The Cambridge History of India, Vol. I: Ancient India v1.c15.182

v1.c15.182

The northern section of the region now inhabited by peoples of this type, that is to say, the country of the north-western tributaries of the Indus, was, in the times of the Rigveda, occupied by Indo-Āryans. The predominant racial character of the whole region is due to the invasion of Mongolo-Altaic peoples from Turkestān on the one hand, and of Persian Āryans or Irānians on the other. The Indus is the ethnographical boundary between the Turko-Irānian and Indo-Āryan types, just as in history it has often been the political boundary between Irān and India.