A bit of Googling yields the following:
Design History: Get to know your ampersands, by Kaitlyn Ellison
It was once the 27th letter of the alphabet, derived from the Roman
word for and: ‘et’. When ancient Roman scribes were scribbling away in
Roman Cursive around the 1st century AD, they had a tendency to
connect to two letters into a ligature.
That symbol evolved over time, and by the Renaissance had developed
into the calligraphic symbol that we’re so familiar with now.
Wikipedia adds the information that the origin was not from 'et' but 'Et'.
Symbols originating as ligatures
The most common ligature is the
ampersand &. This was originally a ligature of E and t, forming the
Latin word "et", meaning "and". It has exactly the same use in French
and in English. The ampersand comes in many different forms. Because
of its ubiquity, it is generally no longer considered a ligature, but