Models based on developmental mechanisms described in mice and shared by most mammals are shown to accurately predict tooth size in extinct hominins, and can explain the small third molars in our species. See Letter p.477
The third molars of modern humans — those often-troublesome molars we sometimes call wisdom teeth — are frequently very small or do not even develop. By contrast, the third molars of other hominin species in our evolutionary tree were huge, with chewing surfaces that were two to four times larger than those in an average modern human. This profound change in tooth proportions is usually explained by dietary and cultural changes considered to be unique to our species. But on page 477 of this issue, Evans et al.1 offer an explanation that may make us feel rather less special. The authors propose that the evolution of our small third molars may be explained by basic developmental mechanisms that we share with most mammals.
In 2007, researchers proposed an inhibitory-cascade model of dental development on the basis of experimental studies in mice2. According to this model, dental proportions are established by the relative amount of inhibitory and activatory molecules that are expressed as one tooth develops after another (the greater the net inhibition exerted by an earlier tooth, the smaller the size of the later-developing tooth). Since then, researchers have explored the extent to which this model can explain tooth proportions in other mammalian species. Evans and colleagues, alongside others3, have extended the application of this model to hominins.
Evans et al. focused on the lower primary postcanine teeth (lower milk molars and permanent molars). Their results show that variation in tooth size and proportions follows a remarkably simple rule that differs slightly between two major hominin groups. In australopiths, the group of African early hominins that includes, in this study, the genera Ardipithecus, Australopithecus and Paranthropus, teeth tend to get bigger towards the back of the primary molar row (up to the second or third permanent molar), with proportions that are constant irrespective of overall dental size. However, in the Homo genus, the proportional size of teeth varies with their overall size: smaller dentitions have disproportionately small third molars and tend to decrease in size from the first to the third molar.
The beauty of this model is that these simple relationships have an astonishing predictive power: by knowing only the size of a single tooth and the group to which it belongs — australopith or Homo — it is possible to infer with considerable accuracy the size of the remaining primary teeth. The model does not always correctly predict the location of the largest tooth, but predicted molar sizes are impressively close to observed values (Fig. 1). Dental anthropologists will be happy about this finding, but why should others care?
Size and proportions are but one aspect of the variation in dental anatomy, which can be studied using morphometric traits (measurable size and shape) and non-metric traits (such as the presence or absence of particular cusps and crests). Together with cranial features, dental traits are the bread and butter of hominin evolutionary studies, because they are thought to contain genetically coded information from which species relationships can be reconstructed. However, it is increasingly recognized that dental anatomy can vary in complex scenarios in which natural selection interacts with developmental and functional constraints4; this complexity may make teeth less useful for inferring phylogenies than we tend to recognize. For example, it is still debated whether the megadontia (extremely large dental size) observed in the genus Paranthropus is indicative of the evolutionary cohesion of this group, as is usually assumed, or whether it represents a convergent trend that was driven by similar ecological and developmental contexts5.
At the other extreme of the dental-size spectrum, fossils from the Sima de los Huesos site in Atapuerca, Spain, which are dated to more than 400,000 years old and are considered to be closely related to Neanderthals6, show an extreme degree of third-molar reduction7. The small third molars of these hominins are surprising for a taxon that is ancestral to classic Neanderthals, who had larger third molars. However, Evans and colleagues' model exactly predicts the strong third-molar reduction of Sima de los Huesos hominins as a result of their small overall postcanine dental size8 (Fig. 1). It remains to be understood why these hominins had such small teeth without an equivalent reduction of their faces and jaws. It is to be hoped that future experimental work will help to unveil models of the interaction between teeth and jaws that will explain this apparent paradox.
Because of the paucity of hominin fossils, Evans and colleagues simplified their analysis by using species mean values. However, activation and inhibition levels are expected to be individual-specific, which may result in tooth proportions that differ between individuals of the same species. It is therefore possible that the rule they describe would not be so simple had interindividual variation been included in the model.
Despite this limitation, Evans and colleagues' paper advances palaeoanthropology in three fundamental ways. First, it draws on experimental data from mice, which are the most common model organism, to explain the variation observed in the hominin fossil record. Second, it develops a rigorous quantitative framework to formally test hypotheses related to that model. And finally, it improves our understanding of the human fossil record by identifying evolutionary changes that are developmentally linked.
More importantly, Evans and colleagues' results are relevant beyond the study of fossil teeth. Many of the developmental constraints that influence dental evolution are shared by other systems formed by the repetition of serially homologous components, such as vertebrae, ribs, limbs and digits. Teeth can therefore be useful in identifying developmental mechanisms operating in these other systems9. By extension, the authors' model has the potential to help us understand the evolution of human traits that are associated with serially homologous structures, including our upright posture (which is influenced by vertebral anatomy), bipedal locomotion (which is linked to limb anatomy) and precision grip (which depends on the anatomy of digits). As with third-molar reduction, we tend to consider these traits the result of human-specific selective pressures, but their evolution is also fundamentally channelled by general developmental rules that humans have not escaped.