Abstract
(July 14, 2015) Human
hands are distinguished from apes by possessing longer thumbs relative to
fingers. However, this simple ape-human dichotomy fails to provide an adequate
framework for testing competing hypotheses of human evolution and for reconstructing
the morphology of the last common ancestor (LCA) of humans and chimpanzees. We
inspect human and ape hand-length proportions using phylogenetically informed
morphometric analyses and test alternative models of evolution along the
anthropoid tree of life, including fossils like the plesiomorphic ape Proconsul
heseloni and the hominins Ardipithecus ramidus and Australopithecus sediba. Our
results reveal high levels of hand disparity among modern hominoids, which are
explained by different evolutionary processes: autapomorphic evolution in
hylobatids (extreme digital and thumb elongation), convergent adaptation
between chimpanzees and orangutans (digital elongation) and comparatively
little change in gorillas and hominins. The human (and australopith) high
thumb-to-digits ratio required little change since the LCA, and was acquired
convergently with other highly dexterous anthropoids.