I thought this book was very interesting, but the subtitle is misleading. Klugar does a good job describing where things fall on the simple to complex arc, but he never really explains how simple things become complex or vice versa. He explains why he's placed things where he has on the simplicity/complexity arc but he never goes any deeper than that. There's no real explanation of why a given thing has evolved to be simple or complex.
Still, it was an interesting read. It reminded me a little of [book:Freakonomics], although I think that book did a much better job of explaining why of things.