The ampersand or esperluette dates to the 1st century A.D. The Old Roman cursive letters E and T were written together to form a ligature. In the 7th century, development of the Latin script and Carolingian minuscule, ligatures in general diminished, but et sign continued to be used and gradually became more stylized and more abstracted.
Though I'm pretty sure I learned this fact at some point during my typography classes and maybe even during my former life as an English minor, I managed to forget it, and the above illustration makes it a little clearer.
So, there's your fact for the day. Enjoy.