How did Carl Woese’s three-domain system change the classification of living things?

Carl Woese’s three-domain system fundamentally transformed the classification of living things by shifting the focus from observable physical traits (morphology) to molecular phylogeny, specifically through the analysis of ribosomal RNA (rRNA) sequences.12

Prior to this, life was often categorized into two primary lineages—prokaryotes and eukaryotes—or the five-kingdom system. Woese’s research revealed that prokaryotes are not a single group but consist of two distinct evolutionary lineages: Bacteria and Archaea. This established a new framework consisting of three domains: Bacteria, Archaea, and Eukarya.321

This system changed biological classification in several key ways:

• Phylogenetic Accuracy: By using molecular data, it replaced systems based on nutrition or physical appearance, which often grouped unrelated organisms together.2
• Reassessment of Protists: The system challenged the validity of the kingdom Protista, as molecular insights showed that many eukaryotic groups are more closely related to multicellular organisms (like animals or plants) than to other protists.12
• Evolutionary Insight: It demonstrated that Archaea are genetically distinct from Bacteria and, in some respects, more closely related to Eukarya.34