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Introduction
An important part of life on earth has remained unnoticed until about 30 years ago and then rapidly been framed as a curious biota predominant in extreme environment. However, the archaea are known to be metabolically diverse organisms coexisting with bacteria and eukarya in the majority of earth environments, both terrestrial and aquatic, including extreme ones such as high or low ph, low temperature, high salinity or pressure.Not only are the archaea very diverse in virtually all environments, but they can also be very abundant.Their predominance in marine plankton,including deep ocean, points to a crucial and still poorly known role in the biogeochemical cycles of our planet. In addition, archaea include so far the sole organisms capable of methanogensis.Archaea to a third domain of life in addition to bacteria and eukarya based on universal small subunit ribosomal RNA and protein trees has been validated by comparative genomics. The distinctive nature of archaea that was realized by early studies on molecular mechanism is still valid today numerous components of archaeal informational processes are more similar to their eukaryotic than bacterial homologues and sometimes are uniquely shared by archaea and eukarya to the exclusion of bacteria. Their similarities to the both bacteria and eukaryal features, the archaea a unique trait that radically dintinguishes them from the other two domain the glycerol backbone of their membrane phospholipids are isoprenoids ethers build on glycerol -1-phosphate while bacteria and eukaryal membranes contain fatty acids esters linked to a stereoisomer glycerol-3-phosphate.

Microbial life can be traced back to archaean based on ratios of biogenic isotopes distinctive of different metabolisms but also on microfossils traces and biomarkers. The most ancient reliable biomarkers for bacteril life are given by presence of hopanes and steranes in archaean shales. On the contrary extended isoprene chains which are good fossil biomarkers for archaeal lipids are less stable and have been found in rocks.However the isotopic record of ultralight carbon indicates the presence of methane of biological origin.Archeal fossil traces has been objected and possibly needs further confirmation.Moreover, the lack of reliable fossil traces for archaea may severely affect any attempt to data the origin of this domain by molecular data. The techniques to identify archaeal fossil traces in old samples should be developed further and will provide reliable calibration points for the molecular dating of archaea are prokaryotes in general.
Archaea are domain of single celled microorganisms.They have no cell nucleus or any organelles inside their cells.In the past archaea were classified as unusual group of bacteria and named archaebacteria, but since the archaea have an independent evolutionary history and manifest numerous differences in their biochemistry from other forms of life they are now classified as a separate domain in the three domain system. In this system the three primary branches of evolutionary descent are the archaea, eukarya and bacteria.Archaea are further divided into four recognized phyla, although other phyla may exist. One of these groups the crenarchaeota and euryarchaeota are most intensively studied.Classifying the archaea is somewhat challenging,since the vast majority have never been studied, and have chiefy been detected by analysis of their nucleic acids in samples from environment.

Very early probable prokaryotic cell fossils have been dated at approximately 3.5 billion years before present day, making them som of the most primitive and ancient life form on earth however prokaryotes generally lack distinctive morphologies thus fossil shapes cannot be used to identify them archaea.Instead,chemical fossils of uique lipids hold greater information since such compounds do not occur in other organisms. Some research indicates archaean or eukaryotic lipid remains are present in shales as old as2.7 billion years. Such lipids have been identified in precambrianformations, the earliest of which are present in the issue greenstone belt of western Greenland.The locale boasts the earth oldest sediments circa 3.8 billion years of age
Need and significance
Extremophile archaea particualy those resistant either to heat or to extremes of acidity and alkalinity, are a source of enzymes that function under these harsh conditions
.These enzymes have found many uses.For example thermostable DNA polymerases, such as the DNA polymerase from pyrococcus furiosus revolutionized molecular biology by allowing the polymerase chin reaction to be used in research as a simple and rapid technique for cloning DNA.In industry, amylases, galactosidases and pullulanases in other species of pyrococcus that function at over 100 temp allow food processing at high temp, such as the production of low lactose milk.
Enzymes from these thermophilic archaea also tend to be very stable in organic compounds. This stability makes them easier to use in structural biology.Bcteria and eubacteria enzymes from extremophile archaea are often used in structural studies.
Mathanogenic archaea are a vital part of sewage treatment.In mineral processing acidophilic archaea display promise for extraction of metals from ores, including gold, cobalt and copper.Archaea host new class of potentially useful antibiotics.
Review of literature
With this special issue on the origin and evolution of archaea we honor and celebrate the life and impactful contributions of Carl Woese. Carl Woese work did not only result in the definition of new urkingdom originally named by him as archaebacteria but his insights prompted an appreciation for the incredible microbial diversity of the biosphere. A review article by A.Spang et al.comprehensively describes current hypotheses on the relationships of the three domains and evaluates archaeal diversity and evolution using recent genomic data.P.Forterre also evaluates the contemporary scenarios for the origins of the three domains. A.Nasir and G.Caetano-Anolles explore a novel comparative genomic framework that makes the vertical horizontal evolutionary contributions explicit and G.Caetano-Anolles et al.advance structural phylogenomic analyses of protein and nucleic acid structures and their associated functions.G.Borrel et al. present a bioinformatics analysis of three genomes from a newly identified order of methanogens and find the pyrrolysine codiny system. L.S.Yafremava et al study amino acid substitution patterns in the protein domains of nonbarophilic and barophilic pyrococcus species and reveal that barophily is a very ancient trait that unfolded with the early evolution of the genetic code during early adaptation to deep ocean environments.
Conclusion
In the past few years, scientists have sequenced the entire genomes of many different species from the domains bacteria, archaea and eukarya. The plethora of data has allowed scientists to compare how thousands of genes differ or similar among organisms from the three domains.
Many of the comparisions corroborate the model of evolution that biologists currently hold tha archaea and eukarya diverged from bacteria long ago.However many surprises have also emerged from vast amount of collected data.
Archaean species have genes that have been recently derived from bacteria and eukarya also have number of genes that are of relatively recent bacterial origin.
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