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), … - Johanna Nichols

It is a basic tenet of migration and homeland theory that the geographical location of a language family’s proto-homeland is to be sought in the vicinity of the root of the family tree (i.e. in the region where the deepest branches come together on a map); or, more generally, that the homeland is to be sought in the region of present greatest genetic diversity of the family.

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.

Along the forest trajectory as well there is evidence of either an early Indie presence or undifferentiated Proto-Indo-Iranian or Proto-Indo-Aryan. Among the Indo-Iranian loans into early Finno-Ugric are some so phonologically archaic that they could well be Proto-Indo-Iranian... Iranian, but not Indo-Aryan, regularly reflects PIE s as h, so this Finno-Ugric form looks more Indie than Iranian. Abaev also cites some less well attested forms that could be specifically Indie... These borrowings would have taken place somewhere in the vicinity of the southern Ural Mountains. They were received from a steppe language and incorporated into Finno-Ugric as it began its spread along the forest trajectory. This linguistic evidence for an Indie or Proto-Indo-Iranian wave preceding Iranian on the steppe is weak but legitimate. In partial confirmation of it, Kuz’mina identifies the Andronovo culture of eastern Kazakhstan in the mid-second millennium BC as Indo-Aryan..... There is also evidence for Indo-Aryan along the steppe trajectory in the form of a set of Crimean place names which Trubač identifies as Indo-Aryan. This evidence is even weaker—place names in general have poor diagnostic value since they lack denotational meaning—but carefully researched and again legitimate. If Trubačv is right there is evidence for an Indie advance to the western steppe. Taken together, the Finno-Ugric and Crimean evidence are consistent with the assumption of a short-lived Indie or Indo-Aryan presence at the frontier of the Iranian spread on the steppe, in addition to the well-known Indie frontier in northeastern Mesopotamia and India.