The archaeological sources on which this summary is based... most often describe the westward cultures as derived from, or extensions from, the easte… - Johanna Nichols

No migrations are required to derive the attested IE distribution from a reconstructed homeland consisting of a locus in western central Asia and a range over the steppe and desert. Sometime in the fourth to early third millennium, PIE spread along the steppe and southern trajectories to occupy the entire reconstructed range: the steppe, the desert of wester central Asia, part of the adjacent mountains, and perhaps some of south- west Asia. At this time its distribution was continuous, and that distribution had been achieved not by migration but by expansion.

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 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.