在一項對非遺傳學家聽來很了不起的研究工作中,，Maria C. Rivera 和 James A. Lake利用種系發(fā)生分析方法來重建三類生物的演化歷史：真核生物、真細菌和古細菌,。他們沒有獲得一棵“生命樹”,，而是獲得一個“生命環(huán)”。有確鑿的統(tǒng)計證據表明,，真核生物是由兩個迥異的原核生物基因組融合而形成的,。一個融合伙伴來自光合作用生物的一個古代種系，另一個與古細菌有關,。隨著“生命樹”的“倒地”,，一些相互沖突的觀測結果開始變得容易理解了。例如,，在一些基因樹中,，真核生物與細菌有關，而在另一些基因樹中,，真核生物則與古細菌有關,。這種現象可通過“生命環(huán)”的作用得到解釋。

The ring of life provides evidence for a genome fusion origin of eukaryotes

Genomes hold within them the record of the evolution of life on Earth. But genome fusions and horizontal gene transfer seem to have obscured sufficiently the gene sequence record such that it is difficult to reconstruct the phylogenetic tree of life. Here we determine the general outline of the tree using complete genome data from representative prokaryotes and eukaryotes and a new genome analysis method that makes it possible to reconstruct ancient genome fusions and phylogenetic trees. Our analyses indicate that the eukaryotic genome resulted from a fusion of two diverse prokaryotic genomes, and therefore at the deepest levels linking prokaryotes and eukaryotes, the tree of life is actually a ring of life. One fusion partner branches from deep within an ancient photosynthetic clade, and the other is related to the archaeal prokaryotes. The eubacterial organism is either a proteobacterium, or a member of a larger photosynthetic clade that includes the Cyanobacteria and the Proteobacteria.

 
 
Figure 1 Conditioned reconstructions provide evidence for the ring of life. The genomes are from two yeasts (Y1, Schizosaccharomyces pombe and Y2, Saccharomyces cerevisiae), a -proteobacterium (P, Xylella fastidiosa), a bacillus (B, Staphylococcus aureus MW2), a halobacterium (H, Halobacterium sp. NRC-1), a methanococcus (M, Methanosarcina mazei Goe1), an eocyte (E, Sulfolobus tokodaii) and an archaeoglobium (not shown; the conditioning genome Archaeoglobus fulgidus DSM4304). The five most probable unrooted trees are shown with leaves pointing upward to emphasize that each is part of a repeating pattern. Cumulative probabilities are shown at the right of each tree. Fully and partially resolved rings are at the lower left and right, respectively. 

 
 
Figure 2 Eubacterial relationships within the ring of life. The genomes are from two yeasts (Y1, S. pombe and Y2, S. cerevisiae), a -proteobacterium (P, X. fastidiosa 9a5c), an -proteobacterium (P, Brucella melitensis 16M), a cyanobacterium (C, Synechocystis sp. PCC6803), a halobacterium (H, Halobacterium sp. NRC-1), an eocyte (E, S. tokodaii) and a bacillus (not shown; the conditioning genome S. aureus MW2). The five unrooted trees consistent with the ring are shown with leaves pointing upward to emphasize that each is part of a repeating pattern. Cumulative probabilities are at the right of each tree. Fully and partially resolved rings are at the lower left, right and centre, respectively. 

 
 
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.

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