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The last universal ancestor (LUA), also called the last universal common ancestor (LUCA), cenancestor, or progenote, is the most recent organism from which all organisms now living on Earth have a common descent. Thus it is the most recent common ancestor (MRCA) of all current life on Earth. The LUA is estimated to have lived some 3.5 to 3.8 billion years ago (sometime in the Paleoarchean era). The earliest evidence for life on Earth is biogenic graphite found in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia. Charles Darwin proposed the theory of universal common descent through an evolutionary process in his book ''On the Origin of Species'' in 1859, saying, "Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed."〔 〕 == Features == Considering what is known of the offspring groups (see phylogenetic bracketing), the LUA is thought to have been a small, single-cell organism. It likely had a ring-shaped coil of DNA floating freely within the cell, like modern bacteria. Morphologically, it would likely not have been exceptionally distinctive among a collection of generalized, small-size, modern-day bacteria. However, Carl Woese ''et al'', who first proposed the currently-used three domain system based on an analysis of the 16S rRNA sequences of bacteria, archaea, and eukaryotes, stated that the LUA would have been a "...simpler, more rudimentary entity than the individual ancestors that spawned the three () (and their descendants)" regarding its genetic machinery. While the gross anatomy of the LUA must be reconstructed with much uncertainty, its internal mechanisms may be described in some detail, based on the properties currently shared by all independently living organisms on Earth: * The genetic code was based on DNA. Note, however, that some studies suggest that the LUCA may have lacked DNA and been defined wholly through RNA. * * The DNA was composed of four nucleotides (deoxyadenosine, deoxycytidine, deoxythymidine, and deoxyguanosine), to the exclusion of other possible deoxynucleotides. * * The genetic code was composed of three-nucleotide codons, thus producing 64 different codons. Since only 20 amino acids were used, multiple codons code for the same amino acids. * * The DNA was kept double-stranded by a template-dependent DNA polymerase. * * The integrity of the DNA was maintained by a group of maintenance enzymes, including DNA topoisomerase, DNA ligase and other DNA repair enzymes. The DNA was also protected by DNA-binding proteins such as histones. * The genetic code was expressed via RNA intermediates, which were single-strand. * * RNA was produced by a DNA-dependent RNA polymerase using nucleotides similar to those of DNA with the exception that thymidine in DNA was replaced by uridine in RNA. * The genetic code was expressed into proteins. * Proteins were assembled from free amino acids by translation of an mRNA by ribosomes, tRNA and a group of related proteins. * * Ribosomes were composed of two subunits, one big 50S and one small 30S. * * Each ribosomal subunit was composed of a core of ribosomal RNA surrounded by ribosomal proteins. * * The RNA molecules (rRNA and tRNA) played an important role in the catalytic activity of the ribosomes. * * Only 20 amino acids were used, to the exclusion of countless other amino acids. * * Only the L-isomers of the amino acids were used. * ATP was used as an energy intermediate. * There were several hundred protein enzymes that catalyzed chemical reactions that extract energy from fats, sugars, and amino acids, and that synthesize fats, sugars, amino acids, and nucleic acid bases using arbitrary chemical pathways. * The cell contained a water-based cytoplasm that was surrounded and effectively enclosed by a lipid bilayer membrane. * Inside the cell, the concentration of sodium was lower, and potassium was higher, than outside. This gradient was maintained by specific ion transporters (also referred to as ''ion pumps''). * The cell multiplied by duplicating all its contents followed by cellular division. * The cell used chemiosmosis to produce energy. It also reduced CO2 and oxidized H2 (methanogenesis or acetogenesis) via acetyl-thioesters 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「last universal ancestor」の詳細全文を読む スポンサード リンク
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