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How, when one considers the absolutely ridiculous odds of a single-celled, self-replicating organism spontaneously evolving on this little planet some three bill ion years ago, can we even contemplate it happening right next door to us on Mars?
Two thoughts...
(1) As was said, we don't know the probability of abiogenesis, so it's an open question (although I think many scientists would be surprised to see evidence of independent life having formed on Mars).
(2) Martian life need not be independent. Bits of rocks from Mars and Earth are periodically exchanged and it is feasible that microbes could hitch a ride from one planet to the other. Large asteroid/comet impacts can send debris from a planet into space & eventually they can find their way to other planets. Remember the news about the Martian "SNC" meteorites found in Antarctica a few years ago? And I'm pretty sure NASA has found that some microbes can survive long periods in space (e.g., through forming a cyst stage).
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