Although life on Earth is relatively well studied, if a shadow biosphere exists, it may still remain unnoticed because the exploration of the microbial world targets primarily the biochemistry of the macro-organisms.
Steven A. Benner, Alonso Ricardo, and Matthew A. Carrigan, biochemists at the University of Florida, argued that if organisms based on RNA once existed, they might still be alive today, unnoticed because they do not contain ribosomes, which are usually used to detect living microorganisms.
[6] Carol Cleland, a philosopher of science at the University of Colorado (Boulder), argues that desert varnish, whose status as biological or nonbiological has been debated since the time of Darwin, should be investigated as a potential candidate for a shadow biosphere.
Existence of a shadow biosphere could mean that life has independently evolved on Earth more than once, which means that microorganisms may exist on Earth which have no evolutionary connection with any other known form of life.
[6][7] Methods used by proponents and conclusions drawn from experiments that purport to show evidence of shadow biospheres have been criticized.