Rig Veda 10.90 describes a cosmic person, Purusha, sacrificed by the gods, from whose dismembered body the world is made: the moon from his mind, the sun from his eye, the sky from his head, the earth from his feet. It is a creation-by-sacrifice cosmology and it has close parallels elsewhere in Indo-European material, most obviously the Norse account of the world made from Ymir.
The verse that made it consequential is the eleventh and twelfth. Asked how the person was divided, the hymn answers that the brahmin was his mouth, the ruler his arms, the producer his thighs, and the servant his feet. This is the earliest appearance in Indian literature of the four varnas as a set, and it presents them as an aspect of the structure of the cosmos rather than as a human arrangement.
That single verse has been cited in defence of the caste order for two and a half millennia, and cited against it, reformers including Dayananda Saraswati and later Ambedkarite writers have read it as descriptive of function rather than birth, or rejected the hymn's authority outright, and Ambedkar's own argument was that the passage is late and interpolated.
The scholarly position on that last point is not merely convenient. Book 10 of the Rig Veda is generally regarded as among the latest material in the collection on linguistic and stylistic grounds, and this hymn is often treated as late even within Book 10. That is a judgement made on philological grounds and reached independently of the political argument, but it lands squarely in the middle of it.