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chondriate protists as well as in presumed mitochondriate ancestors, minimally consists of a single nucleus, a single kinetosome and their protein connector. As predecessor of standard mitosis, the karyomastigont preceded free (unattached) nuclei. The nucleus evolved in karyomastigont ancestors by detachment at least five times (archamoebae, calonymphids, chlorophyte green algae, ciliates, foraminifera). This specific model of syntrophic chimeric fusion can be proved by sequence comparison of functional domains of motility proteins isolated from candidate taxa.

Archaeprotists / spirochetes / sulfur syntrophy / Thiodendron / trichomonad

TWO DOMAINS, NOT THREE

All living beings are composed of cells and are unambiguously classifiable into one of two categories: prokaryote (bacteria) or eukaryote (nucleated organisms). Here we outline the origin of the nucleus, the membrane-bounded organelle that defines eukaryotes. The common ancestor of all eukaryotes by genome fusion of two or more different prokaryotes became “chimeras ” via symbiogenesis (Gupta and Golding, 1995). Long term physical association between metabolically dependent consortia bacteria led, by genetic fusion, to this chimera. The chimera originated when an archaebacterium (a thermoacidophil) and a motile eubacterium emerged under selective pressure: oxygen threat and scarcity both of carbon compounds and electron acceptors. The nucleus evolved in the chimera. The earliest descendant of this momentous merger, if alive today, would be recognized as an amitochondriate protist. An advantage of our model includes its simultaneous consistency in the evolutionary scenario across fields of science: cell biology, developmental biology, ecology, genetics, microbiology, molecular evolution, paleontology, protistology. Environmentally plausible habitats and modern taxa are easily comprehensible as legacies of the fusion event. The scheme that generates predictions demonstrable by molecular biology, especially motile protein sequence comparisons (Chapman et al., 2000), provides insight into the structure, physiology, and classification of microorganisms.

Our analysis requires the two- (Bacteria/Eukarya) not the three-(Archaea/Eubacteria/Eukarya) domain system (Woese et al., 1990). The prokaryote vs. eukaryote that replaced the animal vs. plant dichotomy so far has resisted every challenge. Microbiologist 's molecular biology-based threat to the prokaryote vs. eukaryote evolutionary distinction seems idle (Mayr, 1998). In a history of contradictory classifications of microorganisms since 1820, Scamardella (1999) noted that Woese's entirely nonmor-



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