Our big brains set humans apart from other animals. Our ability to walk upright also distinguishes us from apes, with whom we share a common ancestor. However, in Darwin's day, and for many decades afterwards, too few fossils had been found to solve the mystery of which came first: intelligence or bipedalism.

Donald Johanson's 1973 discovery of "Lucy," a half-complete skeleton of a previously unknown species of hominid, proved critical to understanding this aspect of human evolution. In offering evidence for how humans diverged from apes, scientists cite specific features in Lucy's anatomy. Although she displayed subtle but important differences in skull structure, she largely retained an apelike upper body. However, it was her lower body, in particular her pelvis and knee joints, that helped scientists determine the order in which changes occurred.

Bipedalism, or walking on two feet, is what separated the earliest human ancestors from the earliest ape ancestors. The shape and position of the pelvis (hip), femur (leg), and tibia (shinbone) distinguish bipeds from quadrupeds, or animals that walk on four feet. As the video states, Lucy was like a human from the waist down. Her knee joints locked straight, a feature characteristic of upright walkers. And in bone structure and orientation, her pelvis more closely resembled a modern human's than a chimp's.

Lucy was clearly capable of walking bipedally. Her funnel-shaped ribcage and broad pelvis indicate that she probably had a large belly, like a modern ape, reflecting an adaptation to a relatively low-quality, high-bulk diet. However, the thick waist this gave her would have hindered her flexibility, and her high shoulders and the shape of her torso suggest it would have been difficult for her to swing her arms as humans do when running.

In this way, a three-million-year-old fossil showed that bipedalism was the first step toward becoming human—long before the earliest stone tools were made or any language was developed. Lucy's species, Australopithecus afarensis, was just one of many apelike human ancestors that walked upright on two legs. Subsequent fossil finds of much earlier bipedal hominids have confirmed this. As a group, small-brained bipedal apes inhabited Earth for millions of years before modern humans, Homo sapiens, evolved.

Interestingly, one compelling hypothesis, proposed by evolutionary biology professor Daniel Lieberman, suggests that bipedalism evolved to preserve energy. As seen by examining the fossil record, several species of upright walkers evolved with subtle anatomical differences between them. Minor increases in hip extension and leg length, for example, would have saved early transitional forms at least some energy. The compound effect of small changes ultimately yielded large energy savings.

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