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Combining Prehension and Propulsion: The Foot of Ardipithecus ramidus

C. O. Lovejoy et al.

The special foot adaptations that enable humans to walk upright and run are central to understanding our evolution. Until the discovery of Ardipithecus ramidus, it was generally thought that our foot evolved from one similar to that of modern African apes. Apes have feet that are modified to support their large bodies and to facilitate vertical climbing, thus allowing them to feed, nest, and seek safety in trees. Our foot differs from theirs in myriad ways, and its evolution from theirs would consequently have required an extensive series of structural changes. Some mid–20th-century comparative anatomists were so impressed with the profound differences between human and extant ape feet that they postulated a deep, pre-ape origin for hominids.

Ar. ramidus brings a new perspective to this old controversy. Its foot turns out to be unlike those of the African apes in many ways. The partial skeleton of Ar. ramidus preserves most of the foot and includes a special bone called the os peroneum that is critical for understanding foot evolution. This bone, which is embedded within a tendon, facilitates the mechanical action of the fibularis longus, the primary muscle that draws in the big toe when the foot is grasping. Until now, we knew little about this bone’s natural history, except that it is present in Old World monkeys and gibbons but generally not in our more recent ape relatives. Monkeys are very accomplished at leaping between trees. They must keep their feet fairly rigid during takeoff when they hurl themselves across gaps in the tree canopy; otherwise, much of the torque from their foot muscles would be dissipated within the foot rather than being transferred to the tree.

The African apes are too large to do much leaping. They have therefore given up the features that maintain a rigid foot and have instead modified theirs for more effective grasping—almost to the point of making it difficult to distinguish their feet from their hands. Indeed, very early anatomists argued that the “quadrumanus” apes were not related to humans because of their hand-like feet. Extant apes lack the os peroneum, and their fibularis tendon, which draws the great toe closed during grasping, has been relocated more toward the front of the foot. This makes the tendon run more parallel to other joints that cross the midfoot, and allows apes to grasp with great power without stiffening these other flexible joints. Apes can thus both powerfully grasp and mold their feet around objects at the same time. However, their feet have become less effective as levers, making them far less useful in terrestrial propulsion.

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Foot skeleton of Ar. ramidus (bottom; reconstruction based on computed tomography rendering shown) lacked many features that have evolved for advanced vertical climbing and suspension in extant chimpanzees (pan, top left). Chimpanzees have a highly flexible midfoot and other adaptations that improve their ability to grasp substrates. These are absent in Ar. ramidus. Credits: Reconstruction; copyright J. H. Matternes; chimpanzee climbing, J. DeSilva; bonobo and human feet, S. Ingham.

The foot of Ar. ramidus shows that none of these ape-like changes were present in the last common ancestor of African apes and humans. That ancestor, which until now has been thought to be chimpanzee-like, must have had a more monkey-like foot. Not only did it still have an os peroneum, it must also have had all of the other characteristics associated with it (subsequently abandoned in chimpanzees and gorillas). We infer this because humans still have these characteristics, so we must have retained them from our last common ancestor. The mid–20th-century anatomists were correct to worry about the human foot as they did: Ours turns out to have evolved in one direction, while those of African apes were evolving in quite another.

One of the great advantages of our more rigid foot is that it works much better as a lever during upright walking and running (as it also does in monkeys). However, Ar. ramidus still had an opposable big toe, unlike any later hominid. Its ability to walk upright was therefore comparatively primitive. Because it had substantially modified the other four toes for upright walking, even while retaining its grasping big toe, the Ardipithecus foot was an odd mosaic that worked for both upright walking and climbing in trees. If our last common ancestor with the chimpanzee had not retained such an unspecialized foot, perhaps upright walking might never have evolved in the first place.

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