Consider the countries Uzbekistan, Kazakhstan, Kyrgyzstan and Turkmenistan. They speak Turkic languages (alongside some Russian). It was primarily Iranic people (Scythians and Persians) who lived in these territories up to the 3rd century BC or so.
Did Turkic languages exist at that time? If so where were they spoken (my guess would be the territory of modern Mongolia or East Russia)?
When did Turkic languages start being spoken in Central Asia? Did the linguistic shift also come with a genetically detectable demographic shift or was it the Iranic people adopting a different language?
I found that the First Turkic Khaganate used an Iranic language for official functions and a Turkic language informally so whenever this shift happened it must have been mostly complete by 550 AD.