Such a discovery would confirm for the first time that life has existed elsewhere in the universe. Big news, for sure — but what would life on Mars really mean for life here on Earth? [7 Theories on the Origin of Life]
If life exists on Mars, then we might be ethnic Martians ourselves, scientists told Life's Little Mysteries. They explained that the small coincidence of having two life-bearing planets right next door to one another gets cleared up if one of the planets actually seeded life on the other — a concept called "panspermia." According to Pavlov, hundreds of thousands of Martian meteorites are strewn across Earth. These were hurled into space during past planetary collisions (such as the bash that left Mars with a crater covering nearly half its surface). One of these chunks of Mars could feasibly have contained spores that lay dormant during the interplanetary commute to Earth, and then blossomed upon arrival, some 3.8 billion years ago.
Alternatively, any Martian microbes we find could be ethnic Earthlings that made the trip from here to there. That's a little less likely, considering the relative locations and gravitational pulls of the planets, the scientists said.
Either way, we can tell if Martians and Earthlings have a common root by determining whether Martian life encodes itself the same way we do — with DNA. DNA breaks down on hundred-thousand-year time scales, so we would need to find living or freshly dead alien microbes in order to be sure that Mars' life arose independently of Earth's. Pavlov says it's very possible that living things are eeking out an existence in Mars' inhospitable modern landscape, if they were ever there in the first place. As attested to by the extremophiles inhabiting Earth's underground volcanoes and frozen tundras, life tends to adapt and persist once it gets started. Following this line of thinking, if Curiosity finds remains of ancient life, NASA's next Mars mission will go in search of extant microorganisms.