Form space principal component analysis.

<p>The first three PCs in Procrustes form space account for approximately 94% of the total form variance for block 1 and 98% of that for block 2. Chimpanzees (blue trajectory) and humans (black trajectory) have distinct curvilinear ontogenetic trajectories. Blue dots: chimpanzee fetuses (a, b) to neonates (c). Black dots: humans from birth (d) to ~1 y.o. (e), red dots: from ~1 to ~2.5 y.o., green dots: from 2.5 to 5.5 y.o (f). Pink dots: regression estimates at age a, b, c, d, e and f visualized in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0081287#pone-0081287-g002" target="_blank">Figure 2</a>. The 3D reconstructions of blocks 1 and 2 show the landmark and semilandmark configurations (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0081287#pone.0081287.s001" target="_blank">Figure S1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0081287#pone.0081287.s003" target="_blank">Table S1</a> for complete description). Note the parallel orientations of trajectories between a) and b) in chimps and between d) and e) in humans demonstrates similar developmental pathways within a different anatomical context. </p

Partial least square analysis.

<p>Plot of the first pair of singular warps (SW1s), accounting for 21.92% of the summed squared covariances between these Procrustes coordinates (p-value=0.005). Humans (dots) and chimpanzees (squares) share the same pattern of covariation between the symphysis and the space at the back of the vocal tract. The thin plate spine deformation grids (exaggerated by factor of 2) show the pattern of independent growth. From the Procrustes mean shape of the pooled sample towards the negative SW1 scores, a convex symphyseal middle axis (mental prominence) correlates with a narrowed pharynx, a relatively short anterior cranial base, a backward upper mid-face and a globular tongue. Conversely, towards the positive SW1 scores, a concave symphyseal middle axis is associated with a broad pharynx, a relatively long anterior cranial base, a forward upper mid-face, and a flatter tongue. r: Pearson product-moment correlation coefficient between the first pair of singular warps.</p

Short Faces, Big Tongues: Developmental Origin of the Human Chin

<div><p>During the course of human evolution, the retraction of the face underneath the braincase, and closer to the cervical column, has reduced the horizontal dimension of the vocal tract. By contrast, the relative size of the tongue has not been reduced, implying a rearrangement of the space at the back of the vocal tract to allow breathing and swallowing. This may have left a morphological signature such as a chin (mental prominence) that can potentially be interpreted in <i>Homo</i>. Long considered an autopomorphic trait of <i>Homo sapiens</i>, various extinct hominins show different forms of mental prominence. These features may be the evolutionary by-product of equivalent developmental constraints correlated with an enlarged tongue. In order to investigate developmental mechanisms related to this hypothesis, we compare modern 34 human infants against 8 chimpanzee fetuses, whom development of the mandibular symphysis passes through similar stages. The study sets out to test that the shared ontogenetic shape changes of the symphysis observed in both species are driven by the same factor – the space restriction at the back of the vocal tract and the associated arrangement of the tongue and hyoid bone. We apply geometric morphometric methods to extensive three-dimensional anatomical landmarks and semilandmarks configuration, capturing the geometry of the cervico-craniofacial complex including the hyoid bone, tongue muscle and the mandible. We demonstrate that in both species, the forward displacement of the mental region derives from the arrangement of the tongue and hyoid bone, in order to cope with the relative horizontal narrowing of the oral cavity. Because humans and chimpanzees share this pattern of developmental integration, the different forms of mental prominence seen in some extinct hominids likely originate from equivalent ontogenetic constraints. Variations in this process could account for similar morphologies.</p> </div