Fossil hominin shoulders support an African ape-like last common ancestor of humans and chimpanzees.
Reconstructing the behavioral shifts that drove hominin evolution requires knowledge of the timing, magnitude, and direction of anatomical changes over the past ∼6-7 million years. These reconstructions depend on assumptions regarding the morphotype of the Homo-Pan last common ancestor (LCA). However, there is little consensus for the LCA, with proposed models ranging from African ape to orangutan or generalized Miocene ape-like. The ancestral state of the shoulder is of particular interest because it is functionally associated with important behavioral shifts in hominins, such as reduced arboreality, high-speed throwing, and tool use. However, previous morphometric analyses of both living and fossil taxa have yielded contradictory results. Here, we generated a 3D morphospace of ape and human scapular shape to plot evolutionary trajectories, predict ancestral morphologies, and directly test alternative evolutionary hypotheses using the hominin fossil evidence. We show that the most parsimonious model for the evolution of hominin shoulder shape starts with an African ape-like ancestral state. We propose that the shoulder evolved gradually along a single morphocline, achieving modern human-like configuration and function within the genus Homo. These data are consistent with a slow, progressive loss of arboreality and increased tool use throughout human evolution.
Department of Orthopaedic Surgery, University of California, San Francisco, CA 94110; nathan.m.young@gmail.com.
Nathan M
NM
Young
Department of Human Evolutionary Biology, Harvard University, Cambridge, MA 02138; Broad Institute of MIT and Harvard University, Cambridge, MA 02142;
Terence D
TD
Capellini
Department of Human Evolutionary Biology, Harvard University, Cambridge, MA 02138; Division of Anthropology, American Museum of Natural History, New York, NY 10024;
Neil T
NT
Roach
Department of Anthropology, California Academy of Sciences, San Francisco, CA 94118.
Zeresenay
Z
Alemseged
D000818:Animals; D005580:Fossils; D015186:Hominidae / Q000033:anatomy & histology*; D006801:Humans; D016018:Least-Squares Analysis; D008954:Models, Biological; D002679:Pan troglodytes / Q000033:anatomy & histology*; D010802:Phylogeny; D025341:Principal Component Analysis; D012782:Shoulder / Q000033:anatomy & histology*; D013997:Time Factors
developmental simulation; geometric morphometrics; phylomorphospace; rotator cuff; scapula