These spines were made for walking.

Human ancestors that lived in Africa around 3 million years ago possessed backbones like those of people today and thus walked much as we now do, says Carol V. Ward of the University of Missouri-Columbia.

Ward and Bruce Latimer of the Cleveland Museum of Natural History analyzed the anatomy of spinal columns from the 3.2-million-year-old Australopithecus afarensis skeleton known as Lucy and a pair of roughly 2.5-million-year-old Australopithecus africanus specimens.

As in modern people, the spines of the three australopithecines bend inward at the middle of the back and curve outward at the lower back. A bony column angled in this fashion positions the torso directly over the hip joints, fostering erect posture and a two-legged gait, Ward says. The shapes of australopithecine vertebrae also correspond closely to those of modern humans, the researchers found.

Even with an upright stance, Lucy and her kind may have spent much time in trees. A. afarensis' short legs, relative to its upper body, drained energy during walking or running but boosted climbing power, contends Karen Steudel-Numbers of the University of Wisconsin-Madison. Climbing skill must have been crucial to A. afarensis, she proposes, because the species retained short legs throughout its 1-million-year evolutionary history.

In laboratory studies, her team found that short-legged people consume substantially more oxygen while walking or running than long-legged people do. Steudel-Numbers estimates that australopithecines required an average of 30 percent more energy to walk a given distance than people do today.--B.B.