LETTER
LETTER
An alternative interpretation of the Australopithecus scapula In PNAS, Young et al. (1) conclude that the scapula of the last common ancestor (LCA) of chimpanzees and humans was African apelike, supporting what they call the “African ape” model. This model was favored over the “ape convergence” model, in which the ancestral condition was more primitive and some morphological similarities shared among modern apes would reflect convergent evolution. The authors suggest that less ape-like morphology in Australopithecus afarensis reflects an adaptive trade-off between arboreality and tool use (1). Young et al.’s (1) A. afarensis sample consists of one juvenile specimen, DIK 1-1, from Dikika, Ethiopia. The authors infer adult shape using ontogenetic trajectories of three nonhuman primates, but do not consider the adult scapula of the A. afarensis partial skeleton, KSD-VP-1/1, from Woranso-Mille, Ethiopia. The KSD-VP-1/1 scapula differs from that of African apes in having a less cranial orientation of the spine, which is the feature that best differentiates human scapulas (2–4). It is further similar to human scapulas in features associated with a primarily manipulatory function of the upper limb, including infraspinous fossa expansion and attachment locations of muscles involved in arm elevation (3, 4). Despite the fact that the KSDVP-1/1 scapula lacks the similarity to Gorilla that is reported for DIK 1-1 (2–4), the magnitude of shape difference between the two fossils does not exceed the level observable
www.pnas.org/cgi/doi/10.1073/pnas.1520902112
in living species, suggesting that the true A. afarensis mean lies between the two (4). In their phylogenetic analyses, Young et al. (1) conclude that the African ape model is more parsimonious than the ape convergence model. However, the difference between the two models is small (tree lengths: 0.085 and 0.100). Furthermore, LCA reconstructions under the two models differ primarily in blade shape, a feature with a predominately phylogenetic signal, and they differ negligibly in spine and glenoid orientation, which are the features of interest in assessing the functional hypotheses associated with each model. Neither model reconstructs the LCA with markedly cranial spine and glenoid orientation, indicating that chimpanzees converge with gibbons in this feature. This result is consistent with aspects of the ape convergence model, in which the skeleton of the LCA was less adapted for suspension than in chimpanzees (5). Young et al. (1) show that A. afarensis scapula morphology is derived with respect to the LCA (under both models), and this result would be more pronounced with the inclusion of KSD-VP-1/1. If this is accepted, scapula morphology did not remain static early in hominin evolution. Young et al. invoke a trade-off between primitive and derived functions to explain the slow and sustained pace of morphological change: in their view, continued arboreality explains why Australopithecus scapula morphology differs in some ways from Homo. However,
slow and sustained morphological evolution is equally well-explained by a progressive increase in the selective advantage of Homo-like morphology alone, as tool use came to have a greater impact on hominin fitness. In the broader context of the extensive changes in the A. afarensis postcranial skeleton that reduced arboreal efficiency, derived morphology in the scapula is similarly best understood as indicating committed terrestriality. Stephanie M. Melillo1 Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Leipzig 04103, Germany 1 Young NM, Capellini TD, Roach NT, Alemseged Z (2015) Fossil hominin shoulders support an African ape-like last common ancestor of humans and chimpanzees. Proc Natl Acad Sci USA 112(38): 11829–11834. 2 Haile-Selassie Y, et al. (2010) An early Australopithecus afarensis postcranium from Woranso-Mille, Ethiopia. Proc Natl Acad Sci USA 107(27):12121–12126. 3 Melillo SM (2011) Anatomy of the Australopithecus scapula and evolution of the human shoulder: Insights from a new fossil specimen, KSD-VP-1/1g. PhD dissertation (Stanford University, Stanford, CA). 4 Melillo SM (2016) The shoulder girdle of KSD-VP-1/1. The Postcranial Anatomy of Australopithecus afarensis: New Insights from KSD-VP-1/1, eds Haile-Selassie Y, Su DF (Springer, Dordrecht, The Netherlands). 5 Lovejoy CO, Suwa G, Simpson SW, Matternes JH, White TD (2009) The great divides: Ardipithecus ramidus reveals the postcrania of our last common ancestors with African apes. Science 326(5949):100–106.
Author contributions: S.M.M. wrote the paper. The author declares no conflict of interest. 1
Email: [email protected].
PNAS | December 29, 2015 | vol. 112 | no. 52 | E7159
LETTER
An alternative interpretation of the Australopithecus scapula In PNAS, Young et al. (1) conclude that the scapula of the last common ancestor (LCA) of chimpanzees and humans was African apelike, supporting what they call the “African ape” model. This model was favored over the “ape convergence” model, in which the ancestral condition was more primitive and some morphological similarities shared among modern apes would reflect convergent evolution. The authors suggest that less ape-like morphology in Australopithecus afarensis reflects an adaptive trade-off between arboreality and tool use (1). Young et al.’s (1) A. afarensis sample consists of one juvenile specimen, DIK 1-1, from Dikika, Ethiopia. The authors infer adult shape using ontogenetic trajectories of three nonhuman primates, but do not consider the adult scapula of the A. afarensis partial skeleton, KSD-VP-1/1, from Woranso-Mille, Ethiopia. The KSD-VP-1/1 scapula differs from that of African apes in having a less cranial orientation of the spine, which is the feature that best differentiates human scapulas (2–4). It is further similar to human scapulas in features associated with a primarily manipulatory function of the upper limb, including infraspinous fossa expansion and attachment locations of muscles involved in arm elevation (3, 4). Despite the fact that the KSDVP-1/1 scapula lacks the similarity to Gorilla that is reported for DIK 1-1 (2–4), the magnitude of shape difference between the two fossils does not exceed the level observable
www.pnas.org/cgi/doi/10.1073/pnas.1520902112
in living species, suggesting that the true A. afarensis mean lies between the two (4). In their phylogenetic analyses, Young et al. (1) conclude that the African ape model is more parsimonious than the ape convergence model. However, the difference between the two models is small (tree lengths: 0.085 and 0.100). Furthermore, LCA reconstructions under the two models differ primarily in blade shape, a feature with a predominately phylogenetic signal, and they differ negligibly in spine and glenoid orientation, which are the features of interest in assessing the functional hypotheses associated with each model. Neither model reconstructs the LCA with markedly cranial spine and glenoid orientation, indicating that chimpanzees converge with gibbons in this feature. This result is consistent with aspects of the ape convergence model, in which the skeleton of the LCA was less adapted for suspension than in chimpanzees (5). Young et al. (1) show that A. afarensis scapula morphology is derived with respect to the LCA (under both models), and this result would be more pronounced with the inclusion of KSD-VP-1/1. If this is accepted, scapula morphology did not remain static early in hominin evolution. Young et al. invoke a trade-off between primitive and derived functions to explain the slow and sustained pace of morphological change: in their view, continued arboreality explains why Australopithecus scapula morphology differs in some ways from Homo. However,
slow and sustained morphological evolution is equally well-explained by a progressive increase in the selective advantage of Homo-like morphology alone, as tool use came to have a greater impact on hominin fitness. In the broader context of the extensive changes in the A. afarensis postcranial skeleton that reduced arboreal efficiency, derived morphology in the scapula is similarly best understood as indicating committed terrestriality. Stephanie M. Melillo1 Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Leipzig 04103, Germany 1 Young NM, Capellini TD, Roach NT, Alemseged Z (2015) Fossil hominin shoulders support an African ape-like last common ancestor of humans and chimpanzees. Proc Natl Acad Sci USA 112(38): 11829–11834. 2 Haile-Selassie Y, et al. (2010) An early Australopithecus afarensis postcranium from Woranso-Mille, Ethiopia. Proc Natl Acad Sci USA 107(27):12121–12126. 3 Melillo SM (2011) Anatomy of the Australopithecus scapula and evolution of the human shoulder: Insights from a new fossil specimen, KSD-VP-1/1g. PhD dissertation (Stanford University, Stanford, CA). 4 Melillo SM (2016) The shoulder girdle of KSD-VP-1/1. The Postcranial Anatomy of Australopithecus afarensis: New Insights from KSD-VP-1/1, eds Haile-Selassie Y, Su DF (Springer, Dordrecht, The Netherlands). 5 Lovejoy CO, Suwa G, Simpson SW, Matternes JH, White TD (2009) The great divides: Ardipithecus ramidus reveals the postcrania of our last common ancestors with African apes. Science 326(5949):100–106.
Author contributions: S.M.M. wrote the paper. The author declares no conflict of interest. 1
Email: [email protected].
PNAS | December 29, 2015 | vol. 112 | no. 52 | E7159
