National Geographic : 2010 Jul
in upright walking, long the hallmark of our lineage. Ardi's big toe instead splayed out to the side, like those of apes---the better to grasp on to limbs when clambering about in the trees. Yet Ardi's foot also contains a small bone called the os peroneum---retained in the hominid lineage from ancient apes and monkeys but almost never seen in chimps and gorillas---that keeps the bottom of the foot more rigid. Lovejoy and his colleagues believe that this rigidity enabled Ar. ramidus to walk upright on the ground, using its four aligned toes to provide the levering "toe off" that propels a bipedal stride. Ardi's pelvis also bears witness to a primitive primate caught in the act of becoming human. The human pelvis has undergone a major overhaul to adapt it for upright walking---a locomotor juggling act requiring one limb or the other to be suspended in the air while the other pushes forward. As far back as Lucy, 3.2 million years ago, our hip bones had become broader and shorter to enlarge attach- ment areas for gluteal muscles that stabilize the supporting hip joint. In contrast, chimp pelvises are narrow and long and provide more rigid support for climbing but force chimps to lurch side to side when walking upright. Ardi's upper pelvis is short and broad and shows other features rarely seen ex- cept in hominids, such as a protrusion on the inside edge of the pelvis where bone was added during development to bolster support for a bipedal stride. Yet the lower pelvis is thoroughly apelike, with at- tachments for massive hind-limb muscles needed for effective climbing. Then there is Ardi's surprising hand. Living Afri- can apes have long fingers and palms adapted to arboreal climbing and strong, stiff joints in their hands to support their weight on their knuckles when they walk on the ground. Since this knuckle- walking adaptation is seen not only in chimps but also in gorillas, which separated from our lineage even farther in the past, it has long been thought that it represents the primitive condition that our own ancestors passed through on their way to walking upright. Ardi's hand utterly confounds that assumption. Though her fingers are long, her palm is short and very flexible. This would have allowed her to walk on her palms on top of tree limbs, more Ardi's foot features an opposable big toe: well suited for grasping branches but a poor arrangement for push off in a bipedal stride, as in later hominids. According to her discoverers, the other four toes carried that load. Moving in the trees was aided by long fingers and highly flexible wrists.