|MadSci Network: Genetics|
Thanks for this interesting question. You raise many points with a question like this one, some of these are: (1) does the presence of DNA mean that life is present, (2) do we know of DNA from places other than the earth and (3) if we did, would that mean that there was life there? Let's look at each of these in turn.
As you say there are "several" life forms on earth-in fact we divide them into 3 groups or kingdoms; the Archaea (primitive, single celled microbes), the Eubacteria (more widespread and numerous bacteria) and the Eukarya (everything else-plants, animals, fungi, you and me). One thing that we all have in common is DNA. All life on our planet is based on DNA, what makes it different is the number and type of genes encoded by that DNA and how they are expressed. The fact that all life here is related through DNA is the best evidence for evolution.
So on earth, DNA means life. You might then ask, has it always been this way and how did it come to be so? Well, experiments in the 1950's showed that the simple chemicals which make up DNA could have been produced on the early earth by simple chemical reactions. This was important, because it meant that it may not be so hard to make life after all-wherever you have the right conditions, the chemicals would form and life would follow. Most scientists assume that life has always been based upon DNA, or something similar (in fact many believe that the first genes were based on a similar chemical called RNA). But there are some alternative theories. In particular, Alexander Cairns-Smith put forward a theory that the first life forms were inorganic-made entirely from complex clay minerals which could perform many of the functions that DNA and proteins perform today, and that DNA and RNA came later. This is an interesting but rather controversial idea! But it's worth remembering that because all we know is DNA, we'll never know if there was something before it.
Now, you ask if there could be DNA elsewhere and if so, could life derive from it? Well, if we found DNA from somewhere else, we would know that there had to have been life there, because so far as we know, DNA is only made through biological processes. In fact we have never found actual DNA in any of the materials that have come to earth from space, for instance in the meteorites from Mars and other places. But we have found simple organic chemicals that could be used to make DNA and other biochemicals like amino acids. So it might be that these materials are quite common in certain objects in space and who knows, perhaps on other planets, the chemistry was right and life could start there. But then if people like Cairns-Smith are correct, there could be life based on something other than DNA. So you could say that not finding DNA doesn't have to mean there is no life-it would just be a life that we don't recognise!
You can see that this is a fascinating topic-the origin of life and whether there is life elsewhere are the biggest unanswered questions in biology. You might like to look at these web sites:
The Talk.origins archive is devoted to origins and evolution.
This is a really good interview with Stanley Miller, who did those experiments in the 1950's.
This is a page discussing life on Mars. You know, some people believe that the Viking spacecraft did detect microbial life there in the 1970's!
The book by Alexander Cairns-Smith is a classic called "Seven Clues to the Origin of Life: a scientific detective story" and is a very good read.
I hope this helps with your question.
Try the links in the MadSci Library for more information on Genetics.