Origins
The word 'who' is the animate interrogative pronoun in English, asking about the identity of persons.βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ It descends from Old English 'hwΔ' (who, anyone, someone), from Proto-Germanic *hwaz, from PIE *kΚ·Γ³s, the nominative singular of the interrogative pronoun stem *kΚ·Γ³- / *kΚ·Γ-.
The relationship between 'who' and 'what' mirrors the PIE distinction between animate and neuter interrogatives. PIE *kΚ·Γ³s (who) asked about persons and animate beings; PIE *kΚ·Γ³d (what) asked about things and abstractions. This animate-neuter split is preserved in English (who/what), Latin (quis/quid), Greek (tΓs/tΓ), and Sanskrit (kΓ‘αΈ₯/kΓ‘d). The consistency across branches confirms that the distinction is ancient and original to PIE.
The phonological history of 'who' is unusual among the English 'wh-' words. In Old English, 'hwΔ' was pronounced with an initial /hw/ cluster, like all the other 'hw-' interrogatives. During the Middle English period, the vowel shifted and the pronunciation evolved toward /hoΛ/ and eventually modern /huΛ/. The critical difference from other 'wh-' words is that 'who' lost its /w/ element entirely, while words like 'what' /wΙt/, 'when' /wΙn/, and 'where' /wΙΙΙΉ/ preserved the /w/ and lost the /h/ instead. This asymmetry β 'who' keeping the /h/ while the others kept the /w/ β is a genuine irregularity in the history of English phonology, likely influenced by the following rounded vowel.
Old English Period
The case system of 'who' preserves ancient distinctions that have disappeared elsewhere in English. 'Who' (nominative, subject), 'whom' (accusative/dative, object), and 'whose' (genitive, possessive) represent three surviving cases of what was once a full Old English declension (hwΔ, hwone, hwΓ¦s, hwΗ£m). English nouns lost their case endings entirely by late Middle English, but the interrogative pronoun partially retained them β a pattern seen cross-linguistically, where pronouns are more conservative than nouns.
The distinction between 'who' (subject) and 'whom' (object) is in active decline in modern English. 'Whom did you see?' is now felt as formal or archaic by most speakers, who prefer 'Who did you see?' This represents a natural continuation of the case-loss that has characterized English for a millennium. Prescriptive grammarians have defended 'whom' since the eighteenth century, but the trajectory is clear: 'whom' is retreating to fixed expressions ('to whom it may concern') and formal registers, while 'who' expands to cover all syntactic positions.
Latin 'quis' (who) produced an enormous derivative family in English: 'question' (a seeking of who/what), 'quest' (a search), 'request' (to seek again), 'inquest' (an inquiry into), 'query' (from Latin 'quaere,' ask!), and 'acquire' (to seek toward). The same PIE root, through different branches, thus gave English both its basic question word and much of its vocabulary of inquiry and investigation.