I think blue-green algae is prokaryotic, thus making it one of the simplest forms of life. The thing that always confuses me is stromatolites. Mats of prokaryotic cells? I wish I had an illustration.

Don't even start with smart comments! I know your brain is holding a LOT more information and jargon that mine -- I simply have the advantage of having read the rest of the book. ^_^ I would highly recommend the rest of the book, too, it's fairly amazing.