Kingdoms of Life

Taxonomic Hierarchy of Life

The modern system for classifying life sorts all organisms into one of three domains and one of six kingdoms.
The current system for classifying life places all organisms in one of three domains: Archaea, Bacteria, or Eukarya. Domain is the largest taxonomic grouping. Kingdom is the second largest taxonomic grouping, under domain. The classification system has changed since the 1700s, when naturalist Carolus Linnaeus first introduced the conventions of modern classification, or taxonomy. Among his ideas for simplifying and standardizing organism names, Linnaeus proposed a two-kingdom system in which all organisms were considered either plants or animals. That classification does not reflect the current understanding and is no longer practical. However, one aspect of Linnaeus's work that still applies today is the use of a system for identifying organisms, called binomial nomenclature. Binomial nomenclature is a two-word naming convention for each type of organism that includes its genus and species name. By tradition, the scientific name of an organism includes two Latin words that are used by all scientists, regardless of their spoken language. The genus and species names are the two most specific categories of the hierarchy of life. These names are italicized with only the genus name capitalized, as in Panthera tigris for tigers. There are eight major categories in the hierarchy of life: domain, kingdom, phylum, class, order, family, genus, and species. There are also some categories that fall in between, such as subphylum. The categories are nested such that each domain includes at least one kingdom, each kingdom includes at least one phylum, and so on, all the way down the list. Each genus includes at least one species. The groups high on the list are more general than those low on the list. For example, the kingdom Animalia includes all animals, but the family Felidae includes the less diverse group of cats.
The categories of the taxonomic hierarchy are nested, such that each level contains at least one of the lower levels. The higher levels are more general, and contain more organisms than the lower levels. For example, the red fox, which as a species is called Vulpes vulpes. But the red fox can also be classified in the order rank (Canidae) because red foxes come from a lineage of carnivores which represent other organisms like coyotes and wolves. All organisms follow this pattern. There are also relatives of the bluejay Cyanositta cristata and the earthworm Lumbricus terrestris that would have the same hierarchy until the species level of organization.
One of the three domains, Eukarya, includes all organisms with a eukaryotic cell type. Eukaryotic cells are complex, with a nucleus and other organelles surrounded by one or two membranes. The other two domains, Archaea and Bacteria, include organisms that seem superficially similar. They are small, single-celled prokaryotes. Prokaryotic cells are smaller and simpler than eukaryotic cells, containing no nucleus or other membrane-bound organelles. However, there are differences in the cell structure and general lifestyles of Archaea and Bacteria that provide clues about their evolutionary distance from one another. Once their genomes were sequenced, it became clear that Archaea are not closely related to Bacteria at all. Furthermore, DNA evidence suggests that Archaea are more closely related to organisms within Eukarya than they are to Bacteria. However, there is some debate over the determination of relative relatedness among the three domains because horizontal gene transfer (the direct transfer of DNA between individuals, without reproduction) throughout evolutionary history has complicated the issue. The direct transfer of genes between organisms (through plasmids, viruses, or other vectors) can cause genetic similarity that is not due to common ancestry. The domains of Bacteria and Archaea each contain a single kingdom, however the kingdom name in Archaea (Archaebacteria) is rarely used anymore and is replaced with just the domain name. The domain Eukarya contains four kingdoms: Plantae, Fungi, Animalia, and Protista. The organisms of these kingdoms are sorted primarily by how they obtain their nutrition.

Characteristics of the Kingdoms of Life

Domain Archaea Bacteria Eukarya
Kingdom Archaebacteria Eubacteria Animalia Fungi Plantae Protista
Cell Type Prokaryotic Prokaryotic Eukaryotic Eukaryotic Eukaryotic Eukaryotic
Multicellular No No Yes Some Yes Some
Autotrophic Some Some No No Yes Some
Cell Wall Yes Yes No Some Yes Some
Movement Some Some Yes No No Some
Examples extremophile methanogen E. coli fish, human, ant yeast, mold, mushroom moss, fern, cactus, pine tree seaweed, Euglena, Paramecium

Each kingdom of life has characteristics that distinguish it from the others. Some organisms are multicellular, meaning they are comprised of more than one cell. Other organisms are unicellular, meaning they have only one cell. Autotrophic organisms make their own energy. Those with a cell wall have a rigid structure around their cells that offers structural support. Movement is defined as being able to relocate under one's own means. Plants and fungi cannot move by themselves, while animals and some protists can.