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The Discovery of the Third Domain of Life

In 1977, Carl Woese overturned one of the major dogmas of biology. Until that time, biologists had taken for granted that all life on Earth belonged to one of two primary lineages:
* the eukaryotes (which include animals, plants, fungi and certain unicellular organisms such as paramecium)
* and the prokaryotes (all remaining microscopic organisms).

Woese discovered that there were actually three primary lineages. Within what had previously been called prokaryotes, there exist two distinct groups of organisms no more related to one another than they were to eukaryotes.

Because of Woese’s work, it is now widely agreed that there are three primary divisions of living systems – the Eukarya, Bacteria, and Archaea, a classification scheme that Woese proposed in 1990.

….In 1996, Woese and colleagues (University of Illinois professor Gary Olsen and researchers from the Institute for Genomic Research) published in the journalScience the first complete genome structure of an archaeon, Methanococcus jannaschii.

Based on this work, they concluded that the Archaea are more closely related to humans than to bacteria. “The Archaea are related to us, to the eukaryotes; they are descendants of the microorganisms that gave rise to the eukaryotic cell billions of years ago,” Woese said at the time.


Learning Standards

Massachusetts Science and Technology/Engineering Curriculum FrameworkLife

Science (Biology), Grades 6–8.
Classify organisms into the currently recognized kingdoms according to characteristics that they share. Be familiar with organisms from each kingdom.

Biology, High School
5.2 Describe species as reproductively distinct groups of organisms. Recognize that species are further classified into a hierarchical taxonomic system (kingdom, phylum, class, order, family, genus, species) based on morphological, behavioral, and molecular similarities.

Benchmarks for Science Literacy, American Association for the Advancement of Science

Students should begin to extend their attention from external anatomy to internal structures and functions. Patterns of development may be brought in to further illustrate similarities and differences among organisms. Also, they should move from their invented classification systems to those used in modern biology… A classification system is a framework created by scientists for describing the vast diversity of organisms, indicating the degree of relatedness between organisms, and framing research questions.

SAT Biology Subject Area Test

Evolution and diversity: Origin of life, evidence of evolution, patterns of evolution, natural selection, speciation, classification and diversity of organisms.

Teaching About Evolution and the Nature of Science, National Academy Press (1998)

Biological classifications are based on how organisms are related. Organisms are classified into a hierarchy of groups and subgroups based on similarities which reflect their evolutionary relationships. Species is the most fundamental unit of classification.

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