Figure 2 Prevalent evolutionary view between 1970 and 1977. The eukaryotic nuclear lineage arose from within the already characterized prokaryotes (eubacteria, perhaps a mycoplasma). Mitochondria descend from endosymbiotic proteobacteria, and plastids descend from endosymbiotic cyanobacteria.

The Woesian Revolution

The consensus represented by Figure 2 rested on comparative ultrastructural, biochemical, and physiological data and on a modest accumulation of primary (protein) sequence information, mostly from cytochromes and ferredoxins. In 1978, Schwartz and Dayhoff summarized this information and the then even-more-limited data from ribosomal RNA (rRNA)—in particular, 5S rRNA (Schwartz and Dayhoff, 1978). The endosymbiotic nature of organelles was well supported, but the origin of the nuclear genome (that is, the genome of the host for these endosymbioses) remained a mystery. A grand reconstruction of all of the main events of evolution with a single molecular chronometer was called for.

Such a grand reconstruction was the goal of Woese, who had begun, in the late 1960s, to assemble catalogs of the sequences of the oligonucleotides released by digestion of in vivo-labeled 16S rRNA with T1



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