What is the theme in Understanding Comics by Scott McCloud?

I think the main theme of this book involves comics as an underappreciated art form. The purpose of Understanding Comics is to stimulate a debate about the inclusion of comics as an art form. McCloud believes that comics are much older than generally thought. He even provides evidence of pre-Columbian art as proof of the origin of comics, although he does not care to speculate as to the exact location and date that comics is born.