History of Tibet is also related to prehistoric Burma. It is situated between the two ancient civilizations of China and India, separated from the former by the mountain ranges to the east of the Tibetan Plateau and from the latter by the towering Himalayas. Tibet is nicknamed the roof of the world or the land of snows. The Tibetan language and its dialects are classified as members of the Tibeto-Burman language family. Humans inhabited the Tibetan Plateau at least twenty one thousand years ago. This population was largely replaced around 3,000 BP by Neolithic immigrants from northern China. However, there is a partial genetic continuity between the Paleolithic inhabitants and the contemporary Tibetan populations. Some archaeological data suggests humans may have passed through Tibet at the time India was first inhabited, half a million years ago. The first documented contact between the Tibetans and the Mongols occurred when Genghis Khan met Tsangpa Dunkhurwa (Gtsang pa Dung khur ba) and six of his disciples, probably in the Tangut empire, in 1215.