Why were archaea once grouped with bacteria in kingdom Monera?

Archaea were once grouped with bacteria in the kingdom Monera because both groups are prokaryotes—organisms whose cells lack a defined nucleus and other specialized organelles.12

Historically, biological classification systems were often based on observable morphology. Because archaea and bacteria share this fundamental structural simplicity, they were long considered to be a single, unified group of microorganisms. In the late 1930s, the kingdom Monera was established specifically to categorize these nucleated-lacking organisms.132

This classification persisted until the late 1970s, when microbiologist Carl Woese utilized ribosomal RNA analysis to demonstrate that prokaryotes actually consist of two distinct evolutionary lineages. Further research revealed that archaea possess unique biochemical and genetic traits—such as specific cell wall compositions and RNA polymerase structures—that differentiate them from bacteria and, in some respects, align them more closely with eukaryotes. Consequently, the scientific community moved toward the three-domain system, recognizing Archaea as a separate domain of life.143