Which addition did Carl Woese make to the taxonomic classification system?

Carl Woese made a significant contribution to the taxonomic classification system by adding the level of "domain." Prior to his work, the classification system consisted of five kingdoms, but Woese's research on ribosomal RNA sequences led to the recognition of three distinct lineages of life: Archaea, Bacteria, and Eukarya. This classification demonstrated that there are fundamental differences between these groups, particularly at the genetic and evolutionary levels. As a result, Woese introduced the domain as a higher taxonomic rank above kingdom, which allowed for a more accurate representation of the relationships among all living organisms and emphasized the evolutionary connections among them. His contributions have fundamentally altered our understanding of the tree of life.