Final answer:
Originally, life was classified into two main kingdoms: Animalia and Plantae. These have been expanded to three domains: Bacteria, Archaea, and Eukarya, which encompass various kingdoms based on deeper genetic relationships.
Step-by-step explanation:
Life used to be classified into two main kingdoms: Animalia and Plantae. These categories were based on Linnaeus's tree of life, which contained just these two main branches for all living things. Later on, the classification system was expanded to include additional kingdoms, such as Protista and Monera, proposed by Ernst Haeckel, to host unicellular organisms with or without nuclei, respectively. Subsequently, the work of Carl Woese and others led to the development of a new system that categorized life into three domains based on comparative genetic analysis: Bacteria, Archaea, and Eukarya, with Eukarya encompassing kingdoms Animalia, Plantae, Fungi, and Protista. This newer system reflects a more fundamental evolutionary relationship between organisms than the traditional kingdom-based system.