The other Nubian languages are found hundreds of kilometers to the southwest, in Darfur and in the Nuba Mountains of Kordofan. For a long time it was assumed that the 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).