The ampersand is a logogram (a character that represents a word) that dates back to the 1st centry C.E. In Roman cursive, the ampersand is “Et,” which is Latin for “and” (“Et tu, Brute?” = “And you, Brutus?”, “et al” = “and all”).
So how did “et” become the modern ampersand? Well, anyone that has dealt with cursive will attest that handwriting can and often is stylized to the point of illegibility.
As time went on, it became more and more stylized–and so it goes (or, et ita fit)! The word “ampersand” itself is thought to come from “& per se and” (“and per se and”).
Want to know more about etymology (the history of words)? Try any of these books.