To quote aldiboronti, from whose Wordorigins thread I swiped this link:
Lovely page from Adobe on the historical development of the ampersand, from the ligature of ET or et to the & of today, with illustrations of various stages, the earliest being a ligature from Pompeian graffiti dated 79AD.
A wonderfully exhaustive treatment of this relic of ancient times.
Addendum. See the wide-ranging discussion of the use of ampersands in poetry going on at Dave Bonta’s Via Negativa.