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] 4 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 evolu- tion 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 J Dentofacial Anom Orthod 2010;13:4-10. 5 PASCAL PICQ 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.