Who was the first śūdra (or the śūdra-couple) to be born?
Likewise, who was the first brāhmaṇa, the first kṣatriya and vaiśya to be born?
Some might argue, the first śūdra could not have been born because that would make his/her parents also śūdra and then they would become the first śūdras. So we end up with an infinite regress. To avoid this problem, assume the first śūdra (or the śūdra-couple) just appeared on Earth and were not borne of a womb, then the question becomes: Who was the first śūdra (brāhmaṇa, kṣatriya, etc.) to appear on Earth?
Note: This question assumes the caste-system to be birth-based, so I'm not looking for answers/comments that argue that varṇa is guṇa-based or varṇa is independent of birth.