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Number (in trillion [yes, with a “t”]) of distinct smells on Earth that humans can distinguish, according to neuroscientist and smell researcher Andreas Keller of Rockefeller University. As reported in Smithsonian magazine, these smells appear to be perceived by some 400 different types of receptors high in the nostrils. Yet exactly how smell works and how perceptions of various smells are interpreted in the brain is not understood.