In the mid-1700s, Swedish botanist Carl Linnaeus was one of the first scientists to develop a taxonomy for living organisms. It is for this reason that he is known as the father of taxonomy. Linnaeus' taxonomy grouped living things into two kingdoms: plants and animals.
By the 1900s, scientists had discovered microorganisms that had characteristics that were dramatically different than those of plants and animals. Therefore, Linnaeus' taxonomy needed to be enhanced to encompass microorganisms.
In 1969 Robert H. Whitteker, working at Cornell University, proposed a new taxonomy that consisted of five kingdoms (see Fig. 9-1). These were monera, protista, plantae (plants), fungi, and animalia (animals). Monera are organisms that lack a nucleus and membrane-bounded organelles, such as bacteria. Protista are organisms that have either a single cell or no distinct tissues and organs, such as protozoa. This group includes unicellular eukaryotes and algae. Fungi are organisms that use absorption to acquire food. These include multicellular fungi and single-cell yeast. Animalia and plantae include only multicellular organisms.
Scientists widely accepted Whitteker's taxonomy until 1977 when Carl Woese, in collaboration with Ralph S. Wolfe at the University of Illinois, proposed a
Gram Positive Bacteria
Gram Negative Bacteria
Fig. 9-1. Whitteker's five-kingdom taxonomy.
new six-kingdom taxonomy. This came about with the discovery of archaea, which are prokaryotes that lives in oxygen-deprived environments.
Before Woese's six-kingdom taxonomy, scientists grouped organisms into eukaryotes animals, plants, fungi, and one-cell microorganisms (paramecia)— and prokaryotes (microscopic organisms that are not eukaryotes).
Woese's six-kingdom taxonomy consists of:
• Eubacteria (has rigid cell wall)
• Archaebacteria (anaerobes that live in swamps, marshes, and in the intestines of mammals)
• Protista (unicellular eukaryotes and algae)
• Fungi (multicellular forms and single-cell yeasts)
Woese determined that archaebacteria and eubacteria are two groups by studying the rRNA sequences in prokaryotic cells.
Woese used three major criteria to define his six kingdoms. These are:
• Cell type. Eukaryotic cells (cells having a distinct nucleus) and prokaryotic cell (cells not having a distinct nucleus)
• Level of organization. Organisms that live in a colony or alone and one-cell organisms and multicell organisms.
• Nutrition. Ingestion (animal), absorption (fungi), or photosynthesis (plants).
In the 1990s Woese studied rRNA sequences in prokaryotic cells (archaebacteria and eubacteria) proving that these organisms should be divided into two distinct groups. Today organisms are grouped into three categories called domains that are represented as bacteria, archaea, and eukaryotes.
The domains are placed above the phylum and kingdom levels. The term archaebacteria (meaning from the Greek word archaio "ancient") refers to the ancient origin of this group of bacteria that appears to have diverged from eubacteria. The archaea and bacteria came from the development of eukaryotic organisms.
The evolutionary relationship among the three domains is:
• Domain Bacteria (eubacteria)
• Domain Archaea (archaebacteria)
• Domain Eulcarya (eukaryotes)
Different classifications of organisms are:
The three domains are archaea, bacteria, and eukarya (see Fig. 9-2).
• Archaea lack muramic acid in the cell walls.
• Bacteria have a cell wall composed of peptidoglycan and muramic acid. Bacteria also have membrane lipids with ester-linked, straight-chained fatty acids that resemble eukaryotic membrane lipids. Most prokaryotes are bacteria. Bacteria also have plasmids, which are small, double-stranded DNA molecules that are extrachromosomal.
• Eukarya are of the domain eukarya and have a defined nucleus and membrane bound organelles.
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