Rg Veda 10.90.12 is mostly misrepresented in colonial translations as the source of the scriptural authority for the "caste system", because the analogy for Purusha is a "man, standing", hence the hierarchy. But what really does the verse say?
Here is the colonial translation:
At the top of the hierarchy were the Brahmins who were mainly teachers and intellectuals and are believed to have come from Brahma's head. Then came the Kshatriyas, or the warriors and rulers, supposedly from his arms. The third slot went to the Vaishyas, or the traders, who were created from his thighs. At the bottom of the heap were the Shudras, who came from Brahma's feet and did all the menial jobs.
And here is the real meaning, as paraphrased:
From this cosmic intelligence, we derive the four most desirable traits of character, namely wisdom, nobility, industry, and talent or
Purusha (energy) and Prakriti (matter) represent the duality of all creation. Purusha in the Rg Veda is not standing man. Just as the constellation of Orion is visualized as the Nataraja, Purusha here represents the sum total of energy in the universe, Prakriti the sum total of matter in the universe. So any representation in the analogy is not associated with a hierarchy, nor does Purusha represent humans alone. Purusha & Prakriti together represent the sum total of the duality of all creation.
Further, a verse from the Skanda Purana [SP 18.VI.239] asserts "janme jayate shudra...", meaning all humans are born with some talent or facility. That, if you wish to go into "public service" to serve the people of the land, then you must train long and hard to qualify to be a dwija! Thus, one could say, that Shudra is just another word for the "talented citizens" of a kingdom or country, essentially 90% of all citizens.
Make what you want of this!
Since some of you asked, "caste" (Portuguese for "race"), has no authority in the Veda. In fact, there is no word that is equivalent to "caste" in any Indian language - Sanskrit or any of the Prakrits.