The only free and comprehensive online etymological dictionary of the Spanish language
yo (Personal Pronoun) "I"
10th cent. From Vulgar Latin eo, from Latin ego 'id.' From Proto-Italic *egō 'id.' From Proto-Indo-European *h1eǵ-(o)H. From earlier *h1eǵ- 'id.'
Asturian yo, Portuguse eu, Galician eu, Catalan jo, French je, Italian io, Aromanian io, Romanian eu, Sardinian eo

Indubitably, the word is especially ancient in Proto-Indo-European, at one point even labelled a "Devonion rock" of the language by Benjamin Fortson (2011). Because of its extreme age, the semantic sense of *h1eǵ- and its many suffixes (Latin points to *-(o)H; Sanskrit to *-om; Germanic *-Hom; and Hittite to a word without a suffix) have inspired significant discussion (see Sihler 2008).