2000-1500 BCE : Indo-european assimilation in India and start of the Vedic era

2000-1500 BCE: 

Even as the Indian subcontinent witnesses a rapidly growing and then equally rapidly declining civilization around the Indus river, a group of pastoralists from the Central Asian steppe region (probably in today's Kazakh, Uzbekistan etc) begin their epic westward migration to Northern Europe, upto Ireland. Further on, some of them (classified now as Indo-Iranians) comes down the Steppe mountains, across Hindu Kush ranges into north western India bringing with them Indo European family of languages to Indian sub continent. 

Indian subcontinent forms one of the eastern most boundary of Indo European languages brought by these people. 

It is clear there was some cultural and spatial intermixing between the sophisticated IVC peoples and these mountain dwelling nomads from up north who we can call Aryans (A word they call themselves in their initial body of literature starting with the Vedas) or  Vedic people (since they are known for some of the highest quality literature of those times, albeit oral, named Vedas which forms the bedrock of Hinduism, for want of a better word though current Hinduism has only little to do with Vedic religion of these people) 

These new comers are unlikely to be same people as those who populated the Indus valley civilisation - IVC had a structured city life, agriculture of a  very high order etc, while the new comers were nomadic, horse born pastoralists.  Proto vedic people knew of Horses - something integral to their livelihood and movements in the hills, something clearly not prevalent in IVC. Since no Indian bloodline of horses exist, these were likely brought in from the central Asian plains which formed their former homes. IVC people also probably imported some horses given their extent of trading, however, none of their seals ever show horses these were likely not central to IVC people as they were for the vedic people.  

Their mythology compares favorably with structure of myths across all Indo-European cultures and includes
1. Myths around agriculture / water for lands
2. Myths around cattle raiding (Cattle being a key aspect of pastoralist lifestyle)
3. Dragon slaying myths (e.g. Indra kills dragons, like many other gods of Indo European stable)

However, it is worth noting that  most scholars discount the Aryan Invasion theory (that the steppe pastoralists invaded NW India decimating IVC and all local natives) as racist bunkum. All evidence currently points to a gradual assimilation of the new immigrants with locals to form a rich cultural heritage that is the bedrock of most of the Indian culture and religion that we know today. Fact that it was an assimilation is proven by the fact that many of the tools, pottery and material artifacts of the IVC period was widely used by the Aryans in India post their inter mixing. On the other hand, the counter narrative pushed by Indian right wing that vedas/ Aryan culture were all wholly created by people who were natives in Indian lands have also been found unworthy of discussion. 

1700-1500 BCE

The proto-vedic people of northwestern India, after assimilating and inter mixing with local people, namely the IVC people and other natives already living in India - Dravidians - Ancient Ancestral South Indians aka AASI, moves further south and east to settle in the lush green plains of today's Punjab in northern India (land of 5 rivers)

Here in the bountiful banks of these rivers, they settle down and creates the first major literary epic of India - Rig Veda in an Indo European language Sanskrit - one of the oldest languages known to humans along with Egyptian, Sumerian etc. 

However, Sanskrit clearly assimilated with existing languages in India. Sanskrit, even the earliest in Rig Veda, has retroflex consonants (tta, Na, da etc) which are absent in Indo European languages but exist in pre Aryan Indian languages like Dravidian especially Tamil, the world's oldest language which is still spoken. This clearly shows a deep assimilation of the Aryans with pre-Aryans like Dravidians. 

However, there are many similarities even today across Indo-European languages. 

Example, pashu in Sanskrit has same root as pecus (Latin), impecunious (English, out of cow or out of money, since in olden days cattle was wealth)

Pada (Sanskrist) for feet, pedis (Latin),

Deva (Sanskrit), deus (Lating), divine (English)