Juhaynah. Both showed physical continuity with the indigenous pre-Islamic population. The former claimed descent from the Quraysh, the Prophet Muhammad's tribe. Historically, the Ja'ali have been involved in the slave trade, making up an important subsection of the nomadic, slave trading jallaba, along with other tribes such as the Danagla. The nomadic Juhayna comprised a family of tribes that included the Kababish, Baqqara, and Shukriya. They were descended from Arabs who migrated after the 13th century into an area that extended from the savanna and semi-desert west of the Nile to the Abyssinian foothills east of the Blue Nile. Both groups formed a series of tribal shaykhdoms that succeeded the crumbling Christian Nubian kingdoms, and were in frequent conflict with one another and with neighboring non-Arabs. In some instances, as among the Beja, the indigenous people absorbed Arab migrants who settled among them. Beja ruling families later derived their legitimacy from their claims of Arab ancestry. Although not all Muslims in the region were Arabic-speaking, acceptance of Islam facilitated the Arabization process. There was no policy of proselytism, however. Islam penetrated the area over a long period of time through intermarriage and contacts with Arab merchants and settlers. The Funj At the same time that the Ottomans brought northern Nubia into their orbit, a new power, the Funj, had risen in southern Nubia and had supplanted the remnants of the old Christian kingdom of Alwa. In 1504 a Funj leader, Amara Dunqas, founded the Kingdom of Sennar. This Sultanate eventually became the keystone of the Funj Empire. By the mid-sixteenth century, Sennar controlled Al Jazirah and commanded the allegiance of vassal states and tribal districts north to the third cataract and south to the rainforests. The Funj state included a loose confederation of sultanates and dependent tribal chieftains drawn together under the suzerainty of Sennar's mek (sultan). As overlord, the mek received tribute, levied taxes, and called on his vassals to supply troops in time of war. Vassal states in turn relied on the mek to settle local disorders and to resolve internal disputes. The Funj stabilized the region and interposed a military bloc between the Arabs in the north, the Abyssinians in the east, and the non-Muslim blacks in the south. The sultanate's economy depended on the role played by the Funj in the slave trade. Farming and herding also thrived in Al Jazirah and in the southern rainforests.