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. 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. 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. “There is growing evidence that the emergence of the genus Homo did not result from the emergence of entirely new behaviors but rather from the accentuation of traits already present in Australopithecus, including tool making and meat consumption”, says Jean-Jacques Hublin, director at the Max Planck Institute for Evolutionary Anthropology.Human hands are distinguished from apes by possessing longer thumbs relative to fingers. These results support previously published archaeological evidence for stone tool use in australopiths and provide skeletal evidence that our early ancestors used human-like hand postures much earlier and more frequently than previously considered. “This new evidence changes our understanding of the behaviour of our early ancestors and, in particular, suggests that in some aspects they were more similar to humans than we previously thought”, says Matthew Skinner of the Max Planck Institute for Evolutionary Anthropology and the University of Kent. The research shows that Australopithecus africanus, a three to two million-year-old species from South Africa traditionally considered not to have engaged in habitual tool manufacture, has a human-like trabecular bone pattern in the bones of the thumb and palm (the metacarpals) consistent with forceful opposition of the thumb and fingers typically adopted during tool use. This unique human pattern is present in known non-arboreal and stone tool-making fossil human species, such as Neandertals. They found clear differences between humans, who have a unique ability for forceful precision gripping between thumb and fingers, and chimpanzees, who cannot adopt human-like postures. The researchers first examined the trabeculae of hand bones of humans and chimpanzees. “Over time these structures adapt in a way that enables them to handle the daily loads in the best way possible“, says Dieter Pahr of the Institute of Lightweight Design and Structural Biomechanics at the Vienna University of Technology where special computer algorithms for the analysis of the computer tomography images of the bones had been developed. Trabecular bone remodels quickly during life and can reflect the actual behaviour of individuals in their lifetime. Matthew Skinner and Tracy Kivell of the Max Planck Institute for Evolutionary Anthropology and the University of Kent used new techniques to reveal how fossil species were using their hands by examining the internal spongey structure of bone called trabeculae. However, it is unclear when these locomotory and manipulative transitions occurred. The distinctly human ability for forceful precision (e.g., when turning a key) and power “squeeze” gripping (e.g., when using a hammer) is linked to two key evolutionary transitions in hand use: a reduction in arboreal climbing and the manufacture and use of stone tools.
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