![]() In these features, humans were argued to more closely resemble cercopithecoid primates than hominoids ( 10– 12). For example, Pan, Pongo, and Colobus have a markedly reduced thumb, and Ateles and Brachyteles have entirely lost an external thumb ( 8, 11). The most suspensory hominoids, cercopithecoids, and platyrrhines tend to display a reduced pollex and narrow, elongated nonpollical rays. The morphology of the highly dexterous human hand, with its intrinsically elongated first ray (pollex or thumb), shortened metacarpals and nonpollical digits, and hypertrophied thenar muscles, contrasts sharply with that of suspensory adapted anthropoid primates ( 8– 10). Keith ( 6) established the hypothesis that human postcranial anatomy was derived from an orthograde ancestor, later interpreted as a “brachiating” ancestor ( 7), by highlighting the aspects of trunk and limb anatomy shared among humans and other hominoids. The recognition that humans are closely related to African apes influenced the range of possible explanations for bipedalism by raising questions about the kind of ancestor from which the human bauplan was derived ( 1). Numerous adaptive explanations for bipedalism rely on an understanding of our place in nature. The morphology and inferred positional behavior of the last common ancestor (LCA) of humans, chimpanzees, and bonobos (hereafter, “LCA”) are critical for understanding the evolution of hominin bipedalism ( 1– 5). Overall, our results suggest that early hominins evolved from an ancestor with a varied positional repertoire including suspension and vertical climbing, directly affecting the viable range of hypotheses for the origin of our lineage. We identify an evolutionary shift in hand morphology between Ardipithecus and Australopithecus that renews questions about the coevolution of hominin manipulative capabilities and obligate bipedalism initially proposed by Darwin. Here, we use morphometric and phylogenetic comparative methods to show that Ardipithecus retains suspensory adapted hand morphologies shared with chimpanzees and bonobos. However, the hand of the 4.4-million-year-old hominin Ardipithecus ramidus purportedly provides evidence that the hominin hand was derived from a more generalized form. Early 20th century anatomical research supported the view that humans evolved from a suspensory ancestor bearing some resemblance to apes. The researchers also identified examples of convergent evolution - in which similar traits develop independently in different lineages - including finger elongation in both chimpanzees and orangutans, and high thumb-to-finger ratios among human ancestors and “other highly dexterous anthropoids such as capuchins and gelada baboons.The morphology and positional behavior of the last common ancestor of humans and chimpanzees are critical for understanding the evolution of bipedalism. The results, reported in Nature Communications, show that the finger and thumb lengths of human hands have actually changed little since the human-chimp LCA, countering the notions that the LCA’s hands were more chimp-like and that higher thumb-to-finger ratios only emerged in later human ancestors. Combining the data with phylogenetic models, they pieced together a map of evolutionary change in primate hands. Sergio Almécija of Stony Brook University in New York and colleagues made detailed measurements of hand proportions among humans, apes and other living primates, as well as of fossil species like the primitive African ape Proconsul heseloni and the early human ancestors Ardipithecus ramidus and Australopithecus sediba, noting a wide range of hand dimensions among modern hominoids. But rather than humans having the more evolved hand - a prevailing hypothesis since the late 20th century - a new study suggests that chimps' hands, with lower thumb-to-finger-length ratios, have changed considerably more. This trait has endowed our ancestors and us with a particular talent for grasping and working with tools, which likely contributed to our evolutionary success over the last several million years since splitting off from the last common ancestor (LCA) shared by the two groups. ![]() Credit: Almécija et al., Nature Communications, 2015, CC BY 4.0.Ĭompared to chimpanzees, our recent evolutionary cousins, humans have long thumbs relative to our fingers. With longer fingers, chimpanzees have lower thumb-to-finger-length ratios than humans. ![]()
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