There most certainly is life on Mars at present though it may not be native. Let me explain
Every spacecraft that goes into space or lands on other planets is heat treated to kill any human based life that may be on its surface. This is fine in principle but it turns out that not all of the components are treated this way, mainly because they instruments/parts themselves would not survive the heat treatment. This has lead to contaminated spacecraft leaving earth.
At present the curiosity rover is currently exploring Mars and test conducted by NASA scientists have shown that there is a strong possibility that bacteria including Rhodococcus and Pseudomonas have survived the flight from earth to Mars. This could have huge implications, firstly it could contaminate the samples collected by the rover and give false positives. But if there are native bacterial species then our earth based bacteria may overrun and destroy any life of Mars that may exist.
There are some fossil records found on our planet that are said to have originated from Mars, from over 4 billion years ago. Scientists have analysed these meteors and detected what they believe to be signs of fossilised bacteria. If they can conclusively show that these fossils are not from earth then it is safe to assume that Mars did once support life.