The application of molecular tools has been useful in establishing relationships and the divergence of major groups within the prokaryotes. Using 16S rRNA phylogenetic trees, Woese and Fox (1977) identified a group of prokaryotes called the Archaea. Although they had many features in common with other prokaryotes, they differed in certain genetic processes, and are generally considered to be more closely related to the Eucarya than to the Eubacteria. Today, many archeans live in some of the most severe environments on Earth and are termed extremophiles. They can be found in hot springs, deep within the ocean, in the digestive tracts of animals, and a variety of other harsh environments. Archeans include halophiles (organisms that live in high concentrations of salinity), thermophiles (live in hot and acidic environments), and methanogens (utilize hydrogen [H2] to reduce CO2 to methane). Many believe that the Archaea evolved from single-celled organisms ~4 billion years ago. Many of the environments they inhabit today are believed to be the types of conditions present in the first billion years of Earth's history, and it has been suggested that life may have arisen in these extreme habitats (Leach et al., 2006) (Chapter 2). As a result of ribosomal sequences of modern halobac-teria, some suggest that these are the most primitive of the archean group. Although no body fossils of Archaea have been confirmed to date, there is evidence of biomarkers from archeans in the fossil record. Perhaps the largest obstacle is distinguishing fossil from living Archaea, since members of these group are widespread in the deep biosphere.

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