"If aliens exist, they may be living right next door..." No, it's not a writer pitching a television pilot, but the premise of a scientific theory called the "shadow biosphere," New Scientist reports. Some astrobiologists say that life forms may have originated on Earth after our own, and endured, unnoticed. "I think if we found a second sample of life on Earth, it would be as big as Darwin's theory of evolution," said one scientist.
Skeptics have issues. New life forms—most likely in molecule form—would be eaten up on our crowded Earth, they say. Or if they did endure, their bizarre biochemistries would be hard to detect. But "shadow biosphere" believers are encouraged by the fact that less than 1% of microbes have been identified. So they're busy hunting for alien life in extreme environments, like ice sheets and ultra-dry deserts.