During the Goguryeo kingdom (37 B.C. to A.D. 668), the Maek people in the northern Korean Peninsula ate maekjeok, a dish of grilled pork marinated in doenjang, a fermented soybean paste. Years later, maekjeok evolved into neobiani, a dish of broad, thin slices of beef tenderized and grilled over flames. A feature of royal court cuisine during the Joseon dynasty (1392 to 1910), neobiani is considered a predecessor to today’s beloved bulgogi.
Source: wiktionary