The past is not always something to be proud about. Deep down, we are all mongrels. There is always an element of shame hidden in history, and such is the case of the Hindu as well.
The first wave of Aryans, who invaded the Harappans of the plains of the Indus valley in about 1800 BC, learnt two lessons that had far-reaching consequences. First, that the Harappans had a clearly superior culture with advanced religion, refined art and a prosperous urban civilisation. Second, that despite their marked superiority, the Harappans ended up as the losers because they despised hierarchy and maintained an egalitarian society.
An inferiority complex arising from the first lesson prompted the Aryans to create a hierarchical social order. The Brahmins were to maintain purity to perform ritual sacrifices. Contamination of any kind was to be forbidden. The Kshatriyas were to be the warrior rulers. To facilitate their function, these protectors were allowed some measure of interaction with the artisan, the farmer, and the trader groups that were to be co-opted from the urban aboriginals. Together, they constituted the ruling triad in the first book of the Rig Veda.
The hierarchy grew with the addition of Shudras in the later hymns. They were to be integrated from the non-urban aboriginals on the pain of servitude. These Untouchables were kept outside the system, either because they refused to surrender to the invaders or because they weren't economically very important to the ruling classes. But they were to be feared nevertheless, because their numbers were significant. Manu then appeared with his iron-clad dictates of purity. The fact that Aryan men had started marrying aboriginal women may have prompted him in his work to lump women together with the dasas (slaves) and animals.