Your logic is backwards. Of course Mother Nature doesn't work according to what we know. We know what we know in accordance with Mother Nature. WE KNOW life can come from liquid water because it does on earth. I could very well exist from other base compounds but we don't know that because we haven't seen it. So the question is, what is the most effecient way of searching for life off of earth? Look for liquid water. Because we know it worked on earth is could work elsewhere. And that is what is so exciting about liquid water on Mars! We now have a place that is relatively close to home that could support life!