In formal history today it is said that the vedics were aryans(which again is a misnomer) and migrants to india through the west from central asia thru iran and the people of IVC(indus valley civilization) were the natives(or earliest migrants) of india and were probablu mundas(of SEAsian origin) or dravidians who later moed south. this is purely based on linguistics theory and doesnot have any proof. just a hypothesis.
after recent archeological, geneological and other logical discoveries this has been completely debunked but the mystery of their origins and identity still remains.

Ancient india even before IVC was already multi ethnic(not race). Each ethnic group had its own faiths of which vedicism(deva worshippers), ashuraism(later zoroastrians) and shaivaism(not the shaivism we know today but a grave culture with lingams as tombstones or phallus worship etc) were prominent. There were also many other cults like human idol(vishnu) worshippers, early forms of jainism or yogic religion (as in some siddhas/yogis/recluse sadhus) and many pagan and tribal beliefs like animal worshipers etc.

over the time these beliefs/cultures intermingled and became hinduism and also more outsiders(from the west mostly) came and their beliefs were also added to this common system.
ofcourse there were also movement outside to the west especially the deva ashura wars and ashuras moving west to iran and further west and the early shaivites moving south.
these movements happened due to internat wars between the various kingdoms of north india as depicted in the deva-ashura mythical wars, dasarajana war and mahabaratha wars. devaashura mythical war of rig veda happened in prevedic times when the rig vedics split as deva and ashura worshippers. the deva worshippers became vedics and ashura worshippers became ashuras. the lived in sarasvati and indus rivers respectively. dasarajana was the second famous war(indicating a second wave) of vedicisation of some groups of the north when the vedic baratha sudas won the war and assimilated the won over groups into vedic religion and those that dint moved to iran. so this time the ashuas were driven out of india who later became zoroastrians of iran. the third major wave was the vedicisation of the shaivaites and other southern clans like the yadus etc was during the mahabaratha times. this was the completion of vedicisation. mot central and southers parts were already partly vedicised before as seen in ramayana and parashurama stories.

the deva/ashura concept was the earliest and happened sometime during the mixing of all these original faiths and based on their belief system some groups took to worldly and dulaistic faiths and others to sacrificial vedic faiths. this split could have happened right during the early rig vedic times which could have been around atleast 3-4k B.C if not earlier and the wars continued on for another 1000 years.
the later vedic period continued with the influence of the faiths which have already mingled and hence we see shiva(early form as in rudra, shiva the benevolent etc) and vishnu mentioned from later rig and yejur veda onwards. thus the concepts of vedas itself is a mix of all these faiths and made by priests from all these clans(hence we have so many ghotras and the 7 saptha rishis must have been each from each one of these clans or ethnic groups)
Jiggys thoughts