English I originates from Old English (OE) ic. Its predecessor ic had in turn originated from the continuation of Proto-Germanic *ik, and ek; the asterisk denotes an unattested form, ek was attested in the Elder Futhark inscriptions (in some cases notably showing the variant eka; see also ek erilaz). Linguists assume ik to have developed from the unstressed variant of ek. Variants of ic were used in various English dialects up until the 1600s.
Germanic cognates are: Old Frisian ik, Old Norse ek (Danish, Norwegian jeg, Swedish jag, Icelandic ég), Old High German ih (German ich) and Gothic ik and in Dutch also "ik".
The Proto-Germanic root came, in turn, from the Proto Indo-European language (PIE). The reconstructed PIE pronoun is *egō, egóm, with cognates including
Sanskrit aham, Hittite uk, Latin ego, Greek ἐγώ egō, Old Slavonic azъ and Alviri-Vidari (an Iranian language) اَز az.
The oblique forms are formed from a stem *me- (English me), the plural from *wei- (English we), the oblique plurals from *ns- (English us) and from Proto-Germanic *unseraz, PIE *no-s-ero- (our, ours).