The Zhou, who unified China in the twelfth century B.C. , themselves came from the western plains at the foot of the mountains; their earliest records identify the Qiang as close allies with whom they may have exchanged women. During this period of early contact, the culture of the lowlands and mountains appears to have been relatively undifferentiated. It was not until the sixth century B.C. , with the rise of intensive agriculture in the east, that the two cultures began to diverge. The Qiang gave way before the Chinese; subsequent records tell of mass migrations from the original points of contact in Gansu south through the mountain corridor. There are accounts of Qiang states in the western part of the corridor during the fifth and sixth centuries; these were overrun with the rise of the Tibetan Empire in the eighth century and were replaced with Tibetanized states. The passage of Mongol armies through the area in the thirteenth century resulted in the tusi system. Under this system, local sovereigns (called "tusi" in Chinese) were given charters in return for nominal recognition of imperial authority. Eventually, this system spread through most of the corridor as less powerful headmen, often brought in from other minority areas, were given charters of their own. Beginning in the eighteenth century, some of the tusi were deposed in an attempt to bring the area under Chinese control. Most Qiang areas were under direct government control by the end of the nineteenth century. However, most states in the Jiarong area managed to maintain their autonomy until Liberation. Other areas, including areas inhabited by Jiarong and Qiang under Jiarong headmen, were able to retain varying degrees of autonomy.