Humans possess a unique neck ligament that stabilizes the head during running, a feature missing in chimpanzees.

The ligamentum nuchae is a specialized elastic structure that extends from the external occipital protuberance of the skull to the spinous process of the seventh cervical vertebra. Research led by Dr. Daniel Lieberman at Harvard University and Dr. Dennis Bramble at the University of Utah suggests this ligament was a critical adaptation for endurance running in early Homo species. Fossil evidence indicates that this feature appeared roughly 2 million years ago in Homo erectus, marking a significant divergence from earlier hominids like Australopithecus.While humans share many skeletal traits with chimpanzees, our closest living relatives lack this ligament because their locomotion is primarily based on climbing and knuckle-walking. When a human runs, the head naturally wants to pitch forward due to its mass and the forces of impact. The nuchal ligament stores elastic energy and provides a passive recoil mechanism that keeps the head upright without requiring massive muscle exertion. This stabilization is vital for 'vestibulo-ocular' reflexes, which allow humans to maintain a steady gaze on the horizon while moving at high speeds.In modern times, the nuchal ligament has gained attention in clinical studies regarding 'text neck' or forward head posture. Prolonged flexion of the neck, often at angles of 45 to 60 degrees while looking at smartphones, places up to 60 pounds of force on the cervical spine. This chronic strain can lead to the calcification of the ligament or the development of bone spurs where the ligament attaches to the skull. Understanding this evolutionary trait helps researchers treat repetitive strain injuries caused by modern sedentary lifestyles.