These principles make it possible to pinpoint the locus in space more or less accurately even for a language family as old as IE. Here it will be sho… - 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...

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.

The vast interior of Eurasia is a linguistic spread zone—a genetic and typological bottleneck where many genetic lines go extinct, structural types tend to converge, a single language or language family spreads out over a broad territorial range, and one language family replaces another over a large range every few millennia. The linguistic geography of the central and western grasslands, from at least the Neolithic until early modern times, has consisted of an overall westward trajectory of language spreads... The central Eurasian spread zone... was part of a standing pattern whereby languages were drawn into the spread zone, spread westward, and were eventually succeeded by the next spreading family.