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

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.

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.

Several kinds of evidence for the PIE locus have been presented here. Ancient loanwords point to a locus along the desert trajectory, not particularly close to Mesopotamia and probably far out in the eastern hinterlands. The structure of the family tree, the accumulation of genetic diversity at the western periphery of the range, the location of Tocharian and its implications for early dialect geography, the early attestation of Anatolian in Asia Minor, and the geography of the centum-satem split all point in the same direction: a locus in western central Asia. Evidence presented in Volume II supports the same conclusion: the long-standing westward trajectories of languages point to an eastward locus, and the spread of IE along all three trajectories points to a locus well to the east of the Caspian Sea. The satem shift also spread from a locus to the south-east of the Caspian, with satem languages showing up as later entrants along all three trajectory terminals. (The satem shift is a post-PIE but very early IE development.) The locus of the IE spread was therefore somewhere in the vicinity of ancient Bactria-Sogdiana. This locus resembles those of the three known post-IE spreads: those of Indo-Iranian (from a locus close to that of PIE), Turkic (from a locus near north-western Mongolia), and Mongolian (from north-eastern Mongolia)... Thus in regard to its locus, as in other respects, the PIE spread was no singularity but was absolutely ordinary for its geography and its time-frame. ... The reason that dialect divisions arising in the locus show up along more than one trajectory is that the Caspian Sea divides westward spreads into steppe versus desert trajectories quite close to the locus and hence quite early in the spread. In contrast, developments that occurred farther west, as the split of Slavic from Baltic in the middle Volga may have, continue to spread along only one trajectory. This is why the Pontic steppe is an unlikely locus for the PIE spread. ...Thus the structure of the IE family tree, the distribution of IE genetic diversity over the map, and what can be inferred of the geographical distribution of dialectal diversity in early IE all point to a locus in western central Asia