1Center for the Study of Human Origins, Department of Anthropology, New York University, 2New York Consortium for Evolutionary Primatology, NYCEP, 3Evolutionary Studies Institute and Centre for Excellence in PalaeoSciences, University of the Witwatersrand, 4Paleoanthropology Group, Museo Nacional de Ciencias Naturales (MNCN-CSIC), 5Biology Department, Universidad Autónoma de Madrid, 6Department of Anthropology, Chaffey College, 7Department of Human Anatomy and Physiology, University of Johannesburg, 8Anthropological Institute and Museum, University of Zurich, 9Department of Anthropology, University of Wisconsin-Madison, 10Department of Evolutionary Anthropology, Duke University
April 16, 2016 9:45, Imperial Ballroom A
We describe the axial skeletal material recovered from the Dinaledi chamber of Rising Star cave that, together with some pelvis and shoulder remains, document the complicated nature of trunk evolution. The axial material includes two near-complete lower thoracic vertebrae found in articulation with an 11th rib, the proximal portion of a 12th rib, and 80 other rib and vertebral fragments. Proximal aspects of the two lower ribs are straight (uncurved), suggesting a broad, australopith-like lower ribcage. The thoracic vertebrae are amongst the smallest yet encountered in the hominin fossil record, but are characterized by large neural canals and moderately-proportioned centra, consistent with opposite scaling trends (negative and positive allometry, respectively) of these features in small-bodied Homo. Given the derived aspects of cranio-dental, hand, and foot morphologies and limb lengths, we suggest that the primitive appearance of the ribcage and other anatomical features (e.g., pelvic breadth, glenoid orientation) either 1) reflects the evolutionary retention of australopith-like features coupled with more derived (Homo-like) aspects of lower rib and thoracic vertebra shape and/or 2) points to the importance of scaling relations among respiratory system elements and other trunk morphologies that may correspond with possible size-related differences recently identified in small- and large-bodied Australopithecus afarensis. In combination with some features that are most similar to Middle Pleistocene hominins (to the exclusion of modern humans), such as robust lower rib cross sections and dorsally sweeping transverse processes, we entertain multiple evolutionary scenarios to explain the presence of this unique combination of morphologies in H. naledi.
MB and DGM recognize CGL2012-27379 (MINECO, Spain) for funding.