"The Indo-Iranian people, like a giant river, started on their nomad career from their now-forgotten land of birth, in some obscure dawn of history. At last the current of emigration divided into two streams, one finding its destination in the west of the Hindukush, and the other pouring into the plains of India ..."
(Rabindranath Tagore, 1923)
The migrations of the Indo-Iranian peoples from the Central Asian steppes to the Indus valley and Iranian plateau occurred over a period of time, probably between the 3rd and 2nd millennium BCE. Features of the old Iranian religion shared with the Vedic religion of India include the ritual sacrifice, performed by trained priests, sacred utterances (Avestan mathra, Vedic mantra), the notion of cosmic/world order (Avestan asha Vedic rta) and the principles of friendship and hospitality. Worship involved the invocation and propitiation of the gods of the Indo-Iranian pantheon that included Mithra, Varuna, and Indra, a divinity who epitomised the warrior of the heroic age - both reckless and bountiful.
As the forces of nature were important factors in the lives of a semi-nomadic pastoral people, so the migrations created the need for new lands. They brought with them the horse-drawn war-chariot and, after taming the wild horses of the steppe, were formidable fighting men. Later still, the attributes of Indo-Iranian gods and goddesses were fused with those from other lands including Mesopotamia, one example being that of the water goddess Aredvi Sura Ahahita with the Mesopotamian Nanai, both goddesses being associated with fertility.