Intermediate Steps

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Science  26 Nov 2010:
Vol. 330, Issue 6008, pp. 1187-1188
DOI: 10.1126/science.1196720

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Identifying the origin of the prokaryote-eukaryote transition has been a long-standing and contentious issue (1, 2) with numerous hypotheses. Over the last decade, “fusion hypotheses” have dominated the field (3). Many of these propose that the fusion of two prokaryotes accounts for the seemingly chimeric nature of the eukaryote gene set. Other theories propose that eukaryotes and archaea shared an ancestor that emerged from a bacterium (4). Thus, eukaryotes may be a mosaicism, combining features (including genes) inherited from a bacterial ancestor with those developed by an archaeal-eukaryotic ancestor before the split between eukaryotes and archaea. The hypotheses and debates about ancient cellular evolution continue as our knowledge of molecular and cellular mechanisms in all three domains of life expands (5). One intriguing curiosity is a “superphylum” of bacteria whose members display features of archaea and eukaryotes. What might these unusual organisms suggest in terms of evolution? It may be that their ancestor served as a “cauldron” for the evolution of eukaryotic and archaeal features, and that the superphylum is indicative of a path of intermediate steps between such an ancestor and an archaeal-eukaryotic ancestor (before the eventual split of the two domains).