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The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking

C. O. Lovejoy et al.

Virtually no other primate has a human-like pelvic girdle—not even our closest living relatives, the chimpanzee and bonobo. Such uniqueness evolved via substantial modifications of a pelvis more originally suited for life in trees. This arboreal primate heritage has left us rather ungainly. Our legs are massive because they continue to house almost all of the muscles originally required for climbing. Our hamstrings, the large muscles in our posterior thighs, must decelerate the swinging limb with each step, and when we run, the limb’s inertia is sometimes too great and these muscles fail (not something one would want to happen on a savanna).

Furthermore, when each limb leaves the ground to be swung forward, it and the pelvis are unsupported and would slump toward the ground were it not for muscles acting on the opposite side of the body (the anterior gluteals). One early anthropologist described human locomotion as a process by which we alternately almost fall on our faces. Chimpanzees and other primates cannot prevent such slumping when walking upright because they cannot reposition these muscles effectively. Their spine is too inflexible and their ilia—the large pelvic bones to which the gluteals attach—are positioned and shaped differently than ours. Modifying a typical chimp or gorilla pelvis to facilitate upright walking would require extensive structural changes.

Until now, the fossil record has told us little about when and how the early hominid pelvis evolved. Even 3 to 4 million years ago (when our brains were still only slightly larger than those of chimpanzees), it had already undergone radical transformation. One of the oldest hominid pelves, that of Australopithecus afarensis (A.L. 288-1; “Lucy”), shows that her species had already evolved virtually all of the fundamental adaptations to bipedality. Even the kinetics of her hip joint were similar to ours. Although the human pelvis was later further reshaped, this was largely the result of our much enlarged birth canal.

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The Ar. ramidus pelvis has a mosaic of characters for both bipedality and climbing. Left to right: Human, Au. afarensis (“Lucy”), Ar. ramidus, Pan (chimpanzee). The ischial surface is angled near its midpoint to face upward in Lucy and the human (blue double arrows), showing that their hamstrings have undergone transformation for advanced bipedality, whereas they are primitive in the chimpanzee and Ar. ramidus (blue arrows). All three hominid ilia are vertically short and horizontally broad, forming a greater sciatic notch (white arrows) that is absent in Pan. A novel growth site [the anterior inferior iliac spine (yellow arrows)] is also lacking in Pan.

Ardipithecus ramidus now unveils how our skeleton became progressively modified for bipedality. Although the foot anatomy of Ar. ramidus shows that it was still climbing trees, on the ground it walked upright. Its pelvis is a mosaic that, although far from being chimpanzee-like, is still much more primitive than that of Australopithecus.

The gluteal muscles had been repositioned so that Ar. ramidus could walk without shifting its center of mass from side to side. This is made clear not only by the shape of its ilium, but by the appearance of a special growth site unique to hominids among all primates (the anterior inferior iliac spine). However, its lower pelvis was still almost entirely ape-like, presumably because it still had massive hindlimb muscles for active climbing.

Changes made in the upper pelvis rendered Ar. ramidus an effective upright walker. It could also run, but probably with less speed and efficiency than humans. Running would also have exposed it to injury because it lacked advanced mechanisms such as those that would allow it to decelerate its limbs or modulate collision forces at its heel. Australopithecus, which had given up its grasping foot and abandoned active climbing, had evolved a lower pelvis that allowed it to run and walk for considerable distances.

Ar. ramidus thus illuminates two critical adaptive transitions in human evolution. In the first, from the human-chimp last common ancestor to Ardipithecus, modifications produced a mosaic pelvis that was useful for both climbing and upright walking. In the second, from Ardipithecus to Australopithecus, modifications produced a pelvis and lower limb that facilitated more effective upright walking and running but that were no longer useful for climbing. Because climbing to feed, nest, and escape predators is vital to all nonhuman primates, both of these transitions would likely have been a response to intense natural selection.

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