Hominoid Scapular Morphology Suggests a Generalized Last Common Ancestor.

African apes have traditionally been considered simple size vicars of one another, and when combined with an assumption of parsimony, have led many to conclude that the last common ancestor (LCA) of hominoids was African ape-like. This conclusion now stands in contrast to the current hominid fossil record, which also suggests that the LCA postcranium was more generalized than that of any living ape. To investigate this contradiction we conducted four discriminant function analyses (DFAs) of hominoid scapulae that also included a broad sample of anthropoids. To limit size issues, we used five scapular angles that reflect overall shape. All DFAs yielded highest loadings for spine orientation, glenoid-vertebral border position, spine-axillary border angle, and inferior angle. At least 74% of cases were correctly assigned. In the initial DFA, apart from a single gorilla specimen grouped as a chimpanzee, African apes and humans were entirely distinct from one another. We then conducted three DFAs in which gorillas, chimpanzees, and humans were each initially unclassified, and then assigned to the distribution post hoc . This procedure identified only one gorilla as a chimpanzee, whereas 16 of 29 were categorized as Lagothrix . All 29 chimpanzees were grouped as Ateles . No human was categorized as an African ape, whereas 15 of 28 were classified as Alouatta . Chimpanzees were similar to Ateles in having a more cranially oriented glenoid, more highly angled spine, and narrower scapula than do gorillas, which are more similar to the predominantly clambering Lagothrix . Group centroids also placed gorillas closer to humans, suggesting that both likely lie closer than chimpanzees to a generalized LCA for extant hominoids, consistent with the postcranium of Ardipithecus ramidus .