It was a helper nanite, a medical cleaner that disposed of organic waste, but they’d programmed it wrong. It was simple code, evaluating cells as ‘alive’ or ‘dead’ and disposing of the latter, only the designers had forgotten to limit the target areas. So it ate hair and fingernails, dead skin, and tooth enamel. And, after escaping the host, fur and feathers, and bark, and shells and horns and hooves– Swarm by swarm, paring down the world to one binary state.
There were no more silks or cotton or furs, no dead meat or harvested vegetables. Humanity wrapped themselves in plastics and ate their meals alive.