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Biologists tacitly assume that all life on Earth descended from a common
origin. This assumption is based on biochemical similarities and gene sequencing, which
enables organisms to be positioned on a common tree of life. However, most terrestrial
organisms are microbes, and it is impossible to deduce their biochemical nature from
morphology alone. The vast majority of microbes remain unclassified, leaving open the
possibility that some of them might be an alternative form of life, arising either from an
independent origin, or representing a hitherto overlooked very ancient branch of the
known tree. Thus there may exist an extinct, or even extant, shadow biosphere. I discuss
various research proposals for locating and identifying "alien" organisms on Earth, both
ecologically separate and ecologically integrated.
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