Facial ontogeny in Neanderthals and modern humans

Abstract

One hundred and fifty years after the discovery of Neanderthals, it is held that this morphologically and genetically distinct human species does not differ from modern Homo sapiens in its craniofacial ontogenetic trajectory after the early post-natal period. This is striking given the evident morphological differences between these species, since it implies that all of the major differences are established by the early post-natal period and carried into adulthood through identical trajectories, despite the extent to which mechanical and spatial factors are thought to influence craniofacial ontogeny. Here, we present statistical and morphological analyses demonstrating that the spatio-temporal processes responsible for craniofacial ontogenetic transformations differ. The findings emphasize that pre-natal as well as post-natal ontogeny are both important in establishing the cranial morphological differences between adult Neanderthals and modern humans.

Figure 1

Ontogenetic form trajectories along PC1 and PC2 for modern humans (orange; subadults, light diamonds; adults, dark squares) and Neanderthals (blue; subadults, light triangles; adults, dark circles). The scatterplot shows divergent post-natal ontogenetic shape trajectories. The rendered surface models show the associated morphological changes from (a) juveniles at scores on PC1: -0.35 and PC2: 0 to (b) adult modern humans (scores on PC1: 0.15 and PC2: 0.025) and (c) Neanderthals (scores on PC1: 0.3 and PC2: −0.1). Note the strong vertical expansion, the relatively narrower and higher rami and chin formation in adult humans. These shape changes contrast with Neanderthals, who show much lower and broader rami, and very strong forward growth of the corpus and even more so at the dental arcade. These ontogenetic changes produce both the retromolar space and a sloping chinless symphysis in adults.

Figure 2

The deformation grids in (a) and (b) show the aspects of post-natal growth and development in humans and Neanderthals as represented by PC1 alone. Both species increase their relative vertical facial height and show an increased angulation between the ramus and the corpus. In (c), the final stage of human ontogeny is shown, which is characterized by a strong supero–inferior expansion of the mandible and a retraction of the dental arcade with respect to the lower part of the symphysis (forming the human chin). In (d), shown are the increasingly forward growth in Neanderthal facial ontogeny, produced by a forward shift of the alveolar process, which is responsible for the typical Neanderthal retromolar space and the sloping symphysis.