I'm just guessing, of course, but if they find any evidence of complex life, it will probably be something that can be identified as having come from earth... or at least extremely similar to past life on earth. The similarities will be so great that all they will prove is that there was at one time a near collision of Mars and the earth that sucked a bunch of Earth's material off Earth and deposited it on Mars.

The only other thing that might have happened is that somebody from Earth's ancient past had space travel, and dropped off some "life" on Mars.