Did life emerge spontaneously on earth, or did it come from outer space? The scientific community is sharply split on the question, and the evidence from Mars not only heats up the debate but also adds a tantalizing third possibility: life-forms may have arisen on Mars first and then hitched a ride on a meteorite to Earth--or vice versa. As Stanford University chemist Richard Zare puts it, "Who is to say that we are not all Martians?"
Sound implausible? Consider the alternatives. Sir Fred Hoyle, the distinguished British astronomer, favors an even more radical theory. The idea, known as panspermia, is...
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