Indigo plants have been used to dye fabric for thousands of years. Unlike other dyes, indigo does not end up chemically linked to textile fibers; rather, it adsorbs to the surface of the threads. This allows the fibers' white cores to show through to various degrees after abrasion. Hence that impossible-to-replicate look of perfectly worn-in jeans.
But indigo plants yield only a small amount of the dye. It's not nearly enough to keep pace with the enormous demand that Levi Strauss unleashed when he invented blue jeans in the 1870s. Now, after over a century of relying on a lot of toxic chemicals to make a synthetic version, researchers have engineered bacteria that will make it.
The demand for blue dye was handled by one of Strauss’ fellow Bavarians—Adolph von Baeyer, of aspirin fame. He found a way to make a synthetic version of indigo, for which he won the Nobel Prize in Chemistry in 1905.