Evolution of the hominoid scapula and its implications for earliest hominid locomotion

• Published in: American journal of physical anthropology Vol. 162; no. 4; pp. 682 - 700
• Main Authors: Selby, Michael S., Lovejoy, C. Owen
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.04.2017
• Subjects: Animals; Anthropology, Physical; Biological Evolution; Fossils; glenoid orientation; Hominidae - anatomy & histology; Humans; Hypotheses; Principal Component Analysis; Scapula - anatomy & histology; scapular spine; serratus anterior; vertebral border; vertebral border; serratus anterior; scapular spine; glenoid orientation
• ISSN: 0002-9483; 1096-8644; 2692-7691
• DOI: 10.1002/ajpa.23158

Objectives The higher primate scapula has been subject to many explanations of the putative “adaptive value” of its individual traits. However, the shift from the bone's position in above branch quadrupeds to its more posterolateral position in recent hominoids obviously required fundamental changes to its general form. We hypothesize that most features argued to be individually adaptive are more likely secondary consequences of changes in its fundamental bauplan, a view more consistent with modern developmental biology. Materials and Methods We tested this hypothesis with scapular metrics and angles from a broad anthropoid sample. Results Our results support our hypothesis. Contrary to earlier predictions, vertebral border length differs little relative to body size in anthropoids, inferior angle position primarily reflects mediolateral scapular breadth, and supraspinous and infraspinous fossa sizes largely reflect scapular spine orientation. Suspensory taxa have cranially oriented glenoids, whereas slow clamberers and humans do not. Australopithecus most closely resembles the latter. Discussion Most scapular features can be explained by only two primary changes: (1) reduction in mediolateral breadth and (2) change in the glenoid position relative to the vertebral border with increased reliance on suspension, which led to a more cranially angled scapular spine. Virtually all other scapular traits appear to be byproducts of these two changes. Based on fossil morphology, hominids1 were derived from a last common ancestor primarily adapted for clambering and not for suspension. Scapular form in early hominids such as Australopithecus is therefore primitive and largely reflects the genus's general clambering heritage.