Short answers, pulled from the story.
The Rigveda states that four social groups emerged from different parts of a primordial being named Purusha. A Brahman formed from his mouth, a Rajanya arose from his arms, Vaishya came from his thighs, and Shudra developed from his feet.
Scholars estimate the Mahabharata reached completion around the fourth century CE. Section 12.181 of this epic discusses the Varna system extensively with color-coded models presented by sages like Bhrigu and Bharadwaja.
Patrick Olivelle argues ancient texts do not support ritual pollution based on birth groups. Purity-impurity discussions focus on individual moral or biological actions like eating meat or bodily functions rather than collective group identity.
Buddhist literature concludes only two qualities remain necessary for recognizing a true Brahmin: being virtuous and being learned and wise. Peter Masefield notes that during the Nikaya period people in any Varna could perform any profession without strict commensality rules.
The term jāti refers to thousands of endogamous groups prevalent across the Indian subcontinent while classical authors scarcely speak of anything other than the four varnas. Indologists sometimes confuse these distinct concepts despite their differences.