I would refer you to a website called "Remembering the Old Songs," which I think may have some very useful information for you on the musical evolution of many songs:

http://www.lizlyle.lofgrens.org/RmOlSngs/OldSongs.html

Another suggestion I might make is to touch upon how two musicians can interpret the same song and come up with drastically different treatments. Case in point, the song "Fell in Love With a Girl" by The White Stripes was also recorded by Joss Stone as "Fell in Love With a Boy."

They are so stylistically different that upon initial listenings very few people would ever conclude this was the same song.

Good luck with your class,