This is a possibility that some people I have heard are considering seriously. We will have to see just how similar any purported martian life forms are to Earth life forms. BUT, if we find them to be similar in their DNA, this could also mean that DNA looks pretty much the same everywhere. Earth has gotten smacked by some pretty big asteroids just in the last 100 million years. This happened during its first billion years of history too, so I would not be surprised if some of this litter made it to Mars.