Limb‑strength as bipedal threshold

Bone cross‑sectional thickness from CT scans can act as a proxy for habitual limb use, and comparing arm vs thigh vs shin strength shows Australopithecus mixed arboreal and terrestrial habits while Homo by 1.8 million years had lower‑limb signatures like modern humans. The authors propose that a relative‑limb‑strength shift marks a discrete evolutionary 'threshold' toward committed bipedalism. — If limb‑strength ratios are a reliable threshold trait, they give a simple, testable marker for when major behavioral changes (range, tool use, hand‑freedom) emerged — reframing debates about the timing and drivers of human uniqueness.