The functional role of Carabelli trait in early and late hominins

Carabelli trait is an accessory dental feature that forms along the lingual margin of the protocone of deciduous and permanent maxillary molars [1, 2]. It is variably expressed, ranging from a small pit or furrow to a large cusp, and its development, at least in Homo sapiens, is associated with crown size and intercuspal distance. This, nevertheless, seems to differ systematically between hominin taxa, and the variation in frequency and degree of expression of Carabelli trait has been used extensively for phylogenetic analysis. However, its functional implications remain unclear [3, 4]. In this preliminary study, we analyze the dental wear pattern of maxillary molars of early (Australopithecus afarensis, Australopithecus africanus, Paranthropus robustus and Paranthropus boisei) and late hominins (Neanderthal, Middle Paleolithic Homo sapiens and recent modern humans) using the Occlusal Fingerprint Analysis method, an approach based on digital models of teeth [5]. We only selected one molar for each species with a clearly visible Carabelli cusp that showed signs of wear. We manually outlined each wear facet onto the polygonal surface, measuring their dip directions (the angle between the projected and the reference vectors). The resulting occlusal movements were described using the occlusal compass, a circular space that ranges from 0° to 360°. In tooth crowns with a small Carabelli cusp, a large facet 5 was generally noticeable which extended cervically, while in teeth characterized by a large Carabelli cusp, two new facets, a mesial 5.1 and a distal 6.1, could be identified. These facets are created by lateroretrusive and/or lateroprotrusive movements, occluding with the lingual groove of the lower molars, between the metaconid and entoconid cusps. It has been suggested that Carabelli cusp provides an additional surface area that extends the molar’s functional life in highly abrasive environments [1, 3]. The facets observed in this study increase occlusal shearing area and this may be useful for a diet rich in tough and fibrous foods.