Final answer:
Classification of life on Earth includes three domains—Bacteria, Archaea, Eukarya—and six kingdoms. Domains are based on cell type, presence of a nucleus, and cell wall differences, with Eukarya comprising both unicellular and multicellular organisms across four of the six kingdoms.
Step-by-step explanation:
The Three Domains and Six Kingdoms of Life
Biological classification of life on Earth extends beyond the familiar kingdom level to include a higher category known as domains. There are three domains of life: Bacteria, Archaea, and Eukarya. The domains Bacteria and Archaea consist of single-celled organisms without a nucleus, whereas Eukarya encompasses organisms whose cells contain a nucleus and other membrane-bound organelles. This latter domain includes not only unicellular but also multicellular organisms and is divided into six kingdoms: Animalia, Plantae, Fungi, Protista, Archaea (sometimes grouped within the domain from which it gets its name), and Bacteria (also a kingdom within its corresponding domain).
The domain Eukarya is further broken down into four kingdoms. Consequently, it is clear that the six kingdoms of life encompass all known living organisms and are categorized based on cellular structure, the complexity of the organism, and the method by which they obtain energy. This comprehensive system ensures that scientists around the world can communicate more effectively regarding the classification and natural relationships among the vast array of life forms on our planet.