A brief historical overview of kingdom classification and related issues from the nineteenth century, through "the evolutionary synthesis" of the 1940s to the prokaryote-eukaryote dichotomy and five kingdoms of the 1960s and 1970s sets the background for understanding the uniqueness of the 16S rRNA approach and the conceptual preparedness for the discovery of the Archaea.
While providing striking testimony of the power of the new approach for exploring microbial evolution, hitherto deemed impossible, the discovery of the Archaea opened up new research programs aimed both at resolving old problems and posing new questions. The nature of the paradigmatic shift signaled by the discovery of the Archaea is further highlighted through the views of its critics.
