Origins
The English word 'one' is among the most fundamental in the language, yet its pronunciation conceals a remarkable phonological history. It descends from Old English 'ān' (pronounced roughly like 'ahn'), from Proto-Germanic *ainaz, ultimately from the Proto-Indo-European root *oi-no-, meaning 'one' or 'unique.' This PIE root is one of the best-attested numerals in comparative linguistics, with reflexes across virtually every branch of the Indo-European family.
In the Germanic languages, *ainaz produced Old English 'ān,' Old High German 'ein' (modern German 'eins'), Old Norse 'einn,' Dutch 'één,' and Gothic 'ains.' All these forms are phonologically regular developments from the proto-form. Outside Germanic, the PIE root *oi-no- gave Latin 'ūnus' (one), Old Irish 'oín' (later 'aon'), Old Church Slavonic 'inŭ' (one, a certain), and Greek 'oinos' as seen in the compound 'oinē' (the ace on a die). The Latin descendant 'ūnus' became the ancestor of French 'un,' Spanish 'uno,' Italian 'uno,' and Portuguese 'um,' and it also entered English through learned borrowings: 'unite,' 'union,' 'universe,' 'unique,' 'uniform,' and 'unicorn' all derive ultimately from the same PIE numeral.
The most striking feature of the English word is its pronunciation. Old English 'ān' was pronounced with a long 'a' vowel and no initial consonant. During the Middle English period, dialects diverged significantly in how they treated this word. In the west and southwest of England, a /w/ glide developed before the rounded vowel, producing forms like 'won' and 'wone.' Meanwhile, eastern and northern dialects preserved pronunciations closer to 'oon' or 'on.' By a historical accident, standard English adopted the western pronunciation /wʌn/ but the eastern spelling 'one' (from 'oon'), creating one of the language's most notorious spelling-pronunciation mismatches. The word 'only' — from Old English 'ānlīc,' literally 'one-like' — preserves the older pronunciation without the /w/ glide.
Old English Period
The word 'one' has generated a surprisingly large family of English words that no longer look related. 'Alone' is a Middle English contraction of 'all one,' meaning 'entirely by oneself.' 'Atone' was originally 'at one,' meaning 'to be in harmony or reconciled,' with the theological sense of reconciliation with God developing in the 16th century. 'None' comes from Old English 'nān,' literally 'not one' (ne + ān). 'Any' descends from Old English 'ǣnig,' meaning 'any one' or 'some one,' itself derived from 'ān.' Even 'once' is simply 'one' with an adverbial genitive suffix.
The grammatical behavior of 'one' in English is also notable. It serves not only as a cardinal number but as an indefinite pronoun ('one should be careful'), an indefinite article in some constructions ('one day'), and a noun ('the one I want'). This pronominal use of 'one' developed in Middle English, partly under the influence of French 'on' (from Latin 'homō,' man), which had acquired a similar impersonal pronoun function.
Cross-linguistically, the numeral 'one' frequently develops into indefinite articles and pronouns — French 'un/une,' German 'ein/eine,' and Spanish 'un/una' all serve double duty as both 'one' and the indefinite article. English is unusual in that its indefinite article 'a/an' is historically the same word as 'one' (both from 'ān') but has diverged so far in pronunciation that the connection is no longer obvious. The form 'an' before vowels preserves the older shape; 'a' before consonants represents further reduction.
Proto-Indo-European Roots
The PIE root *oi-no- itself may be related to the demonstrative pronoun *ey-/*i- ('that one'), suggesting that the concept of 'one' may have originally been expressed as 'that (specific) one,' a deictic rather than purely numerical notion. This would align with a broader pattern in PIE where low numbers seem to have emerged from demonstrative and pronominal roots rather than from a fully abstract counting system.