In addition, human toe I is larger than the lesser toes, whereas monkey toes are similar in size. Monkey hand, foot and human hand are similar in shape except for the monkey heel (grey). Both monkey ( Macaca fuscata, left) and human ( Homo sapiens, right) have five physically independent fingers (top) and five toes (bottom), although the human foot is irregular in shape. The shape of the hand and foot in two primate species. One possible way to throw new light on this conundrum is to infer limb adaptation and concomitant neurophysiological changes from a more primitive ancestral model than living apes, and to compare the cortical representation of the hands and feet of humans with that of Old World monkeys. This paradox of the chimpanzee (and other extant apes) hand being apparently too specialized for the LCA is known in comparative anatomy as ‘the riddle of man's ancestry’. In contrast to the hand, the feet of both apes and monkeys differ from the bipedal human foot ( figure 1). Old World monkeys, however, have hands that are in many respects more similar to the human hand than are those of the chimpanzees, even though they diverged from humans much earlier. They lacked features related to advanced tool use, such as a robust thumb and other anatomical changes associated with human-like manual dexterity, widely considered to have occurred after approximately 2.5 Ma. Relative to a chimpanzee-like LCA, species of Australopithecus would have had a greatly shortened palm and fingers relative to the thumb, but nevertheless retained aspects of an ape-like hand for arboreal climbing. If this is correct, it means that some drastic morphological changes occurred in hominins (all species on the human side of the human–chimpanzee split), a view which derives from observations of modern humans and apes, and hominin fossils dated approximately 4.2 million years ago (Ma) and younger. Recent studies that model the hand of the last common ancestor (LCA) of humans and chimpanzees suggest that it was similar to those of a chimpanzee or a modern African ape. We have limited information regarding our early ancestors, however, and the evolutionary path of the human hand is not well known. The early human ancestral lineage(s) has, therefore, commonly been evaluated with respect to the extant chimpanzee. With the advent of genome-wide molecular studies, chimpanzees have been shown to be genetically the most similar, and phylogenetically closest, to humans (and equally so is the bonobo). This accords with the known fossil evidence, including the recently reported hominin fossils which have been dated to 4.4 million years ago. In hominins, a separate adaptation, involving the neural separation of the big toe, apparently occurred with bipedality. This early adaptation laid the foundation for the evolution of manual dexterity, which was preserved and enhanced in hominins. These observations suggest that the brain circuits for the hand had advanced beyond simple grasping, whereas our primate ancestors were still general arboreal quadrupeds. Humans, by contrast, had an independent neurological representation of the big toe (hallux), suggesting association with bipedal locomotion. In the monkeys, the somatotopic representation of the toes was fused, showing that the digits function predominantly as a unit in general grasping. The neural mapping of the subjects’ toes differed, however. This reflects the ability to use each digit independently, as required for the complex manipulation involved in tool use. In both humans and monkeys, we found that each finger was represented separately in the primary sensorimotor cortex just as they are physically separated in the hand. Contrary to the ‘hand-in-glove’ notion outlined above, our results suggest that adaptations underlying tool use evolved independently of those required for human bipedality. In this study, we sought to shed new light on the origins of manual dexterity and bipedalism by mapping the neural representations in the brain of the fingers and toes of living people and monkeys. Either way, it is commonly thought that one led to the other. People have long speculated whether the evolution of bipedalism in early hominins triggered tool use (by freeing their hands) or whether the necessity of making and using tools encouraged the shift to upright gait.
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