In a discovery that has scientists rethinking where they came from, a groundbreaking study has revealed that living organisms could emigrate through the solar system in the relatively cool womb of a space rock, spreading life with little more fanfare than the arrival of a shooting star. The finding, hidden from scientists for more than 15 years in the magnetic structure of a well-studied meteorite found in Antarctica, presents a serious alternative to the idea that life on Earth arose spontaneously out of some primordial soup. The bottom line: Our ancestors may have been Martians.