Based on designs as we find them on Earth, yes. But still, there's no way to be certain if there aren't other lifeforms out there (unless we find them, or they find us), completely different from any mammals we know. Which could also possibly ruin the theory of needing an Earth-class-like planet. That said, it is also unknown whether or not life is 'uncommon' in the universe. What is you definition of common then? We can only see a small sand particle in a desert of stars and planets. Just because we haven't found them yet, doesn't mean they are uncommon. For all we know, intelligent life might be hiding themselves from us, choosing not to interfere in a slowly evolving race who has yet to perfect interstellar travel.