One Giant Step For Mankind

Meet your newfound ancestor, a chimplike forest creature that stood up and walked 5.8 million years ago

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The only trouble with this theory is that it's wrong. The earliest humans, it turns out, didn't live in grasslands. Dry climate or not, a companion paper published last week in Nature shows on the basis of the other fossilized flora and fauna, as well as the chemistry of the ancient soil, that Ardipithecus ramidus kadabba lived in a well-forested environment. That's also the case with other extremely ancient hominids found during the past several years, including Ardipithecus ramidus ramidus and a species called Orrorin tugenensis, announced last December by French and Kenyan researchers. And while the ability to walk on two legs probably started out as an increasingly frequent behavior, evolution demands an explanation for why it persisted. On first blush, bipedalism just doesn't make much sense. For our earliest ancestors, it would have been slower than walking on all fours, while requiring the same amount of energy. Says Lovejoy bluntly: "It's unnatural. It's bizarre."

Yet the advantages of walking upright were somehow so great that the behavior endured through thousands of generations. Indeed, the anatomy of our ancestors underwent all sorts of basic changes to accommodate this new way of moving. Many of the changes help the body stay balanced by stabilizing the weight-bearing leg and keeping the upper torso centered over the feet. Lovejoy, who studies the anatomy and biomechanics of locomotion, thinks the changes may have improved coordination as well. "To walk upright in a habitual way, you have to do so in synchrony," he says. "If the ligaments and muscles are out of synch, that leads to injuries. And then you'd be cheetah meat."

By far the most crucial changes, according to Lovejoy, were those in the spine. The distance between chest and pelvis is longer in humans than in apes, allowing the lower spine to curve, which locates the upper body over the pelvis for balance. The pelvis grew broader, meanwhile, and humans developed a hip joint and associated muscles that stabilize the pelvis. Explains Lovejoy: "That's why a chimp sways from side to side as it walks upright and humans don't."

Changes also had to take place in the femur, or thighbone. For example, the femoral neck--the bent portion at the top of the bone--is broader in humans than it is in apes, which improves balance. The human knee is specialized for walking upright too: to compensate for the thighbone's being at an angle, there's a lump, or groove, at the end of the femur that prevents the patella from sliding off the joint. "A chimp doesn't have this groove because there is no angulation between the hip and the knee," Lovejoy says. "This change says you're a biped."

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