"Moreover, the expanses of interplanetary space are not the forbidding barrier they once seemed. Over the past 20 years scientists have determined that more than 30 meteorites found on Earth originally came from the Martian crust, based on the composition of gases trapped within some of the rocks. Meanwhile biologists have discovered organisms durable enough to survive at least a short journey inside such meteorites. Although no one is suggesting that these particular organisms actually made the trip, they serve as a proof of principle. It is not implausible that life could have arisen on Mars and then come to Earth, or the reverse. Researchers are now intently studying the transport of biological materials between planets to get a better sense of whether it ever occurred. This effort may shed light on some of modern science's most compelling questions: Where and how did life originate? Are radically different forms of life possible? And how common is life in the universe?"

http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=00073A97-5745-1359-94FF83414B7F0000