The number of kingdoms recognized in taxonomy, the scientific classification of living things, has varied since the 1700s, when Carolus Linnaeus divided organisms into Animalia (animals) and Plantae (plants). Following the development of the microscope, scientists created a new kingdom, Protista. Later, when Protista proved too broad, the kingdom Prokaryote, later called Monera, was created for organisms that have cells with no nuclei. More recently, taxonomists divided Monera into Bacteria and Archaea, based on RNA studies. Meanwhile, Fungi was reclassified as a kingdom separate from Plantae (indeed, many experts describe fungal organisms as closer to animals).
Animals are multicellular organisms that consume other organisms or organic matter for energy. They also can move independently for at least part of their life cycle, and have thin cell membranes. The kingdom Animalia includes, among other species, mammals, birds, insects, reptiles and arthropods.
Plant organisms have chlorophylls a and b and produce energy from sunlight and carbon dioxide. In addition, members of the kingdom Plantae have rigid cell walls containing cellulose, and do not move independently. Plants include trees, bushes, grasses, ferns and green algae.
Fungi, like plants, have rigid cell walls and do not move independently; and like ferns, they reproduce via spores. However, taxonomists eventually distinguished fungi from plants. Like animals, fungi consume external organic matter for energy, and unlike plants, they have no chlorophyll and no cellulose. Mushrooms, yeasts and molds are members of the kingdom Fungi.
Protista includes organisms of only one cell, or cell colonies that do not form differentiated tissues and whose cells contain distinct nuclei and organelles. Protozoa, red algae. slime molds and water molds belong to the kingdom Protista.
Some taxonomists use the name Eubacteria for the kingdom commonly called Bacteria. Others occasionally give Bacteria the old Monera label. This kingdom is composed of single-celled organisms that do not contain distinct nuclei and rarely contain organelles. Bacterial organisms are ubiquitous on Earth, found everywhere from deep-sea hot springs to the human gut.
Sometimes called Archaebacteria, Archaea comprises single-celled organisms that contain no distinct nuclei or organelles. Although very similar in appearance to bacteria, these organisms have different genetic structures and metabolic processes. Archaea, initially thought to inhabit only extreme environments like sulfur springs, have been found all over the planet and are particularly abundant in plankton.