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But have organisms actually made such a journey? If scientists find living microbes on Mars, they may be able to answer this question. Should Martian life-forms base their genetic code on dna, for example, researchers could decipher the script and determine whether they were related to the microbes that populate Earth. And if those life-forms didn't rely on dna at all, "that would be even more fantastic," says molecular biologist Jack Szostak of Massachusetts General Hospital. For such a discovery would not only prove that life arose on Mars and Earth independently and, therefore, spontaneously, but it would also strongly suggest that life--unicellular life, at least--is not something rare and special in the universe but an ordinary event that occurs wherever there is enough water and light from a sunlike star.
--By J. Madeleine Nash