LETTER Reply to Melillo: Woranso-Mille is consistent with an australopithecine shoulder intermediate between African apes and Homo

In reference to our recent paper (1), Melillo (2) said, Melillo offers that “...the magnitude of slow pace of scapular evolution supports makes three claims: (i) the adult Woranso- shapedifferencebetweenthetwofossilsdoes the continued, but diminished, importance Mille (KSD-VP-1/1) scapula, which we not exceed the level observable in living of arboreality in australopithecines. did not consider in our analyses, suggests species” (2). Consequently, whereas includ- Australopithecus afarensis shoulders may be ing a more complete Woranso-Mille would Nathan M. Younga,1, Terence D. more derived than we report; (ii)ourrecon- inevitably increase the morphospace occu- Capellinib,c, Neil T. Roachb, structions indicate homoplasy, consistent pied by Dikika, the australopithecine shoul- and Zeresenay Alemsegedd with an “ape convergence” model; and (iii) der is still less derived than Homo and aDepartment of Orthopaedic Surgery, Australopithecus shoulder shape is best ex- modern humans. University of California, San Francisco, CA “ ” plained by committed terrestriality and tool Second, support for an African ape model 94110; bDepartment of Human Evolutionary use and not a trade-off with arboreal efficiency. is not contingent upon the absence of ho- Biology, Harvard University, Cambridge, None of these claims alter the findings of our moplasy. As we discuss in our paper, both MA 02138; cBroad Institute of MIT and paper (1), that the australopithecine shoulder is models predict that chimpanzee/gibbon and Harvard University, Cambridge, MA 02142; intermediate between African apes and Homo, human/orangutan similarities in spine orien- d and that the hominin shoulder underwent tation convergently evolved (1). Instead, we and Department of Anthropology, a slow, sustained evolutionary transformation based our conclusion on the well-established California Academy of Sciences, San from an African ape-like last common ances- principle that the simplest explanation is Francisco, CA 94118 tor to modern humans. preferred. Because the ape convergence First, the Dikika juvenile is currently the model posits living hominoids evolved from 1 Young NM, Capellini TD, Roach NT, Alemseged Z (2015) Fossil best-preserved example of A. afarensis shoul- a more primitive shared ancestral morpho- hominin shoulders support an African ape-like last common ancestor der blade anatomy (3), and its adult shape type, similar blade shapes would have had of humans and chimpanzees. Proc Natl Acad Sci USA 112(38): can be modeled using conservative assump- to independently evolve five times: once 11829–11834. 2 Melillo SM (2015) An alternative interpretation of the tions about growth (1, 4). As Melillo notes (2), each between gorillas, chimpanzees/bono- Australopithecus scapula. Proc Natl Acad Sci USA 112:E7159. blade shape has a strong phylogenetic signal bos, and humans, and twice between gibbons 3 Green DJ, Alemseged Z (2012) Australopithecus afarensis scapular ontogeny, function, and the role of climbing in human evolution. (4), yet the Woranso-Mille scapula is missing and orangutans. In contrast, the African ape Science 338(6106):514–517. critical portions of the inferior angle and model predicts one eventtoevolvetheAfrican 4 Young NM (2008) A comparison of the ontogeny of shape supraspinous fossa (5), limiting its utility for apebladeshapefromamoreprimitiveone variation in the anthropoid scapula: Functional and phylogenetic signal. Am J Phys Anthropol 136(3):247–264. ancestral state reconstruction. Although diffi- shared by Asian apes, a much more parsimo- 5 Haile-Selassie Y, et al. (2010) An early Australopithecus afarensis cult to discern from the two published pic- nious explanation. postcranium from Woranso-Mille, Ethiopia. Proc Natl Acad Sci USA tures, spine orientation in Woranso-Mille Finally, we explicitly argue for a scenario in 107(27):12121–12126. 6 Young NM (2006) Function, ontogeny and canalization of shape may be somewhat more lateralized than which both reduced reliance on arboreality variance in the primate scapula. J Anat 209(5):623–636. Dikika, but it also overlaps with African apes and selection on more lateralized activities, in a number of other metrics (5). Such differ- such as tool-use in australopithecines and Author contributions: N.M.Y., T.D.C., N.T.R., and Z.A. wrote ences between Dikika and Woranso-Mille are throwing in later Homo, served as sustained the paper. not surprising, because hominoid scapulae are selective forces acting on shoulder shape The authors declare no conflict of interest. known to be significantly more variable in (1). We do not dispute the importance of 1To whom correspondence should be addressed. Email: nathan.m. shape compared with other primates (6). That terrestrial bipedality, but simply note that the [email protected].

E7160 | PNAS | December 29, 2015 | vol. 112 | no. 52 www.pnas.org/cgi/doi/10.1073/pnas.1521824112 Downloaded by guest on October 2, 2021