In the past few years my colleagues and I have been led to propose a fundamental 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 organisms we have been studying are prokaryotic 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 prokaryotes as they are from eukaryotes. Phylogenetically 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 archaebacteria. The name reflects an untested conjecture about their evolutionary 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 earth is four and a half billion years old, and on the basis of the macroscopic fossil record it would appear to have been inhabited for less than a seventh of that time: the entire evolutionary progression from the most ancient marine forms to man spans only 600 million years. The fossil imprints of unicellular organisms too small to be seen with the unaided eye tell a different story. 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 bacteria 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 nature of the evolutionary events that took place over those billions of years.
( via scientificamerican.com )