I think Lee Daniel Crocker is quite right in pointing out that we have few clues how to maintain a biosphere capable of supporting a human society indefinitely offplanet. More fullscale biosphere closure experiments are needed, and *much* more information is needed about species (bacterial and otherwise) symbiotic with human beings, and our ability to get along without them in the Long Run. Symbiosis includes not only endosymbiosis, but for example the "air-scrubbing" soil bacteria that were used in Biosphere 2.
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arkuat