What did the ancestors of textile moths eat?
Modern textile moths actually eat a wide variety of material, including fur, feathers, shed exoskeletons, bran, and grain. humans just pay more attention to them when they eat textiles. They prefer human handled material because we tend to create larger concentrations than occur in nature. 

if you are interested in changes in biology in response to humans you may want to look at starch digestion in dogs, nylon eating bacteria, and body lice.