The Iranian family, which was next to sweep across the steppe and deserts, finds its region of greatest diversity in the central Asian mountains, and… - Johanna Nichols

Prior to the Turkic expansion, at the beginning of the Iron Age, Iranian spread from somewhere in the vicinity of Bactria, Sogdiana, and the eastern steppe to cover most or all of western central Asia and the entire steppe, much of the Near East at least to eastern Anatolia, and, at least intermittently, the Danube plain, where Slavic vocabulary and ethnonyms attest to a major Iranianization at about the fifth century AD, and where there is good archaeological evidence of a Scythian presence in the mid-first millennium BC...

Prior to the Iranian expansion, in the early Bronze Age, IE spread to cover the entire steppe and the Danube plain (and subsequently all of Europe), with substantial speech communities also in Anatolia (Hittite and congeners) and northern Mesopotamia (surviving in Armenian) and, in all probability, coverage of much or all of western central Asia (probably by ancestral Indo-Iranian). What is historically attested of the IE spread fits closely the pattern followed later by Iranian, Turkic, and Mongolian.

Approximately every two millennia, then, there has occurred a spread of a language family from a locus in the eastern part of the central Eurasian spread zone to cover the steppe and central Asia, extending partially or intermittently to the Danube plain, Anatolia, and northern Mesopotamia. The loci of the historically attested spreads are near the edge of the spread zone rather than in the centre of it: the piedmont to the south (Bactria-Sogdiana) for Iranian, the north of Mongolia for Turkic and Mongolian. The trajectories of language spread run east to west along the steppe and through the desert to the Near East as shown .... To take clear and historically well-attested examples, the locus, trajectories, and range of IE must have been much like those of Iranian or Turkic. ... The placement of the locus specifically in the vicinity of Bactria-Sogdiana is justified in .... A homeland reconstructed as locus, trajectory and range removes the dilemma: a locus in the vicinity of Bactria-Sogdiana implies a spread beginning at the frontier of ancient Near Eastern civilization and a range throughout the steppe and central Asia, following the east-to-west trajectory, with occasional or periodic spreads into the Danube plain and Anatolia.