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, 2012, at the age of 84. 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 divided in the same way. The large and motile ones were considered to be animals and the ones that appeared not to move, including the bacteria, were considered 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. Ultimately, however, a new simplification took hold. It seemed that life might be dichotomous 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 (including the cyanobacteria, which were formerly called blue-green algae).