Overview
Haplogroup R-B138 is an influential downstream branch of R-Z2123 and represents a key paternal lineage within Iron Age Iranian and Indo Aryan related populations. Its formation coincides with the southward movement of steppe derived cultures into Bactria, Sogdia and the wider Iranian Plateau. B138 exhibits strong evidence of having participated in the formation of early Saka, Sogdian and Bactrian groups, all of which played major roles in the ethnic and cultural development of eastern Iranic peoples. The internal substructure of B138 shows clear regional differentiation. Some branches correlate with historically nomadic Saka related populations, while others appear among settled communities of Bactria, Sogdia and northern Afghanistan. Additional downstream clades show distribution into northwest India and Pakistan, reflecting the eastward extension of Indo Aryan populations from Central Asia. This makes B138 one of the most regionally informative subclades of the Z2123 macrostructure. Modern occurrences of B138 appear in Iran, Afghanistan, Pakistan, Tajikistan, Uzbekistan and northern India. Smaller pockets appear in the Caucasus and Turkmenistan. The mosaic nature of its distribution underscores its historic role within mobile and settled societies of the Iranian world. The lineage persisted through major demographic transitions from the Bronze Age through the early medieval period.