Figure 2. Ontogenetic shape changes.

Lateral view: shape changes along the ontogenetic trajectories of chimpanzees (a-c) and humans (d-f) seen in Figure 1. The 3D morphs are regression estimates scaled to unit centroid size in order to focus on the shape changes. The mandible (translucent) was brought into the space of the cervico-craniofacial complex in order to visualize simultaneous shape changes of the tongue and the symphyseal midline. Sagittal plane: the thin plate spline deformation grids illustrate the shape changes of the sagittal structures of block 1 (left) and the symphysis of block 2 (right) from each age stage to its next older stage (exaggerated by factor of 1.5). Red dots: landmarks (listed in Table S1, Figure S1a).

Similar shape changes in chimpanzees (a-b) and humans (d-e) simultaneously include: 1) the forward projection of the mental region; 2) the relative horizontal reduction of the tongue and that of the oral cavity; 3) the cranial base flexion; 4) the relative shortening of the anterior cranial base; 5) the clockwise rotation of the posterior region of the upper mid-face; and 6) the forward positioning of the cervical column and the hyoid bone.

Growth divergence in chimpanzees (b-c) and in humans (e-f). In humans: 1) vertical growth of the upper mid-face; 2) the rotation of the ramus towards the corpus; 3) lowering of the base of the tongue and the hyoid bone; 4) and the relative anteroposterior narrowing of the pharynx. In chimpanzees: 1) the inferior transverse torus starts to grow posteriorly; 2) the cranial base retroflexes; 3) the upper mid-face rotates forwards; and 4) the cervical column, the hyoid bone and lower part of the pharynx displace backwards.