What if they are not made of dna. In fact, an alien lifeform could be made of something we have never discoved.
Well, technically life isn't made of DNA, but I get your meaning. Sure, there could be completely different forms of life that aren't anything like the carbon-based flora and fauna we're used to.
Scientists base all of their discoveries on what they know, but what about the stuff they don't know?
Actually, they don't. Making baseless assumptions is kinda
the opposite of science.