1Department of Anatomical Sciences, Stony Brook University, 2Paleoprimatology and Human Paleontology, Institut Català de Paleontologia Miquel Crusafont
March 26, 2015 2:45, Grand Ballroom C
The human hand is a unique region of the postcranial skeleton that appears derived in comparison to “the ape condition” by exhibiting long thumbs relative to the fingers (thereby facilitating enhanced manipulation). However, this ape-human dichotomy is too simplistic and does not provide an adequate framework for interpreting hominin evolution –was there digital reduction, thumb elongation, or both? We test these competing options by analyzing the length proportions of the hand (long bones of the thumb and ray IV) relative to each other and to overall body mass. We also model the evolution of these proportions in apes and humans using maximum likelihood with fossils as calibration points, and including several non-hominoid anthropoids as out-groups.
Our results show that extant apes are heterogeneous in terms of hand and thumb proportions, and these differences are not simply explained by allometric scaling. Moreover, our evolutionary modeling reveals that the last common ancestor of chimpanzees and humans (LCA) exhibited a moderate hand length (closer to humans than to chimpanzees). Under different phylogenetic hypotheses, our results imply that hand elongation was achieved independently and to different degrees in the extant and fossil ape lineages, and through distinct evolutionary pathways (possibly in response to adaptations for suspension). The hypothesis that we favor is that (relative to the LCA) humans have modified their hand proportions just slightly, probably in response to manipulative selective pressures. These results highlight the prevalence of homoplasy and mosaicism in ape and human evolution, and are congruent with the available fossil record.
Funded by the National Science Foundation (NSF-BCS 1316947, NSF-BCS-1317047, NSF-BCS 1317029), the AAPA Professional Development Grant, and the Spanish MINECO (CGL2011-27343).