"toothbrush were commonly made from the coarse hairs on the back of a hog's neck."
think of it like "Before the nylon-bristled toothbrush was invented in 1938" has the word before in it so it can't just be a sentence by itself. However, you can say this: "toothbrush were commonly made from the coarse hairs on the back of a hog's neck" as a sentence. It is independent.
