Here's a thought - do compelling characters HAVE to be human?

Right then, here's an interesting thought. I was pootling about the other day, doing lots of ever so important and useful things (er, okay, I was on Facebook), and I happened to see that someone had left me a new review for The Non-Random Dog. It was a five star review, so I was pretty damn chuffed, but the really nice part was what the reviewer actually wrote about the book. Specifically, the part that made me really happy was where they said about the characters being the key thing in the book.

I was really rather happy to see that highlighted, 'cos that's kind of the point of what I'm doing. Y'see, the thing is, I loved sci-fi all my life, and while I love big technology and stars blowing up as much as the next nerd, the thing that's always hooked me in is the characters. From Deckard, to Ripley, to Optimus Prime, to Han Solo, to Lt. Cmdr. Susan Ivanova, to... well, you get the point. But here's a question for you guys and gals out there: do compelling characters absolutely have to be human?