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Archaea - Metabacteria Controversy |
| : On the first report that the probable ancestor of eukaryotes is so-called "the archaebacteria", and not the eubacteria |
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In 1977, based on the 16S rRNA catalog method (= SAB method*), Woese and Fox recognized a series of methanogenic bacteria as the most ancient group of bacteria yet detected and named them the "archaebacteria**". However, their SAB phylogenetic tree suggested apparently equal evolutionary distances among three superkingdoms, i.e., eukaryotes, eubacteria and "archaebacteria" (click the right figure), and these "bacteria" appear to be no more related to typical bacteria than they are to eukaryotic cytoplasms ( Woese and Fox 1977). Thus the authors claimed "It (= the ssu rRNA tree) is not structured in a bipartite way along the lines of the organizationally dissimilar prokaryotes and eukaryotes. Rather, it is tripartite " (tripartite hypothesis). In spite of this hypothesis, they named methanogens as "archaebacteria" without any rigid evidence. Only one reason they mentioned was that the methanogenic phenotype seems well suited to the type of environment presumed to exist on earth 3-4 billion years ago. Note here, however, that many eubacteria such as Thermus aquaticus and Thermatoga species, also live in such extreme environments.
In 1979, Hori and Osawa constructed a phylogenetic tree of 5S ribosomal RNA sequences from 54 eukaryotes and prokaryotes available at that time prior to development of the DNA sequencing techniques. One of our main conclusions was that Halobacterium (one of the so-called Archaebacterial species) and eukaryotes are sister groups with eubacteria as an outgroup. The phylogenetic position of Halobacterium is closer to eukaryotes than to eubacteria (click the left tree). This was the first report that the probable ancestor of eukaryotes is so-called "the archaebacteria", and not the eubacteria.
We therefore proposed the name "metabacteria " instead of "archaebacteria" (= archaea)(Hori and Osawa, 1982). The difference between these two is that the archaebacteria mean the most ancient group of organisms, whereas the metabacteria are closer to eukaryotes, and are in all probability younger than the eubacteria . Since "meta" means "after", the use of metabacteria is more reasonable to reflect the evolutionary history of this bacterial group. The term "archaea" greatly confuses the situation.
About ten years after (Hori and Osawa, 1987), a tree of major groups of organisms was constructed from the 352 5S rRNAs. Finally, using over 550 complete nucleotide sequences of 5S ribosomal RNAs from cytoplasm, chloroplasts, and mitochondria, we could construct a phylogenetic tree providing former support for our hypothesis (Hori et al. 1991). (see all 5S rRNA trees shown in the next page). The phylogenetic relationships proposed first by Hori and Osawa outlined above are in line with the results of analyses of many other biological macromolecules, and no one believes that the archaebacteria are the most ancient group of organisms.
#LUCA: Last Universal Common Ancestor