Bipedalism and the human skeleton

Bipedalism is the first hominin flag. The ‘why’ is a set of models; the ‘how’ is the skeleton.

1. No single agreed cause. Teaching list: carrying, seeing over grass, thermoregulation, foraging (reach and haul), display, walking efficiency on the ground after trees thin.
2. Costs: blood column and varicose veins, difficult birth through a narrow pelvis, lumbar strain — bipedalism is a trade, not an upgrade sticker.

• Vault bones: frontal, parietals, occipital, temporals. Coronal, sagittal, lambdoid, squamous sutures — ageing and trauma in forensic work use these lines.
• Foramen magnum moves under the skull as the spine stacks vertically.
• Pelvis: basin of ilium–ischium–pubis; human blade is shorter and wider than an ape’s for viscera and birth.
• Foot: platform, aligned toes, hallux not for grasping. Non-opposable big toe is a bipedal signature.
• Face trend in later Homo: smaller ridges, vertical forehead, smaller jaws and teeth, a chin — diet and speech-muscle stories both circulate.