topic. In one, he deduced a correspondence table between what was then known as "soluble RNA" and DNA based upon their respective base pair ratios. He then re-evaluated experimental data associated with the hypothesis that viruses used one base, rather than a triplet, to encode each amino acid, and suggested 18 codons, correctly predicting one for proline. Other work established the mechanistic basis of protein translation, but in Woese's view, largely overlooked the genetic code's evolutionary origins as an afterthought. In 1962, Woese spent several months as a visiting researcher at the Pasteur Institute in Paris, a locus of intense activity on the molecular biology of gene expression and gene regulation. While in Paris, he met Sol Spiegelman, who invited Woese to visit the University of Illinois after hearing his research goals; at this visit Spiegelman offered Woese a position with immediate tenure beginning in the fall of 1964. With the freedom to patiently pursue more speculative threads of inquiry outside the mainstream of biological research, Woese began to consider the genetic code in evolutionary terms, asking how the codon assignments and their translation into an amino acid sequence might have evolved. Discovery of the third domain For much of the 20th century, prokaryotes were regarded as a single group of organisms and classified based on their biochemistry, morphology and metabolism. In a highly influential 1962 paper, Roger Stanier and C. B. van Niel first established the division of cellular organization into prokaryotes and eukaryotes, defining prokaryotes as those organisms lacking a cell nucleus. Adapted from Édouard Chatton's generalization, Stanier and Van Niel's concept was quickly accepted as the most important distinction among organisms; yet they were nevertheless skeptical of microbiologists' attempts to construct a natural phylogenetic classification of bacteria. However, it became generally assumed that all life shared a common prokaryotic (implied by the Greek root πρό (pro-), before, in front of) ancestor. In 1977, Carl Woese and George E. Fox experimentally disproved this universally held hypothesis about the basic structure of the tree of life. Woese and Fox discovered a kind of microbial life which they called the “archaebacteria” (Archaea). They reported that the archaebacteria comprised "a third kingdom" of life as distinct from bacteria as plants and animals. Having defined Archaea as a new "urkingdom" (later domain) which were neither bacteria nor eukaryotes, Woese redrew the taxonomic tree. His three-domain system, based on phylogenetic relationships rather than