The first rib of modern humans and chimpanzees are distinctly different from one another. The modern human first rib has a univertebral articulation. meaning it only articulates with the first thoracic vertebrae (T1). In chimpanzees, the first rib has a bivertebral articulation, meaning that it articulates with both the last cervical (C7) and the first thoracic vertebrae (T1).

The morphological understanding of Lucy’s first rib remains controversial. While some anthropologists suggest that Lucy’s first rib is bivertebral, others argue Lucy has only a single vertebral facet similar to modern humans. Dual articulation of the first rib would correlate with some of Lucy’s other Chimpanzee-like features of the upper limb. However, if Lucy’s first rib is indeed univertebral, like modern humans, then this articulation would be an adaptation for upright posture and changing shoulder girdle morphology.