Turkic speakers have probably settled in the Amu-Darya, Syr-Darya and Zeravshan river basins since at least AD 600–700, gradually ousting or assimilating the speakers of Eastern Iranian languages who previously inhabited Soghdiana, Bactria and Chorasmia. The first Turkic dynasty in the region was that of the Karakhanids in the 9th–12th centuries AD, who were a confederation of Karluks (Qarluq), Chigil, Yaghma and other tribes.
Uzbek is a language which can be considered the direct descendant or a latter form of Chagatay, the language of great Turkic Central Asian literary development in the realm of Chagatai Khan, Timur (Tamerlane), and the Timurids. The language was championed by Mir Ali-Sher Nawa’i in the 15th and 16th centuries. Ultimately based on the Qarluq variant of the Turkic languages, it contained large numbers of Persian and Arabic loanwords. By the 19th century it was rarely used for literary composition.
The term “Uzbek” as applied to language has meant different things at different times. Prior to 1921 “Uzbek” and “Sart” were considered to be different dialects; “Uzbek” was a vowel-harmonised Kipchak dialect spoken by descendants of those who arrived in Transoxiana with Shaybani Khan in the 16th century, who lived mainly around Bukhara and Samarkand, although the Turkic spoken in Tashkent was also vowel-harmonised; “Sart” was a Qarluq dialect spoken by the older settled Turkic populations of the region in the Ferghana Valley and the Kashka-Darya region, and in some parts of what is now the Samarkand Province; it contained a heavier admixture of Persian and Arabic, and did not use vowel-harmony. In Khiva Sarts spoke a form of highly Persianised Oghuz Turkic. After 1921 the Soviet regime abolished the term Sart as derogatory, and decreed that henceforth the entire settled Turkic population of Turkestan would be known as Uzbeks, even though many had no Uzbek tribal heritage. The standard written language that was chosen for the new republic in 1924, however, despite the protests of Uzbek Bolsheviks such as Faizullah Khojaev, was not pre-revolutionary “Uzbek” but the “Sart” language of the Samarkand region. All three dialects continue to exist within modern, spoken Uzbek.