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 animal