re: What is life ?
This is not really a satisfactory answer to your question, but minutes before reading your message above; I had seen this article on the BBC web. A physicist speculates that there may be multiple origins of life (highly probable) on Earth (or on Earth via Mars), and that some may still exist. He speculates either that they are in environments so hostile to our life form that we have never properly searched them for life, or so different from us that we cannot recognize them although they may be all around us (or both, they may be hidien and unrecognisable). Something to consider.
Alien life 'may exist among us
http://news.bbc.co.uk/2/hi/science/nature/7893414.stm