perforate, pierce and kill tion – since they parted from their prey as you can easily see if you look “reptilian ancestors” includes a at the morphology, size and shape of reduction in the number of dentitions the canines in modern carnivores that to just two, the deciduous teeth and are blunted and oval or round in cross- the adult dentition, a reduction in the section. This hardly makes them well number of teeth, and the differentia- suited for cutting flesh and even less tion of types of teeth, which is called for breaking bones! Carnivores are heterodontia. These varied types of the mammals which masticate the teeth in humans and primates are the least, if at all. They shear meat with incisors, the canine-first premolar their carnassial teeth and swallow it complex, and the molars. We shall whole, the stomach taking care of the see that, even in primates, these rest. As for the ability to break marrow fields may or may not embrace neigh- bones, only hyenas, bears and wolver- bouring teeth. From the point of ines can manage this, but to do it they view of evolutionary and adaptive use conical premolars, not canines. genetics, the canine is distinguished Therefore, in carnivores, the canines by its propensity to be more devel- are only weapons and do not con- oped and prominent than the other tribute to mastication, which, in any teeth – though not always, and to case, is not a function that these ani- form complicated functional complex- mals employ. Developed canines are es, in response to natural as well as found in several species of herbivores to behavioural and sexual selection and omnivores. The most surprising factors. case is that of young musk deer. The The most commonly accepted males have fine, very prominent notion today links a developed canine canines which are certainly not useful with carnivory, as in the canidae family. in catching and masticating tree But this is wrong, because the order leaves. Both sexes have horns, but of carnivores is not defined by the only males have highly prominent presence of well-developed canines canine teeth. It is a secondary sexual but by its carnassial teeth, formed by characteristic which has developed the first upper molar and the last within a context of sexual competition. lower premolar (P4-M1). Because, The display in individuals of one sex of

6 Picq P. The human canine: Its evolution and adaptive significance THE HUMAN CANINE: ITS EVOLUTION AND ADAPTIVE SIGNIFICANCE

characteristics of size and shape do not routinely use their canines, as well as secondary bony and/or which represent secondary sexual skeletal appendages, as well as in characteristics, to obtain food – even their coats, that the other sex does if they do occasionally catch small not have, is called sexual dimorphism animals with them. Males in the (see Picq P., Le Sexe, l’Homme suida group utilize their canines pri- et l’Evolution. Odile Jacob, 2007). marily in intrasexual competition to Sexual dimorphism is especially threaten rivals and as weapons marked when individuals of one sex against them. Since the suida are attempt to possess, forcibly, a harem omnivorous, their mastication is a of members of the opposite sex. The complex function adapted to the intrasexual competition that arises in breakdown of different types of food. this struggle, which aims to exclude And, of course, the large size of their members of the same sex also canines does not interfere in mastica- searching partners, favours powerful tory kinematics in any way. individuals, armed with dissuasive The most extreme case of sexual weapons, usually antlers, horns dimorphism is found in sea elephants, and/or canine teeth. The well-devel- where the males display the most fla- oped canines in musk deer clearly grant macho characteristics as they then have nothing to do with their corral and control dozens of females folivorous diet. We should note in for their exclusive sexual pleasure. passing, that these prominent canine Their body mass is several times teeth do not interfere with the medial- greater than that of the females and lateral components of the masticatory their canines, which are enormous, cycle. (The temporomandibular joint in are used only to impress rivals, as carnivores has a pintle-hinge type they cannot readily be employed in anatomy which only allows move- combat. But their enormous bulk can ment in the sagittal plane; so it is appear redoubtable when a huge not the canine that restricts the male sea elephant bursts out of the medial-lateral component of a form of water. mastication restricted to vertical This brief overview of the size and shearing). Sexual dimorphism of function of canines reveals an adap- canines is found in horses, being well tive relationship with respect to natu- developed in males and usually ral and sexual selection. In carnivores absent in females; but mares some- they are used like daggers and the times have small canines. These are difference in size between males vestigial characteristics, gradually dis- and females is simply correlated to appearing. body size. (It was the same for the Males in the group suida, such as impressive sabre-tooth tigers at the wild boar and wart hogs, develop end of the prehistoric period). The large, curved canines. In old individu- function and adaptation of canines is als, the tip of the canine may grow so therefore simply a factor of natural exuberantly that it pierces the ani- selection: to kill prey. Animals never mal’s snout. These omnivorous use canines to tear flesh before eat- species search for the underground ing it. This role is played by the car- parts of plants which they dig out nassial teeth. The canines are even with their powerful muzzles. But they less adapted for breaking up marrow

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bones, a manoeuvre that would females in intersexual competition. require an impossibly extreme open- Finally, these species have strong ing of the jaws and a lengthening of masticatory functions and it is clear muscle fibres that would leave feeble that these large canines do not inter- covering of the sarcomeres, thus fere with chewing cycles. If you want reducing the action of the muscles of to validate this contention, all you mastication, even of the powerful have to do is visit a zoo and watch temporal. Prominent canines cannot them eat. You’ll wonder how these be blamed for the absence of kine- animals could have survived to adult- matic mastication in this species hood and reproduce successfully if because it is the morphology of the they were unable to protect them- temporomandibular joint that pre- selves against enemies. The answer vents this process from taking place. to this false enigma is in ontogenesis: Prominent canines are found in their large canine teeth emerge into several mammalian lines, in herbi- the arch later than the rest of the den- vores (graminae), folivores (shrub tition because of sexual and hormonal leaves) and omnivores. They may be factors, at a time when masticatory fine, long and fragile as in musk deer, kinematics has already been well or powerful as in suidae. These established in a process that began canines are not used for obtaining or when the first adult teeth erupted. eating food. Their development is a During their growth period, the large response to sexual selection factors, canine teeth are subjected to the particularly in species where the pressures of orthodontic forces which males set up harems and have to guide their development. Individuals fight off other males (intrasexual com- for whom this procedure malfunc- petition). Their fragility, as in musk tions will perish in the struggle for sur- deer, or complex shapes, as in suidae, vival and thus be prevented from indicate that these are secondary sex- passing on their maladaptive charac- ual characteristics used to menace teristics to future generations. other males in intrasexual competi- Darwin taught us how this works one tion, but may also serve to attract hundred and fifty years ago.

3 - THE ADAPTIVE SIGNIFICANCE OF THE CANINE IN SIMIANS

Human beings and other hominids, maintained slight sexual dimorphism even the males, have small canine of the canines, those of the male teeth. This is one of the chief charac- being only slightly larger than those of teristics that allows us to place the the female. We can see that in the Toumai (Sahelanthropus tchadensis) as human line a trend has developed for an ancestor of our emerging line bodily differences between the sexes while many scientists rule out a rival to remain marked while sexual dimor- contender group, the Orrorin, because phism of the canines has almost dis- of their larger ape-like canine teeth. appeared. The first Australopithecines, who lived This detachment has been noted between 4 and 3 million years ago, in more recent Hominids like the

8 Picq P. The human canine: Its evolution and adaptive significance THE HUMAN CANINE: ITS EVOLUTION AND ADAPTIVE SIGNIFICANCE

Paranthropes, descendents of Lucy, human canine has not become and the first humans, Homo habilis, incisiform in order to eat meat, but Homo rudolfensis and Homo has found a secondary application in ergaster. It is a development that is doing it. especially remarkable because, on We do not yet know why canines the one hand, our relatives the became small and incisiform in Paranthropes evolved by accentuating hominids at the end of the tertiary era. their sexual dimorphism, while our The only coherent explanation is that Homo genus has diminished it (see canines changed in the framework of Picq P., Au Commencement était the evolution of the face and dental l’Homme. Odile Jacob, 2003). arches. With short, wide dental arches Therefore, for the past 2 million years, and the gap between the two tem- the human canine has not played a poromandibular joints at the base of role in sexual selection. the skull, the medial-lateral compo- Neither is there any correlation with nents of the masticatory cycles had to diet. Although paranthropes developed be more accentuated (see Picq P., op. an increasingly impressive masticatory cit. CNRS, 2003). One consequence is apparatus, with a small incisor-canine possible interference between masti- arch and very large, strong premolars catory kinematics and prominent and molars, the reverse trend is true of canines. Natural selection factors take Homo, in whom the incisor-canine arch empirical priority over sexual selection is expanding as the premolars and factors – viability vs. reproductive suc- molars tend to regress. Paranthropes cess –; our immediate ancestors and humans are both omnivores, but experienced an evolutionary modifica- our relatives can chew huge quantities tion in the size of the canines and of tough plant material, especially the age at which they erupt (known as roots, while most humans include a allochrony). All monkeys and large considerable amount of meat in their apes have large canines that erupt diets, although we still subsist primari- after the third molars have appeared ly on fruits and vegetables. It is incor- in the arch. But in paranthropes and rect to claim that the human canine is Homo, the canines erupt at the same linked to the consumption of meat, time the second molars do. This because all hominids, as well as mod- modification in ondontogenesis con- ern chimpanzees, eat meat regularly. firms the antagonistic interactions Thanks to increasingly incisor-like mor- which may result from large canines phology and its powerful root, the emerging before masticatory dynam- human canine tooth tears flesh very ics are in place. This clearly demon- effectively. But, as we have seen, nei- strates that canines must adapt to ther carnivores nor apes ever employ the masticatory environment. Either their canines for alimentary purposes. they appear later and grow under And it is also true that no adaption the effect of orthodontic forces so that of the human canine to tear flesh they interfere closely with masticatory was necessary, because our earliest function, or it becomes a problem, in cave man ancestors could do this with which case they grow smaller and their razor-sharp stone tools. We can more incisiform. But under no consider this to be secondary “pseu- circumstances do the canines guide do-adaptation”; in other words, the mastication!

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4 - CONCLUSION: THE CANINES IN MODERN MAN AND DENTAL PRACTICE

So how can we explain that the enough to validate the notion of canine can sometimes play a major canine protected occlusion in masti- role in dental practice? Approaching a cation. From a phylogenetic and onto- common subject by confronting relat- genetic point of view, masticatory ed scientific disciplinary fields, does functions developed well in advance not mean that one discipline prevails of the appearance of large canines. over the other. I have reviewed – So how can we interpret this relation- briefly – what we know of the evolu- ship between the canine and the tion and adaptation of the canine in rehabilitation of occlusal functions? I mammals in general, apes with have suggested an answer: since the greater accuracy and hominids in par- canine is subjected to a masticatory ticular. No matter how fervently den- context, its eruption is influenced by tal occlusal theorists claim that their the orthodontic forces linked to masti- notions are based on observation and cation. Consequently, its position and experimentation in a context of mod- orientation in the dental arch result ern evolutionary theory, they do not from a previous situation which stand up under rational scrutiny. imposed these stresses. Another Instead we find that occluso- consequence is that through its posi- Platonists rely on a kind of religious tion and the orientation of its crown hominisation and intelligent design, a and facets, characteristics resulting belief totally incompatible with evolu- from dynamic balances conditioned tionary anthropology. In science, it is by this functional environment, the not acceptable to assert, as they do, canine was, in any case, a functional that Man is a separate species, espe- marker, but is not the cause of this cially when they insist on ignoring the function but its consequence. This is other species as they propound their the perspective in which the human pitiful argument that is based on canine can be extremely interesting in theological-philosophical certainties rehabilitating occlusal and masticatory saturated with metaphysics. On function, not because it is the matrix, the other hand, practitioners have but because it is the result. To perse- empirical practices and knowledge vere in this belief we risk making the validated by their clinical experience. canine responsible for masticatory Paleoanthropologists cannot contest malfunction, which clearly does not techniques that give satisfactory mean that the canine is responsible results. Even so, experience is not for masticatory function.

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