A few months ago the first Avenger was officially announced.
And as you may have noticed from the title "The First Avenger" is Captain America, starring Chris Evans.
According to the original comics, Captain America was not a part of the original Avengers, but rather discovered by the original Avengers.
He was trapped in ice, which was Marvels way of bringing back Captain America after 20 years of being gone, so naturally everyone were surprised to find him since he had been missing for 20 years.
This appearance was made in the 4th issue of The Avengers comic.
The original Avengers consisted of Ant-Man (Dr. Hank Pym), Wasp (Janet Van Dyne), Thor, Iron Man (Tony Stark), and the Hulk (at a time when the Hulk was more intelligent(Bruce Banner)).