Collaborative Research: Comparative morphological analysis of the hand and wrist in Ardipithecus ramidus and Miocene hominoids

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The 4.4 million year old fossil species Ardipithecus ramidus may represent one of the oldest human ancestors, and it holds potentially critical insights into the initial evolutionary divergence of humans and chimpanzees from their last common ancestor. Establishing the primitive condition (or 'starting point') for that divergence would have transformative implications for nearly every aspect of research on human origins and evolutionary biology. Unexpectedly, the skeleton of Ar. ramidus has been described as differing from that of the living apes more than had been predicted - instead resembling that of more primitive apes from the early Miocene epoch. In particular, some have argued that the hand and wrist retain features related to cautious climbing and palmigrady (use of an extended wrist and weight-bearing palm, adapted for moving above branches). Much of the anatomy, however, has not been examined in a rigorous, quantitative context, and comparisons with Miocene apes have been limited. The project will analyze Ar. ramidus and a broad comparative sample of extant primates and fossil species using three- dimensional (3D) morphometrics to test the hypotheses that: 1) Ar. ramidus used its hands in a manner inconsistent with advanced vertical climbing, suspension, and/or knuckle-walking as used by living apes; and 2) Ar. ramidus is a hominin (i.e., more closely related to humans than chimpanzees). If these hypotheses are upheld, then models of hominin origins cannot assume a highly suspensory or knuckle- walking ancestor, or one that engaged in vigorous vertical climbing. Consequently, explanations for many human adaptations would have to be drastically modified. All metric data and the generated 3D digital models of most specimens will be available via a public online database or provided to museums housing the original material. Importantly, the 3D models constitute a record of irreplaceable fossils and skeletal material from several domestic and international institutions. This award supports early-career scientists, including one Asian American principal investigator (PI), and one of the co-PIs will foster collaboration between American and Spanish researchers. Finally, the project provides vital opportunities for students to learn skills in anatomical imaging and computer modeling, which are indispensable in the basic sciences and biomedicine.