A few billion years ago, Mars may have been a planet covered with oceans. We’re not sure what happened (NOTE: Subscribers can still listen to this show), but we do know that the liquid has mostly burned away. Curiosity will probe the soil that they left behind in order to look for tiny fossils.
In LiveScience.com, Natalie Wolchover quotes NASA’s Alexander Pavlov as saying, "Organic molecules can last for billions of years."
If two planets that are next door to each other, it could be more than a coincidence. If one planet seeded that life to the other one via microbes riding on an asteroid, it would start to make sense–even if life evolved on our planet but stayed dormant on Mars.
According to Pavlov, hundreds of thousands of Martian meteorites are strewn across Earth. These were hurled into space during past planetary collisions. If one of these "chunks of Mars" contained "spores that lay dormant during the interplanetary commute to Earth, (they could have) "blossomed upon arrival, some 3.8 billion years ago, " Evidence of this is a huge crater that covers almost half the surface of Mars.
Wolchover quotes SETI’s Seth Shostak as saying that if Curiosity DOES find signs of life, "It would be like finding 2-ton blocks of limestone in the desert in Egypt and saying, hmm, these might be leftover pieces of a structure around here somewhere."