Re: Prometheus, Continued
We don't really know if DNA is common. We DO know that amino acids are common, so it is reasonable to assume alien life will also rely on amino acids for its protein and genetic applications, but there may be alternatives to DNA we can't conceive of.
Non-DNA lifeforms may have existed on the primitive earth, but they could not compete and were selected against. Today, every organism on earth is descended from one genetic design, probably a virus-like bug.