The Study of the Human Spine and Its Evolution: State of the Art and Future Perspectives

The cranium directly articulates with the rest of the body via the cervical spine. At the “cranio-cervical interface, " the morphology of the hominoid cranial base offers a wealth of information regarding posture and locomotion. Compared to great apes, modern humans exhibit more anteriorly positioned and anteroinferiorly oriented foramina magna, more anteriorly positioned and flatter occipital condyles, and a reduction and reorganization of the nuchal musculature. Researchers have long inquired about the functional significance of these features in humans as they relate to the adoption of bipedal stance and locomotion using comparative and experimental approaches. Anteriorly positioned foramina magna and occipital condyles presumably confer a mechanical advantage for holding the head above an upright torso in humans rather than in front of a more horizontal torso as in great apes. Reductions in neck extensor musculature suggest a decreased emphasis on head-neck extension for maintaining neutral head posture in humans. Great apes, by contrast, require a comparatively enlarged neck extensor musculature or bony modifications that affect the length of their force arms to hold up heads with greater anterior mass. In the absence of direct evidence of the postcranial skeleton in the human fossil record, the utility of these features as indicators of human-like posture and locomotion in extinct hominins has received considerable attention from researchers. Early hominin taxa resemble modern humans in some (e.g., forward migration of the foramen magnum) but not all (e.g., nuchal plane architecture) aspects of cranial base morphology. Research on the “cranio-cervical interface” will continue to inform our understanding of how hominoid cranial anatomy relates to posture and locomotion and, in particular, how the modern human cranium evolved in relation to our unique reliance on bipedalism.