When Texas A&M University’s Cody Prang was taking his first biological anthropology course as an undergraduate at the University of Delaware in the fall of 2009, the first analyses of Ardipithecus were published. “It’s about placing in context what we know about the morphology of the last common ancestor of humans and chimpanzees, which is a long-standing question in evolutionary anthropology.” “This is a nice, complete study of the hand of Ardipithecus, but it’s more than that,” says Campbell Rolian, a biological anthropologist at the University of Calgary who did not participate in the work. But a new analysis of the hand of Ardipithecus, published today (February 24) in Science Advances, indicates that the hominin was likely capable of swinging beneath the branches of trees, as chimps do today. In previous work, researchers deduced that Ardipithecus moved through the trees over the tops of branches, but didn’t swing from them, meaning that the last common ancestor of chimps and people probably moved in this way as well. One of the best clues about the last common ancestor of people and chimps is the oldest discovered hominin skeleton, that of a human ancestor named Ardipithecus ramidus that lived about 4.4 million years ago. The chimpanzee ( Pan troglodytes) is the closest living relative of humans, with the two species diverging about 6 million years ago. At present, no single skeletal feature can be safely relied upon as an indicator of distinctively human capabilities for precision gripping or tool making in fossil hominids.ABOVE: According to a new study, the hands of the hominin Ardipithecus are much more like chimpanzee hands than human hands (illustrated here). However, functional and behavioral implications of Sterkfontein and Swartkrans hand morphology are less clear. Also, Olduvai hand morphology strongly suggests a capacity for stone tool making. Examination of evidence for these reveals that three of the eight features occur in Australopithecus afarensis, but limited thumb mobility would have compromised tool making. Among these features are substantially larger moment arms for intrinsic muscles that stabilize the proximal thumb joints. (3) Morphological studies reveal eight featured distinctive of modern humans which facilitate use of these grips. (A connection is not found between the "fine" thumb/index finger pad precision grip and early tool making.) (2) Manipulative behavior studies of chimpanzees, hamadryas baboons, and human show that human precision grips are distinguished by the greater force with which objects may be secured by the thumb and fingers of one hand (precision pinching) and the ability to adjust the orientation of gripped objects through movements at joints distal to the wrist (precision handling). Findings from a three-pronged investigation answer this question in the affirmative, as follows: (1) Experimental manufacture of early prehistoric tools provides evidence of connections between distinctive human precision grips and effective tool making. This study asks whether there are discernable links between precision gripping, tool behaviors, and hand morphology in modern hominoids, which may guide functional interpretation of early hominid hand morphology.
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