Eastern Iranian is thought to have separated from Western Iranian in the course of the later 2nd millennium BC, and was possibly located at the Yaz culture.
With Greek presence in Central Asia, some of the easternmost of these languages were recorded in their Middle Iranian stage (hence the "Eastern" classification), while almost no records of the Scytho-Sarmatian continuum stretching from Kazakhstan west across the Pontic steppe to Ukraine have survived.
Middle Persian/Dari spread around the Oxus River region, Afghanistan, and Khorasan after the Arab conquests and during Islamic-Arab rule. The replacement of the Pahlavi script with the Arabic script in order to write the Persian language was done by the Tahirids in 9th century Khorasan. The Persian Dari language spread and led to the extinction of Eastern Iranic languages like Bactrian, Khorezmian with only a tiny amount of Sogdian descended Yaghnobi speakers remaining among the now Persian speaking Tajik population of Central Asia, due to the fact that the Arab-Islamic army which invaded Central Asia also included some Persians who later governed the region like the Samanids. Persian was rooted into Central Asia by the Samanids.