Because of Tibet's geographical isolation it preserved a distinct form of Buddhism. The roots of Tibetan Buddhism lie in the Mahayana and Vajrayana Buddhism of Northern India of the first millennium CE which was lost after the Muslim invasions. When that form of Buddhism was carried into Tibet, Tibetan translators took great care to be faithful to the original Indian texts, and thus preserved much that was being lost in the land where it originated. Buddhism spread in Tibet, and under royal patronage and the guidance, initially, of great teachers from India, became essentially the only religion in Tibet, with the exception being small groups of Bon practitioners, and a certain amount of folk religion. This was the situation until the Chinese take-over of Tibet in 1959. Until that time, just as Tibet itself was an exotic, little known land, closed to most foreigners, so Tibetan Buddhism was more a subject of myth than of knowledge in the west. After the Chinese arrival, many Tibetan Buddhists left, going into neighboring Northern India, Sikkim, Nepal, and Butan. Through them, the Tibetan form of Buddhism made the return trip into the country of its origin, and then began a migration into the West. Though identified with the name and culture of Tibet, this form of Buddhism is in fact, not strictly dependent on Tibetan culture as its only environment. Lineages once held exclusively by Tibetans are now being passed on to people of other national and ethnic backgrounds.