The singular event in human evolution that first separated us from our simian ancestors was long held to be brain enlargement, but the tipping point was a more pedestrian development.1 Relatives of Lucy (Australopithecus afarensis) walked in the volcanic ash of Laetoli, Tanzania, 3.6 million years ago. Analysis of their fossil footprints strongly suggests these still partly-arboreal ancestors walked like modern humans, fully erect with extended limbs, rather than with the less efficient apelike, bent-knee, bent-hip gait.2 By 2 million years ago, genus Homo had abandoned arboreal life altogether; freeing of the hands associated with permanent adoption of bipedalism thus preceded, and likely drove, brain enlargement. As Darwin suggested, bipedal walking is the defining feature of our human lineage.

The 21st century is witnessing a worldwide epidemic of diabetes, which is accompanied by one of its most feared sequela: limb amputation. Limb loss often reduces quality...

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