The Korean peninsula juts south from the east Asian mainland. To the north and southwest lies China, and to the east lies Japan. Just like Japan, Korea had its own features that made it stand out from flat lands. Rugged mountains. Rugged mountains made it almost impossible for agriculture and migration.
The first Koreans were nomadic peoples from northeast Asia. Just as Japan did, they made clans and culture. They also adopted a lot of Chinese culture such as, Confucianism, writing, and political institutions.