I originates from Old English (OE) ic, which had in turn originated from the continuation of Proto-Germanic *ik, and ek;[3] the asterisk denotes an unattested form, but 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.
[4] The Proto-Germanic root came, in turn, from the Proto Indo-European language (PIE) *eg-.
[3] Old English me and mec are from Proto-Germanic *meke (accusative) and *mes (dative).
In some varieties of English (particularly in formal registers), those rules also apply in coordinative constructions such as "you and I".