Archaebacteria

Life on Earth can divided into three large collections, or domains. These are the Eubacteria (or "true" bacteria), Eukaryota (the domain that humans belong to), and Archae. The members of this last domain are the archaebacteria.

Most archaebacteria (also called archae) look bacteria-like when viewed under the microscope. They have features that are quite different, however, from both bacteria and eukaryotic organisms. These differences led American microbiologist Carl Woese to propose in the 1970s that archaebacteria be classified in a separate domain of life. Indeed, because the organisms are truly separate from bacteria, Woese proposed that the designation archaebacteria be replaced by archae.

Archae are similar to eukaryotic organisms in that they lack a part of the cell wall called the peptidoglycan. Also, archae and eukaryotes share similarities in the way that they make a new copy of their genetic material. However, archae are similar to bacteria in that their genetic material is not confined within a membrane, but instead is spread throughout the cell. Thus, archae represent a blend of bacteria and eukaryotes (some scientists call them the "missing link"), although generally they are more like eukaryotes than bacteria.