Modern Mongolian evolved from Middle Mongol, the language spoken in the Mongol Empire of the 13th and 14th centuries. In the transition, a major shift in the vowel-harmony paradigm occurred, long vowels developed, the case system changed slightly, and the verbal system was restructured. Mongolian is related to the extinct Khitan language. It was believed that Mongolian is related to Turkic, Tungusic, Korean and Japonic languages but this view is now seen as obsolete by a majority of (but not all) comparative linguists. These languages have been grouped under the Altaic language family and contrasted with the Mainland Southeast Asia linguistic area. However, instead of a common genetic origin, Clauson, Doerfer, and Shcherbak proposed that Turkic, Mongolic and Tungusic languages form a "Sprachbund", rather than common origin. Mongolian literature is well attested in written form from the 13th century but has earlier Mongolic precursors in the literature of the Khitan and other Xianbei peoples. The Bugut inscription dated to 584 CE and the Inscription of Hüis Tolgoi dated to 604-620 CE appear to be the oldest substantial Mongolic or Para-Mongolic texts discovered.