Modern Korean descends from Middle Korean, which in turn descends from Old Korean, which descends from the Proto-Koreanic language which is generally suggested to have its linguistic homeland somewhere in Manchuria. Whitman (2012) suggests that the proto-Koreans, already present in northern Korea, expanded into the southern part of the Korean Peninsula at around 300 BC and coexisted with the descendants of the Japonic Mumun cultivators (or assimilated them). Both had influence on each other and a later founder effect diminished the internal variety of both language families.
The oldest Korean dictionary.(1920)
Chinese characters arrived in Korea (see Sino-Xenic pronunciations for further information) together with Buddhism during the Proto-Three Kingdoms era in the 1st century BC. 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 a high literacy rate of Hangul during the 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 or North Korea opposes the learning of Hanja, though they are not officially used in North Korea anymore, and their usage in South Korea is mainly reserved for specific circumstances, such as newspapers, scholarly papers, and disambiguation.
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.