1. Evidence that supports the theory that life existed on Mars.
The most promising evidence for past life on mars comes from evidence that liquid water once existed on the surface of mars. Liquid water is considered the 'essential ingredient' for the evolution of life. Evidence for the existence of liquid water has been gathered in abundance by the rovers we sent to Mars - Spirit and Opportunity.
Additional evidence of past life on Mars comes from meteorites recovered in Antarctica. These meteorites were chemically analzyed and proven to come from Mars. Within these meteorites micro-fossils appeared to be present, suggesting that bacterial life once existed on Mars.
2. Current conditions on Mars are not as favorable for Life as they would be if freely running and abundant standing water were available on the surface. That said, living organisms are extraordinarily adaptive. If life ever did exist on Mars, it's possible that it found small micro-environments (underground???) in which to survive.
Also, because the atmosphere on Mars is very thin, there is little protection from Cosmic radiation and solar flares - this can be a problem for unprotected life forms.
Also - its possible that microscopic life on Mars may exist today, transported to the Red Planet by our own space probes from Earth.
3. Is it possible that life is currently on Mars?
Not likely, but not impossible. That's why we need to send robots, experiments, and hopefully one day astronauts to look into this.
KEEP STUDYING SCIENCE - It's the greatest hope for your species.
Stupid politicians, like President George W. Bush, try to suppress scientific inquiry - your future depends on voting for candidates that embrace Science and can read, speak and write.