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What skeletal evidence shows that Lucy walked upright?

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Illustration showing side-by-side reconstructions of five hominins in the human lineage: Australopithecus afarensis, Homo habilis, Homo erectus, Homo neanderthalensis, and Homo sapiens, with each figure labeled and depicted in a walking pose to highlight evolutionary changes in posture and body structure. A scale bar at the top right indicates relative height. — human evolution An artist's depiction of five species of the human lineage.

The fossil specimen known as "Lucy," classified as Australopithecus afarensis, provides significant skeletal evidence for bipedalism despite retaining some arboreal traits.12

The primary evidence for Lucy’s upright walking is her pelvis, which is described as relatively humanlike. In bipedal hominins, the pelvis underwent specific evolutionary changes to support the transfer of upper-body mass to the lower limbs and to stabilize the body during a stride. Key features of the A. afarensis pelvis include:23

Short ilia: Unlike the long, blade-shaped ilia of great apes, Lucy possessed short, broad ilia that allowed for efficient muscle attachment.13
Pelvic configuration: The ilia are curved forward, which situates gluteal muscles on the side of the hip joint to steady the pelvis during walking, preventing excessive side-to-side wobbling.3
Stout sacrum: The presence of a wide, stout sacrum is a hallmark of a bipedal locomotor skeleton.1

Additionally, the femoral angling—where the thighbones slant toward the midline to keep the knees close together—is a diagnostic feature of bipedalism found in A. afarensis remains, indicating that Lucy was an adept walker.13