a, The three-domain proposal based on the ribosomal RNA tree, as rooted with data from anciently duplicated protein genes. b, The two-empire proposal, separating eukaryotes from prokaryotes and eubacteria from archaebacteria. c, The three-domain proposal, with continuous lateral gene transfer among domains. d, The ring of life, incorporating lateral gene transfer but preserving the prokaryoteeukaryote divide. (Redrawn from refs 2, 3, 6 and 1, respectively.) [
FIGURE 2. Four schemes of natural order in the microbial world.
[William Martin and T. Martin Embley: "Evolutionary biology: Early evolution comes full circle", Nature 431, 134-137(9 September 2004) doi:10.1038/431134a]
REVISED “TREE” OF LIFE retains a treelike structure at the top of the eukaryotic domain and acknowledges that eukaryotes obtained mitochondria and chloroplasts from bacteria. But it also includes an extensive network of untreelike links between branches. Those links have been inserted somewhat randomly to symbolize the rampant lateral gene transfer of single or multiple genes that has always occurred between unicellular organisms. This “tree” also lacks a single cell at the root; the three major domains of life probably arose from a population of primitive cells that differed in their genes.
改訂された"Tree of Life"は、真核生物領域の最上位でツリーライクな構造を持っていて、真核生物が細菌からミトコンドリアと葉緑体を得たことを示す。しかし、枝の間では、ツリーライクではない広いネットワークを含んでいる。これらのリンクは、単細胞生物の間で常態的に起きていた、単一もしくは複数の遺伝子の水平伝播を象徴するために、ランダムに挿入した。このツリーの根元には単細胞がない。3つの主要な生物ドメインはおそらく、遺伝情報の違う原始的な細胞の集団から現れたと思われる。 [
W.Ford Doolittle: "Uprooting the Tree of Life"
]
Figure 3 A schematic diagram of the ring of life. The eukaryotes plus the two eukaryotic root organisms (the operational and informational ancestors) comprise the eukaryotic realm (see Supplementary Discussion). Ancestors defining major groups in the prokaryotic realm are indicated by small circles on the ring. The Archaea49, shown on the bottom right, includes the Euryarchaea, the Eocyta and the informational eukaryotic ancestor. The Karyota5, shown on the upper right of the ring, includes the Eocyta and the informational eukaryotic ancestor. The upper left circle includes the Proteobacteria49 and the operational eukaryotic ancestor. The most basal node on the left represents the photosynthetic prokaryotes and the operational eukaryotic ancestor.