Fossil hominin shoulders support an African ape-like last common ancestor of humans and chimpanzees
Nathan M. Younga,1, Terence D. Capellinib,c, Neil T. Roachb,d, and Zeresenay Alemsegede
aDepartment of Orthopaedic Surgery, University of California, San Francisco, CA 94110;
bDepartment of Human Evolutionary Biology, Harvard University, Cambridge, MA 02138;
cBroad Institute of MIT and Harvard University, Cambridge, MA 02142;
dDivision of Anthropology, American Museum of Natural History, New York, NY 10024;
eDepartment of Anthropology, California Academy of Sciences, San Francisco, CA 94118
Edited by Richard G. Klein, Stanford University, Stanford, CA, and approved August 12, 2015 (received for review June 9, 2015)
Significance
Knowing the direction and pace of evolutionary change is critical to understanding what selective forces shaped our ancestors. Unfortunately, the human fossil record is sparse, and little is known about the earliest members of our lineage. This unresolved ancestor complicates reconstructions of what behavioral shifts drove major speciation events. Using 3D shape measurements of the shoulder, we tested competing evolutionary models of the last common ancestor against the fossil record. We found that a sustained shift in scapular shape occurred during hominin evolution from an African ape-like ancestor to a modern human-like form, first present in our genus, Homo. These data suggest a long, gradual shift out of the trees and increased reliance on tools as our ancestors became more terrestrial.
Abstract
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.
geometric morphometrics developmental simulation phylomorphospace scapula rotator cuff
Footnotes
1To whom correspondence should be addressed. Email: nathan.m.young{at}gmail.com.
Author contributions: N.M.Y., T.D.C., and N.T.R. designed research; N.M.Y., T.D.C., N.T.R., and Z.A. performed research; Z.A. contributed new reagents/analytic tools; N.M.Y. analyzed data; and N.M.Y., T.D.C., N.T.R., and Z.A. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
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