Of teeth, feet, and feed. The unexpectedly rapid evolution of mandibular shape in hominins

Members of the hominins – the ‘australopiths’ and the species of Homo –possess short and deep mandibles, and relatively small front teeth. This is believed to form an adaptation to life in open habitats and the consumption of though foods. With the appearance of Homo, the functional meaning of the mandible has been presumably deemphasized, thanks to technological innovations culminating with the control over fire. Hence, the selective pressure on Homo mandible should have been weakened in the process. To verify this hypothesis, we analysed mandibular shape variation in a large sample of primates by applying geometric morphometrics (GMM) to the primate mandible shape under a new phylogenetic comparative method (PCM) approach. We assembled a dataset of 735 primate mandible images belonging to 211 different species, and built a phylogenetic tree for those species. We implemented and applied a new PCM, RRphylo, to the shape data ordinated by GMM. RRphylo allows calculating the rate of shape evolution for all the branches in the phylogenetic tree, and verifies the existence of shifts in the rate across the phylogeny. Contrary to our expectations, we found that mandible shape evolution in hominins is exceptionally rapid as compared to any other primate clade, and that the direction of shape change (from the ape ancestor) is no different between australopiths and Homo. We deem some biomechanical constraints related with the acquisition the upright posture (bipedalism), loss of honing complex, and canine reduction, which is shared by all members of the clade, could be responsible for such surprisingly high evolutionary rates. This exaptive condition occurred early in hominin evolution, and generated “cascading effects” that were recruited for a number of different adaptations along and across the history of the human clade, in response to the rapid environmental changes recorded in Africa from the Upper Miocene through the Plio-Pleistocene. This study reveals the evolution of mandibular mandible shape in bipedal hominins has strong morpho-functional and ecological significance attached.