No one knows if there is presently life on Mars or if there was life in the past there that has become extinct. What we do know is that IF there is life, it is microbial. That is there are no plants or animals. Basically, we will not really know until we go there or study it with very sophisticated robots.

Nobody knows. That's the reason why we're looking for it. Of all of the places in the universe where there might be life, apart from Earth of course, Mars looks like the most likely possibility. And the possibility is very real, but we've not found it yet.

There is no proof that life exists on Mars but there is evidence that may suggest there is or was life on Mars.

A meteorite was found in Antarctica that came from Mars and some of the chemicals and fossilized patterns found on the meteorite could possibly have originated from a living organism that was similar to bacteria. However, there is much controversy on whether or not the chemicals and fossilized patterns actually originated from Mars or are contaminants from the environment in Antarctica.

More recently scientists have discovered that the gas methane exists in the Mars atmosphere which suggests that either a volcano had recently erupted causing the methane gas or a life-form similar to some bacteria on Earth is metabolizing carbon dioxide and hydrogen, producing the methane gas. Other gases and chemicals in the Mars atmosphere are metabolic waste-products of certain life-forms on Earth, suggesting that life may have caused these chemicals to exist on Mars.

I think there is a pretty good chance of life having existed on Mars and maybe still existing today. But I think that any life forms on Mars will be very primitive, similar to bacteria and microbial life on Earth.