There is no evidence of an 'invasion'. But Indo Aryans were racially different from Dravidians and other racial groups that inhabited the subcontinent.
Were they the same people? How did they have so many differences around the same time in history?
If they were so different, how did they co exist at the same areas at the same time?
Truth is, there is continuity in civilizational evidences at Indus-Sarasvati-Ganga range from before 6000 BC to after 1900 BC. The excavation findings of Indus valley era sites have Shaivic and Vishnu traits - both Vedic tradions.
I would call the vedic and indus valley guys from the same roots who lived in broadly the same belt but shifted their epicentre and social/infra structuring due to environmental changes posed by ecological factors and today came to be called as Indus valley people. Cultural changes are not denied but they aren't alien, they move from one tendency to the other.
Dr. Natwar Jha has provided an interpretation of the ancient script of the numerous recovered seals of the Indus Valley civilization. He has concluded that the Indus Valley seals, which are small soapstone, one-inch squares, exhibit a relation to the ancient form of Brahmi. He found words on the seals that come from the ancient Nighantu text, which is a glossary of Sanskrit compiled by the sage Yaksa that deals with words of subordinate Vedic texts. An account of Yaksa's search for older Sanskrit words is found in the Shanti Parva of the Mahabharata. This may have been in relation to the Indus Valley seals and certainly shows its ancient Vedic connection.
Indus Valley civiisation_
1.Urban,
2.farmers and traders,
3.idol worshippers,
4.gave importance to Bull,
5.Did not use horse
Early Vedic civilisation(indo aryan)_
1.Rural,
2.Pastoral,
3.nature worshippers(indra, varuna, agni ets),
4.gave importance to Cow,
5.Used horses in large scale.
You mean Urban people with little form of writing and Rural people with sophisticated texts that are difficult to even distort?
Indus valley sites were well developed even in 3100 BC.
Mehrgarh excavations of Indus valley sites have proven their antecedents to have gone back as early as 6000 BC and before.
Majority of Indus writing signs at seals etc. are identical to the one's of Brahmi scripts. Both have the same base and there is organic development between the two.
Seals in Indus bore dated 3100 BC have names of Vedic seers.
Horses were found in excavations at Indus valley sites as well as pre Indus civilization sites.
Aryan cities are mentioned in many verses of Rig Veda.
Bull is a symbol of Shaivistic traditions and many findings of excavations at Indus valley civilization sites like Lothal in Gujrat and Kalibangan in Rajasthan show not only Shaivite signs/remains but also have vedic fire altars, oxen, postherds, shell jewellery and other items lining to the vedic brahminic rituals.
Vedic people weren't nomadic as suggested by some. The vedas talk of Oceans and Ships on numerous ocassions. Vedic ancestors like Manu, Turvasha, Yadu and Bhujyu are flood figures, saved from across the sea.
Vedic civilization was spanned on what is known as Sapta-Sindhu. Seven ancient rivers from Indus in west to Ganga in east, with Sarasvati (found recently in Rajasthan) being the epicentre. Sarasvati hailed as as lifeline all through the Vedas is a known fact.
Majority of Indus valley civilization sites are to the east of Indus.
Sarasvati by the way is the host of most clustered Indus valley civilization settlements per recent decades archeological findings. What does that tell you?
Sarasvati is estimated to have started around 8000 BC. Its gradually falling health was discussed in Mahabharata (3138 BC) and Lord Balram's pilgrimage at Sarasvati bank sites too. Eventually the river disappeared between 1900 BC and 2200 BC.
There's evidence of ploughed agricultural fields revealed via excavation in Rajasthan sites at old as 2800 BC and there's an archeologically recorded earthquake at those sites during 2600 BC.
Earthquakes and other Tectonic activities are scientifically known to have changed river courses in past and present.
At places, there are settlements even at some part of the dried river bed of Sarasvati. Suggesting that as the river kept changing its course and kept drying, people changed settlements accordingly.
There's more to come and the disconnect would eventually melt away with further research & findings.
Regards,
Virendra