The Barnhart Dictionary of Etymology (Wilson, 1988, ISBN 0-8242-0745-9) says: "~I~ pron. 1137 i; later I (about 1250, in The Story of Genesis and Exodus); developed from the unstressed form of Old English (about 725) ic singular pronoun of the first person (nominative case). Modern and Middle English I developed from earlier i in the stressed position. I came to be written with a capital letter thereby making it a distinct word and avoiding misreading handwritten manuscripts. In the northern and midland dialects of England the capitalized form I appeared about 1250. In the south of England, where Old English ic early shifted in pronunciation to ich (by palatalization), the form I did not become established until the 1700's (although it appears sporadically before that time)."