Definition
Bipedalism is habitual locomotion on two hind limbs while holding the trunk upright. In the hominin context, it specifically means terrestrial striding bipedalism — walking and running on flat or undulating ground with an extended-knee, extended-hip gait, distinct from the occasional bipedal locomotion of chimpanzees, gibbons, or other primates.
The anatomical signature is unmistakable: a foramen magnum centred under the skull, a short broad pelvis, a valgus knee (femur angled inward), a stiff plantigrade foot with a non-divergent big toe and a longitudinal arch, and a spine with a lumbar curve. These traits, combined, distinguish a habitual biped from any other primate.
Why it matters
How it works
A bipedal gait requires balancing the trunk over alternating single feet. Hominin pelves widen and shorten to position hip abductor muscles for that balancing act; the femur angles inward so the knees stay under the body's centre of mass; the foot becomes a stiff lever with arched bones. The skull's foramen magnum migrates forward and downward so the spine enters from below rather than behind.
These changes are reciprocal — every one of them constrains and is constrained by the others. A pelvic fossil with a hominin-like ilium implies a hominin-like femur, knee, and foot, even when those are not preserved.