What archaea have to tell biologists.

thank the Genetics Society of America for this opportuportunities to gain insight into a number of fundamennity. In addition, we recognize the contributions of our tal problems in biology. As one of the most ancient colleagues, Charles Daniels (Ohio State University) and lineages of living organisms, the archaea set a boundary Michael Thomm (Universitaet Kiel), who along with for evolutionary diversity and have the potential to offer the authors served as coeditors of papers on archaea in key insights into the early evolution of life, including this volume. the origin of the eukaryotes. Many archaea are also More than two decades after the initial proposal, the extremophiles that flourish at high temperature, low or archaeal hypothesis remains the best explanation for high pH, or high salt and delineate another boundary the unexpected diversity of molecular and biochemical for life, the biochemical and geochemical boundary, properties found in the prokaryotes. This hypothesis which sets the physical limits of the biosphere. Finally, states simply that the prokaryotes are not a monophysome archaea are fundamental components of the bioletic group but contain two very ancient phylogenetic geochemical cycles on earth or dominate special ecosyslineages (Woese and Fox 1977). In addition to the welltems that are of great interest. characterized bacteria, there is a second prokaryotic Prokaryotes have been present if not abundant on lineage of very remote ancestry called the archaebacteearth for more than 3.5 billion years, while evidence ria or, more recently, the archaea. Although originally for eukaryotes is limited to the last 2.1 billion years. controversial, the archaeal hypothesis has gained wide Thus, early life during the Archaean Eon was probably acceptance among prokaryotic biologists, in part due entirely prokaryotic. During this period, the major orto a thorough examination of many aspects of archaeal ganizing principles of modern cells evolved and the biology, including lipid composition, cell envelope biosphere formed. In the absence of an informative structure, ribosomal structure, tRNA and rRNA strucfossil record, comparative biology represents the major ture, elongation factors, DNA-dependent RNA polymerapproach to investigating life during this era. The arases, and antibiotic sensitivity. These investigations, chaea, as representatives of one of the deepest lineages, which were conducted in many laboratories worldwide, offer special insights into the origin of cellular life and were summarized in Woese and Wolfe (1985). Perhaps, the ancestry of eukaryotes. For instance, most of the the most stunning confirmation of the archaeal hypothbasic biosynthetic pathways for small molecules, such as esis was provided by the first archaeal genomic seamino acids and nucleic acids, appear to be conserved quence, where even after extensive analyses less than between the bacteria and archaea, suggesting that these half of the open-reading frames (ORFs) discovered pathways were inherited from a common ancestor. Likecould be assigned a specific function based upon similarwise, many aspects of the central paradigm of cellular ity to known bacterial and eukaryotic genes in the datainformatics including the genetic code, transcription, bases and analyses of motifs and other structural feaand translation appear to be highly conserved, which tures (Bult et al. 1996; Koonin et al. 1997). also suggests that these features were established in the