Nubian peoples dispersed from the Nile Valley to the south, probably at the time of the downfall of the Christian kingdoms. However, comparative lexicostatistic research in the second half of the twentieth century has shown that the spread must have been in the opposite direction. Joseph Greenberg (as cited in Thelwall 1982) calculated that a split between Hill Nubian and the two Nile-Nubian languages occurred at least 2500 years ago. This is corroborated by the fact that the oral tradition of the Shaigiya tribe of the Jaali group of arabized Nile Nubians tells of coming from the southwest long ago. The speakers of Nobiin are thought to have come to the area before the speakers of the related Kenzi-Dongolawi languages (see classification below). Since the seventh century, Nobiin has been challenged by Arabic. The economic and cultural influence of Egypt over the region was considerable, and, over the centuries, Egyptian Arabic spread south. Areas like al-Maris became almost fully Arabized. The conversion of Nubia to Islam after the fall of the Christian kingdoms further enhanced the Arabization process. In what is today Sudan, Sudanese Arabic became the main vernacular of the Funj Sultanate, with Nobiin becoming a minority tongue. In Egypt, the Nobiin speakers were also part of a largely Arabic-speaking state, but Egyptian control over the south was limited. With the Ottoman conquest of the region in the sixteenth century, official support for Arabization largely ended, as the Turkish and Circassian governments in Cairo sometimes saw Nobiin speakers as a useful ally. However, as Arabic remained a language of high importance in Sudan and especially Egypt, Nobiin continued to be under pressure, and its use became largely confined to Nubian homes. Classification Nobiin is one of the about eleven Nubian languages. It has traditionally been grouped with the Dongolawi cluster, mainly based on the geographic proximity of the two (before the construction of the Aswan Dam, varieties of Dongolawi were spoken north and south of the Nobiin area, in Kunuz and Dongola respectively). The uniformity of this 'Nile-Nubian' branch was first called into doubt by Thelwall (1982) who argued, based on lexicostatistical evidence, that Nobiin must have split off from the other Nubian languages earlier than Dongolawi. In Thelwall's classification, Nobiin forms a "Northern" branch on its own whereas Dongolawi is considered part of Central Nubian, along with Birged (North Darfur) and the Hill Nubian languages (Nuba Mountains, Kordofan).