The bifurcation of Indo-Iranian is well known to be evident not only in South Asia, where all three of Indic, Nuristani and Iranian sub-branches are … - Johanna Nichols

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.

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 its ancestral Indo-Iranian family finds its own greatest diversity in the mountain region from central Asia to northern India (i.e. Bactria- Sogdiana and parts just south).

The location of the Anatolian branch of IE (Hittite and its sisters) is a problem, or at least a puzzle, for IE homeland studies. The Anatolian languages are attested very early in Asia Minor, removed from Europe and far from the steppe; Gamkrelidze and Ivanov ... offer as a strength the ability of their proposed homelands to account for the location of Hittite with minimal migration. Alternatively or additionally, the location of Tocharian—attested in the early centuries AD well to the east of most IE territory in present-day Xinkiang (Chinese Turkestan)—is a problem or a puzzle... Accounting for the locations of both Hittite and Tocharian is usually presented, at least rhetorically, as a major problem.