The Proto-Indo-Iranian tribes existed around the second half of the third millennium. They lived in the vast open steppes of Southern Russia. By 2000 BCE, they had split into two groups–the Indo-Aryans and the Iranians.
During and in the course of the second millennium, the greater part of both peoples left the steppes that had been their home. The reasons attributed to this departure are many: the loss of pasturage due to drought, prolonged frost and even the needs of a growing population. Whatever the reason for their departure may be; what is known is that: whole tribes migrated and took their herds with them.
The main body of Indo-Aryans moved through Central Asia, Afghanistan and the Hindu Kush; and down into the Indus Valley. It was there where they arrived in successive waves, over a period of several centuries.
The Indus Valley, at the time, was rather similar to the Nile. It was easy to clear and was fertilised by an abundance of flood-borne silt. The Indus Valley was a cradle of civilisation. Excavations have revealed that the cities of Harappa and Mohenjo-daro controlled an empire that lasted from at least 2300 to 1750 BCE.
After or around 1750 BCE, some disaster befell this prosperous civilisation. While in the past, the consensus was that it was caused by the arrival of a new wave of immigrants; it is now thought to have been due to an earthquake. It was the earthquake that changed the land itself; causing the Indus to change its course and flood the agricultural land till it could no longer be used.
When the Indo-Aryan tribes arrived, they brought with them the bow and the arrow as well as the axe. Most importantly, however, they carried with them (or were carried by) chariots with spoked wheels that were drawn by two horses that could carry two strong warriors.
Due to the chariot, they were able to establish minor strongholds as conquerers in the land. The Indo-Aryans were not quite a nation. Rather, they united or settled with the non-Aryan people. In the early period, the kings of this region were primarily warriors. Over time, the priests took on this role.
Compared to the people of the Harappa and Mohenjo-daro, the Aryans were not technologically advanced. Nothing remains of their buildings and they left no statues behind. On the steppes, they had lived from the breeding of cattle and horses and they knew little of city life. They continued living in tribal villages; with their herds of cattle, horses, sheep and goats that grazed in nearby pastures.
While the economy did change to a mixed one; from pastoral to mixed–the cattle retained its central role. Prayers were recited for the increase of cattle. The warrior’s wealth consisted mostly of cattle. The sacrificial priest received cattle in payment for his services.
Our only source of information about the invading tribes can be found in The Rig Veda.