Newman (1977) classified the languages into the four groups which have been accepted in all subsequent literature. Further subbranching, however, has not been as robust; Blench (2006), for example, only accepts the A/B bifurcation of East Chadic.
Several modern genetic studies of Chadic speaking groups in the northern Cameroon region have observed high frequencies of the Y-ChromosomeHaplogroup R1b in these populations (specifically, of R1b's R-V88 variant). This paternal marker is common in parts of West Eurasia, but otherwise rare in Africa. Cruciani et al. (2010) thus propose that the Proto-Chadic speakers during the mid-Holocene (~7,000 years ago) migrated from the Levant to the Central Sahara, and from there settled in the Lake Chad Basin.
Newman, Paul and Ma, Roxana (1966) 'Comparative Chadic: phonology and lexicon.' Journal of African Languages, 5, 218–251.
Newman, Paul (1977) 'Chadic classification and reconstructions.' Afroasiatic Linguistics 5, 1, 1–42.
Newman, Paul (1978) 'Chado-Hamitic 'adieu': new thoughts on Chadic language classification', in Fronzaroli, Pelio (ed.), Atti del Secondo Congresso Internazionale di Linguistica Camito-Semitica. Florence: Instituto de Linguistica e di Lingue Orientali, Università di Firenze, 389–397.
Newman, Paul (1980) The Classification of Chadic within Afroasiatic. Leiden: Universitaire Pers Leiden.
Herrmann Jungraithmayr, Kiyoshi Shimizu: Chadic lexical roots. Reimer, Berlin 1981.
Schuh, Russell (2003) 'Chadic overview', in M. Lionel Bender, Gabor Takacs, and David L. Appleyard (eds.), Selected Comparative-Historical Afrasian Linguistic Studies in Memory of Igor M. Diakonoff, LINCOM Europa, 55–60.