The Tajiks are the direct descendants of the Iranian peoples whose
continuous presence in Central Asia and northern Afghanistan is
attested from the middle of the 1st millennium BCE. The ancestors
of the Tajiks constituted the core of the ancient population of
Khwārezm (Khorezm) and Bactria, which formed part of Transoxania (Sogdiana).
They were included in the empires of Persia and Alexander the Great, and they
intermingled with such later invaders as the Kushāns and Hepthalites in the
1st–6th centuries CE. Over the course of time, the eastern Iranian dialect
that was used by the ancient Tajiks eventually gave way to Farsi, a western
dialect spoken in Iran and Afghanistan.
The Arab conquest of Central Asia that began in the mid-7th century brought
Islam to the region. But tribal feuds weakened the Arabs, and, with the rise
of the Sāmānids (819–999), the Tajiks came under the rule of an Iranian dynasty.
The first Turkic invaders (from the northeast) seized this area of Transoxania
in 999, and, because both conquered and conquerors were Muslim, in time many
Tajiks—especially those in the valleys of the Syr Darya and Amu Darya—became
Turkicized. This resulted in the transformation of a formerly purely Iranian
land into “Turkistan.” The name Tajik, originally given to the Arabs by the local
population, came to be applied by Turkic invaders and overlords to those elements
of the sedentary population that continued to speak Iranian languages.