In humans, the lumbar vertebrae are much larger than those seen in the great apes. In addition, modern humans have five lumbar vertebrae while the apes typically have only four. An increase in the number of vertebrae enhances flexibility of the trunk, reducing the distance the hips must swivel forward during bipedal locomotion, and producing a more efficient stride. In addition, the human spinal column demonstrates a very distinct curve, known as the lumbar curvature. This helps to bring the body’s center of gravity closer to the midline and above the feet. Also, the relatively larger size of the human lumbar vertebrae to that of apes indicates an adaptation to the greater amount of stress transferred through the trunk and into the hips.

Lucy’s vertebrae articulate to form a distinct lumbar curve, as is seen in modern humans. The fragmentary remains also indicate that Australopithecus most likely had five or six lumbar vertebrae.