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The Proto-Turkic language is the hypothetical proto-language of the family of Turkic languages that predates the separation of the Turkic peoples and separation into Oghuz and Oghur branches. A separate Turkic family is believed to have existed since approximately 4500-4000 BCE though its separation into its modern branches may have taken as recently as 500 BCE. Its speakers are usually connected with the early archaeological horizon of west and central Siberia and in the region south of it.
The oldest records of a Turkic language, the Old Turkic Orkhon inscriptions of the 7th century Göktürk khaganate, already show characteristics of the Eastern branch of Turkic, and reconstruction of Proto-Turkic must rely on comparisons of Old Turkic with early sources of the Western branches, Oghuz and Kypchak, as well as the Oghur branch (Bulgar, Chuvash, Hunnic, Khazar, Turkic Avar). Since attestation of these non-Eastern languages is much more sparse, reconstruction of Proto-Turkic still rests fundamentally on East Old Turkic of the Göktürks.
The consonant system had a two-way contrast of stop consonants (fortis vs. lenis), k, p, t vs. g, b, d, with verb-initial b- becoming h- still in Proto-Turkic. There was also an affricate consonant, č; at least one sibilant s; and sonorants m, n, ń, ŋ, r, ŕ, l, ĺ with a full series of nasal consonants.
The sounds denoted by ń, ĺ, ŕ refer to palatalized sounds and have been claimed by Altaicists to be direct inheritances from Proto-Altaic. The last two can be reconstructed with the aid of the Oghur languages, which show /r, l/ for *ŕ, *ĺ, while Common Turkic has *z, *š. Oghuric is thus sometimes referred to as Lir-Turkic and Common Turkic as Shaz-Turkic. An alternate analysis reconstructs Proto-Turkic *z, *š, holding that Common Turkic remains closer to the original state of affairs. The glottochronological reconstruction based on analysis of isoglosses and Sinicisms points to the timing of the r/z split at around 56 BCE–48 CE, associated with "the historical situation that can be seen in the history of the Huns' division onto the Northern and Southern: the first separation and withdrawal of the Northern Huns to the west has occurred, as was stated above, in 56 BC,...the second split of the (Eastern) Huns into the northern and southern groups happened in 48 AD" from which time the Northern branch steadily migrated from western Mongolia through southern Xinjiang into the north's Dzungaria and then finally into Kazakhstan's Zhetysu until the 5th century.
- Barış Kabak: Acquiring phonology is not acquiring inventories but contrasts. Turkic migrations and primary long vowels, A brief sketch of the history of Turkic, p. 353.
- Quote: »Turkic languages are the westernmost branch of the Altaic family, which is hypothesized to have come into existence around 4500–4000 BC.«
- Róna-Tas, András. “The Reconstruction of Proto-Turkic and the Genetic Question.” In: The Turkic Languages, pp. 67-80. 1998.
- Quote: »In the region concerning us here, the Neolithic Period started at about 4500-4000 BC. If Proto-Turkic reflects a language of the first period, it began around this time. If there was an earlier proto-language, e.g. a language in the sense of a reconstructed Proto-Altaic, Proto-Turkic came into existence only after its dissolution. The lower limit of Proto-Turkic is the time of the appearance of the first direct data from existing Turkic languages, in fact after the separation of the branches of Turkic, i.e. about the middle of the first millenium BC. ... In early Ancient Turkic, from about 3000 until 500 BC, no stabilised dialects can be assumed.«
- M. Zakiev, Origin of Turks and Tatars, Moscow, Publishing house ”Insan”, 2002, p.76, ISBN 5-85840-317-4
- Quote: »...the split of the Türko-Mongolian unity onto Türkic and Mongolian languages happened in the 4th millennium BC, i.e. 60 centuries ago.«
- Dybo A.V., "Chronology of Türkic languages and linguistic contacts of early Türks", Moskow, 2007, p. 770,  (In Russian)
- Gyula Décsy, The Turkic Protolanguage: A Computational Reconstruction (1998).
- Edward J. Vajda, review of Décsy (1998), Language (2000), 473-474.
- Gerard Clauson, Etymological dictionary of pre-thirteenth-century Turkish, Oxford, Clarendon Press (1972).
- Vilhel Gronbech, Preliminary Studies in Turkic Historical Phonology (Uralic & Altaic), RoutledgeCurzon (1997), ISBN 0-7007-0935-5.
- András Róna-Tas, 'The Reconstruction of Proto-Turkic and the Genetic Question', in The Turkic Languages, edited by Lars Johanson and Éva Csató, Routledge Language Family Descriptions, London: Taylor & Francis, Inc. (1998), ISBN 0-415-08200-5, pp. 67–80.