I always understood that Sikhism sort of just appeared out of nowhere in the 15th century under the leadership of Guru Nanak and the developed subsequently under the following Gurus. But isn't it likely that there was some kind of religious ferment prior to the 15th century that was "pre-Sikh"? Otherwise what factors led to its appearance and popularity out of nowhere? Was Sikhism under the first gurus simply the most popular of a bunch of movements that were reacting to aspects of Hinduism that were unsatisfactory as more Indians encountered Islam??