Modern Korean descends from Middle Korean, which in turn descends from Old Korean, which descends from the language spoken in Prehistoric Korea (labeled Proto-Korean), whose nature is debated, in part because Korean genetic origins are controversial (See Koreans for archaeological and genetic studies of the Koreans). A relation of Korean (together with its extinct relatives which form the Koreanic family) with Japanese (along with its extinct relatives which form the Japonic family), has been proposed by linguists such as William George Aston and Samuel Martin. Roy Andrew Miller and others suggested or supported the inclusion of Koreanic and Japonic languages (because of a certain resemblance) in the purported Altaic family (a macro-family that would comprise Tungusic, Mongolian and Turkic families); the Altaic hypothesis has since been largely rejected by most linguistic specialists.
Chinese characters arrived in Korea (See Sino-Xenic pronunciations for further information) together with Buddhism during the Proto-Three Kingdoms era. It was adapted for Korean and became known as Hanja, and remained as the main script for writing Korean through over a millennium alongside various phonetic scripts that were later invented such as Idu, Gugyeol and Hyangchal. Mainly privileged elites were educated to read and write in Hanja; however, most of the population was illiterate. In the 15th century, King Sejong the Great personally developed an alphabetic featural writing system known today as Hangul. He felt that Hanja was inadequate to write Korean and that this was the cause of its very restricted use; Hangul was designed to either aid in reading Hanja or replace Hanja entirely. Introduced in the document "Hunminjeongeum", it was called "eonmun" (colloquial script) and quickly spread nationwide to increase literacy in Korea. Hangul was widely used by all the Korean classes but often treated as "amkeul" (script for female) and disregarded by privileged elites, whereas Hanja was regarded as "jinseo" (true text). Consequently, official documents were always written in Hanja during the Joseon era. Since most people couldn't understand Hanja, Korean kings sometimes released public notices entirely written in Hangul as early as the 16th century for all Korean classes including uneducated peasants and slaves. By the 17th century, Korean elites Yangban and their slaves exchanged Hangul letters; that indicates high literacy rate of Hangul in Joseon era. Today, Hanja is largely unused in everyday life due to its inconvenience, but it is still important for historical and linguistic studies. Neither South Korea nor North Korea opposes the learning of Hanja even though neither uses it officially anymore.
Since the Korean War, through 70 years of separation, the North–South differences have developed in standard Korean, including variations in pronunciation and vocabulary chosen, but these minor differences can be found in any of the Korean dialects and still largely mutually intelligible.