To summarize : In the 6th or 7th century, Lating scribes shortened the word ad (the Latin word for at or to) to an a with the d sort of curled around it.  Jump ahead a ways, to the 1800’s, and @ appeared on typewriters, to signify a price “I’ll take 3 @ $2.50.” 

“Then, one day in late 1971, computer engineer Ray Tomlinson
grappled with how to properly address what would be history’s very
first e-mail. After 30 seconds of intense thought, he decided to
separate the name of his intended recipient and their location by using
the “@” symbol. He needed something that wouldn’t appear in anyone’s
name, and settled on the ubiquitous symbol, with the added bonus of the
character representing the word “at,” as in,
hey_you@wherever_you_happen_to_work.com.”

http://www.realtechnews.com/posts/3148