Morphometrics and hominoid phylogeny: Support for a chimpanzee-human clade and differentiation among great ape subspecies

Abstract

Taxonomic and phylogenetic analyses of great apes and humans have identified two potential areas of conflict between molecular and morphological data: phylogenetic relationships among living species and differentiation of great ape subspecies. Here we address these problems by using morphometric data. Three-dimensional landmark data from the hominoid temporal bone effectively quantify the shape of a complex element of the skull. Phylogenetic analysis using distance-based methods corroborates the molecular consensus on African ape and human phylogeny, strongly supporting a Pan-Homo clade. Phenetic differentiation of great ape subspecies is pronounced, as suggested previously by mitochondrial DNA and some morphological studies. These results show that the hominoid temporal bone contains a strong phylogenetic signal and reveal the potential for geometric morphometric analysis to shed light on phylogenetic relationships.

Fig. 1.

Phylograms representing preferred trees. (a) NJ tree using female means. (b) NJ tree using male means. (c) LS tree using female means. (d) LS tree using male means. The topology of a, b, and d is the same as the molecular phylogeny for these taxa, whereas that of c differs as described in the text. Analyses of pooled-sex means result in branching patterns identical to analyses of female means.

Fig. 2.

Principal components analysis of Procrustes coordinates for all hominoid males and females (see also ref. 20). The analysis was done by using full tangent space projection in morphologika (21). The first principal component (39.4% of the total variance) primarily distinguishes humans from apes, whereas the second (12.8%) distinguishes chimpanzees and bonobos from gorillas and orangutans. Both of these axes are important in supporting the Pan–Homo clade, because similarities between gorillas and orangutans are reconstructed as primitive (the third principal component separates gorillas and orangutans). This plot also illustrates the positions estimated for the common ancestor of Homo and Pan (B) and the common ancestor of African hominoids (A). Values for these ancestral nodes were estimated only for illustrative purposes by using linear parsimony analysis of female operational taxonomic unit means. The values show that a shift from the primitive condition to the derived Pan–Homo condition involves change in characters related to both principal components 1 and 2. Some of the characters that influence these axes are reduced projection of the entoglenoid process, medial position of the temporomandibular joint, less projecting postglenoid process, a shorter tympanic element, and reduced relative size of the temporomandibular joint (20).