I'm curious about the common ancestor.

How is it determined that there was one common ancestor? Would the DNA of life on Earth look different if life had originated more than once? Would it look different if the common ancestor had originated on Mars? What about both Panspermia and life originating independently on Earth, would we see any evidence in the DNA or fossil record today?

I love the idea of Panspermia; we really would all be children of the universe. (My two little boys love to be told they are made of stardust!)