How to undo all changes in a buffer made since it was open in Vim?

Question

How can I undo all changes since opening a buffer? I imagine there may be some form of :earlier that does this.

Update: Many are suggesting

Answer 1

A graphic solution:

The Gundo plugin allows visual comparison of changes in the undo history.

Open Gundo's "undo history pane", type G go to the last line, then we can back to the original file.