Until we understand how self replicating chemical systems occur
- ie life - then any estimates on whether life could or couldn't have occured on mars or anywhere else in the universe is just complete speculation. For all we know life on earth may be the only example in the universe due to some incredibly unlikely occurance happening in the proto earth chemical soup.
Either way , earth had huge deep oceans which were around for billions of years for this to occur in. Mars at best had a few warm shallow seas which didn't hang around for very so frankly if any life is disovered on mars that didn't hitch a ride on a meteorite from earth then I'll be amazed. My gut feeling is its a sterile desert planet and always has been.