From about the fourth century B.C., Jōmon culture is gradually superceded by the more advanced Yayoi culture. Stimulated by the arrival of agrarian peoples from continental Asia, the Yayoi culture first emerges in northern Kyūshū. It then spreads northward and flourishes until the third century A.D. The Yayoi people are primarily farmers who live in thatched huts clustered to form villages; they engage in wet-rice cultivation, the introduction of which is of momentous importance. Metallurgy is also introduced from the Asian continent during this time, making possible the production of iron and bronze objects. This is a period of increased affluence, and a class society begins to emerge.