Archaebacteria: The Third Domain of Life Missed by Biologists for Decades

These unusual bacteria are genealogically neither prokaryotes nor eukaryotes. This discovery means there are not two lines of descent of life but three: the archaebacteria, the true bacteria and the eukaryotes

By Carl R. Woese

Methanogens, anaerobic bacteria that generate methane from hydrogen and carbon dioxide, make up the largest group of archaebacteria identified so far. Four genera of methanogens that differ widely in size and morphology are seen here in scanning electron micrographs made by Alexander J. B. Zehnder of the Swiss Federal Institute of Technology. Shown here is Methanosarcina. The cells are shown enlarged 2,500 diameters. The methanogens are found only in oxygen-free environments. Image: Scientific American

Editor's Note: Microbiologist Carl R. Woese, a recipient of the Crafoord Prize, Leeuwenhoek Medal, and a National Medal of Science, died December 30 at the age of 84. We are making this classic, definitive essay that outlines the evidence for archaebacteria as a domain of life (independent of eukaryotes and true bacteria) free online for the next 14 days. This story was originally published in the June 1981 issue of Scientific American.

Early natural philosophers held that life on the earth is fundamentally dichotomous: all living things are either animals or plants. When microorganisms were discovered, they were di?vided in the same way. The large and motile ones were considered to be ani?mals and the ones that appeared not to move, including the bacteria, were con?sidered to be plants. As understanding of the microscopic world advanced it became apparent that a simple twofold classification would not suffice, and so additional categories were introduced: fungi, protozoa and bacteria. Ultimate?ly, however, a new simplification took hold. It seemed that life might be dichot?omous after all, but at a deeper level, namely in the structure of the living cell. All cells appeared to belong to one or the other of two groups: the eukaryotes, which are cells with a well-formed nucleus, and the prokaryotes, which do not have such a nucleus. Multicellular plants and animals are eukaryotic and so are many unicellular organisms. The only prokaryotes are the bacteria (in?cluding the cyanobacteria, which were formerly called blue-green algae).

In the past few years my colleagues and I have been led to propose a funda?mental revision of this picture. Among the bacteria we have found a group of organisms that do not seem to belong to either of the basic categories. The or?ganisms we have been studying are pro?karyotic in the sense that they do not have a nucleus, and indeed outwardly they look much like ordinary bacteria. In their biochemistry, however, and in the structure of certain large molecules, they are as different from other prokary?otes as they are from eukaryotes. Phylo?genetically they are neither prokaryotes nor eukaryotes. They make up a new "primary kingdom," with a completely different status in the history and the natural order of life.

We have named these organisms ar?chaebacteria. The name reflects an untested conjecture about their evolution?ary status. The phylogenetic evidence suggests that the archaebacteria are at least as old as the other major groups. Moreover, some of the archaebacteria have a form of metabolism that seems particularly well suited to the conditions believed to have prevailed in the early history of life on the earth. Hence it seems possible that the newest group of organisms is actually the oldest.

The evolutionary record

The earth is four and a half billion years old, and on the basis of the macro?scopic fossil record it would appear to have been inhabited for less than a sev?enth of that time: the entire evolutionary progression from the most ancient ma?rine forms to man spans only 600 mil?lion years. The fossil imprints of unicellular organisms too small to be seen with the unaided eye tell a different sto?ry. Microfossils of bacteria in particular are plentiful in sediments of all ages; they have been found in the oldest intact sedimentary rocks known, 3.5-billion? year-old deposits in Australia. Over an enormous expanse of time, during which no higher forms existed, the bac?teria arose and radiated to form a wide variety of types inhabiting a great many ecological niches. This age of microorganisms is the most important period in evolutionary history not only because of its duration but also because of the na?ture of the evolutionary events that took place over those billions of years.

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