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An organism is any individual living entity. Organisms range in size and complexity from microorganisms to multicellular plants and animals. Modern biologists classify Earth's organisms into five kingdoms on the basis of common patterns of the design of life, that is, in their cellular and sub-cellular organization, metabolism, reproduction, and behavior. Listed in order of their earliest appearance in the fossil record of life, these kingdoms are:

  1. Monera or prokaryotic microorganisms, which do not have their genetic material organized within a bounded organelle called a nucleus, along with other distinctive characteristics. Earth's simplest organisms occur in this group, in particular viruses, which consist of little more than a proteinaceous shell containing nucleic acids. Viruses are incapable of reproduction without parasitizing the metabolism of an unrelated host cell. Other major groups of monerans are blue-green bacteria and true bacteria.
  2. Protista are a diverse group of microorganisms, containing the simplest of the eukaryotic organisms, which have an organized nucleus, one or more flagellae, and generally contain mitochondria and plastids. The most representative group is the protozoans, but some flagellated fungi and algae are placed within this group.
  3. Fungi are a diverse group of non-flagellated, unicellular or multicellular organisms, ranging in complexity from single-celled yeasts, through multicellular but microscopic fungi growing as a thread-like mycelium, to relatively complex fungi that develop large mushrooms as their reproductive structures.
  4. Plantae, or green plants, utilize solar radiation trapped by chlorophyll or other pigments to fix simple mineral nutrients into energy-rich biochemicals in a metabolic process called photosynthesis. Organisms in this diverse group range from unicellular algae, through multicellular but non-vascular algae, liverworts, and mosses, to vascular plants such as ferns, conifers, and flowering plants.
  5. Animalia, or multicellular animals, are heterotrophic organisms that are capable of movement, often in response to sensory stimuli, and with other distinctive characteristics. Animals range in size and complexity from small sponges and arthropods to large vertebrates weighing tons.

All of Earth's organisms are related to varying degrees, sharing certain commonalities of physiology and other functions. Moreover, it is clear that some of Earth's distinctive organisms have a relatively ancient lineage that extends far back into the geological past, while other organisms are enormously more complex in biological organization than others. However, the modern, evolutionary interpretation of life suggests that none of Earth's organisms are "higher" or more "primitive" than any others, and that none have greater intrinsic value. Evolution has not occurred as a progression of types of organisms that represents a logical, directed succession from simple organisms (such as viruses and bacteria) to much more complex organisms (such as birds and mammals). Earth's diversity of living organisms utilizes body and metabolic plans of varying complexity, but all species represent successful adaptations to the planet's habitable environments.

See also Animal; Plant; Protozoa; Virus.

Bill Freedman

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