The Fetal Origin of the Human Chin

The chin prominence is a hallmark of the modern human face and bears on its labial surface an inverted-T bony relief. Evolutionarily, whether the human chin is an adaptation for mastication or speech is debated but there is little compelling data supporting either claim. Furthermore, some suggest that the inverted-T relief is more important for phylogenetic inference than the chin prominence. However, there is no evidence for the developmental independence of the inverted-T relief and chin prominence. This debate requires empirical data on fetal development of the human chin. Using 3D imaging of the musculo-cervico-craniofacial skeleton of human fetuses and geometric morphometric methods, we discovered a developmental sequence leading to a chin prominence during early fetal development that is very similar to that which we previously observed in postnatal modern humans and in chimpanzee fetuses. Furthermore, we provide the evidence that the inverted-T relief is developmentally integrated with the chin prominence. The evolution of the human chin is constrained by cervico-craniofacial developmental that maintain an unobstructed fetal airway. Finally, the inverted T-relief should be neither treated independently from the chin prominence in phylogenetic analysis, nor is it a relevant taxonomic trait that defines the symphysis of modern humans.