Shadow biosphere
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A team of astrobiologists at the University of Colorado at Boulder have defined a shadow biosphere as, "...a microbial biosphere that is so chemically and molecularly different from life as we know it that it wouldn't be in direct competition with familiar life; familiar life couldn't metabolize it and it would occupy ecological niches that were underpopulated by familiar organisms. Such organisms might have proteins made of completely different amino acids or amino acids with the opposite chirality or it might have nucleic acids whose sugars have the opposite chirality, to name a few possibilities."1
The term was also mentioned in the "Jargon" section of the May 2006 issue of Wired magazine where it was described as, "alternative microbial life that evolved from chemistry entirely unknown to modern science..."