Three Domains Of Life

Phylogenetic analyses of rRNA genes gave scientists new insights about the relationships between major groups and suggested a new "tree of life." Three of the most important insights were as follows:

1. The data are consistent with the hypothesis that all living organisms inherited their rRNA genes from an ancient organism or form of life. Scientists refer to this unknown ancestor as the last universal common ancestor.

2. At the broadest level, all living things seem to be related by ancestry to one of three major lineages, or domains. The three domains are named Bacteria, Archaea, and Eukarya. Most of the organisms that we are familiar with, such as plants and animals, belong to just one of the domains—Eukarya. Figure 17-8 shows a phylogenetic diagram for the three domains.

3. The most surprising insights were those related to the domain Archaea. The species in this domain were identified and studied more recently than those in the other two domains. At first, the archaea were classified as bacteria. However, scientists have found that archaea differ greatly from bacteria in many important ways. More recently, scientists hypothesized that modern archaea descended from a unique kind of prokaryotes that existed early in Earth's history.

Domain Bacteria

The domain Bacteria is made up of small, single-celled prokaryotic organisms that usually have a cell wall and reproduce by cellular fission. Each bacterium has a cell wall, a plasma membrane, a cytoplasm that lacks complex organelles, and at least one circular chromosome. Bacteria do not have a membrane-bound DNA and thus lack a true nucleus. Most bacteria are small—many are just 2 pm long. By comparison, human cells can be 6 pm long or more. The oldest known fossils of cells appear to be bacterial cells.

Domain Archaea

The second domain also consists of prokaryotes and is named Archaea. The archaea have distinctive cell membranes and other unique biochemical and genetic properties. Some archaeal species are auto-trophic and are able to produce food by chemosynthesis. Some species produce flammable gases, such as methane, as waste products. Many archaeal species inhabit harsh environments, such as sulfurous hot springs, deep-sea thermal vents, salty lakes, wastewater from mining, and the intestines of some animals. Earliest cells

Word Roots and Origins domain from the Latin dominium, meaning "right of ownership"

figure 17-8

This phylogenetic diagram represents hypotheses of the evolutionary relationships between the major recognized groups of organisms. Notice the alignment of the three domain names (Bacteria, Archaea, and Eukarya) with three major "branches" of the "tree" of all life. For updates on phylogenetic information, visit and enter the keyword HM6 Phylo.

Bacteria Archaea

Bacteria Archaea

Plants Fungi Animals v_y







Because of the unique adaptations of the archaea, scientists think that archaea were among the earliest organisms on Earth. Furthermore, scientists think that archaea and bacteria have co-evolved during Earth's history, meaning that the cells have interacted closely within environments and that the cells could have exchanged nutrients and genetic material. These possibilities, along with biochemical evidence, have led to the theory of endosymbiosis, which holds that eukaryotic cells arose when ancient prokaryotic cells began to live together as one cell.

Domain Eukarya

The most familiar groups of organisms are members of the domain Eukarya (yoo-KAR-ee-uh), which consists of eukaryotic organisms. The cells of these organisms are large and have a true nucleus and complex cellular organelles. The domain Eukarya includes plants, animals, fungi, and a variety of single-celled organisms.

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