Since walking was the first thing I lost when I got sick, I spent much of the months that followed contemplating this most elemental of human acts. Walking upright, or bipedalism, is considered the threshold of being human, the skill that most distinguishes us from our ancestors. It’s also immune to improvement. Ever since humans began walking four million years ago, the act has been essentially unchanged. Every step, my physical therapist observed, is a tragedy waiting to happen: You nearly stumble with one leg, then catch yourself with the other. It’s a constant struggle against gravity,
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