I also used to believe that, but as I learned more, it became evident that this is not really possible. Mars has been absolutely sterile for at least 3 billion years. That's a long time in biological and geological terms. Even on Earth it took a long time for life to start going, let alone evolve into something more complex. If Mars ever had any kind of life form, it was for a very brief period and very simple organisms. Most evidence point to a period between 3.6 and 3.8 billion years ago, which is exactly the age of Gale Crater where Curiosity is in right now. By the way the rover confirmed through soil analysis that the place had all the necessary ingredients for life in that period, with fresh water flowing on the surface.
If life was a common thing on Mars we would have evidence of that all over, including in the hundreds of Martian Meteorites we collected so far, but that is not the case. Only one or two of them seemed to show some evidence of very primitive bacterial life but that evidence is so weak that is was never confirmed.