One popular theory on the origin of life holds that it is a “cosmic imperative”—inevitable on any Earth-like planet. That theory is being tested in a novel way, Scientific American reports: by searching for evidence of a “second genesis”—life on Earth that in some fundamental way differs from life as we know it.
If such “aliens” are among us they are probably microbes, the most common form of life, perhaps existing as a “shadow biosphere” hiding in plain sight. Scientists need to analyze the genetic sequences of a microbe to determine if it belongs on the tree of known life. Only a tiny fraction of microorganisms have been classified.