A Million Ways to Die on Mars #1 Martian Life
How long was life present on Mars? A billion years would not be unreasonable.
A complex Martian ecology may have lived and died long before Earth cooled down. Since Mars has gone through a longer and more complex life cycle, any survivors would be a lot more sophisticated than evolution based on the current conditions would allow.
Mars likely has some very nasty diseases lying in wait. Most knowledgeable people consider a journey to Mars as a suicide mission because of the technical and support issues. If a random sample of Martian soil tested positive for Martian life (Viking landers) then the whole planet could be a deathtrap.