In the 1960s and 1970s, organisms were classified into five kingdoms, one of which was prokaryotes
In the 1980s, organisms were reorganized into three domains, with the prokaryotes split into Archaea and Bacteria, and all of the other four kingdoms lumped together into Eukarya. Why did this happen?
In the original classification, prokaryotes were separated from all other organisms because of cell structure. With the advent of DNA sequencing, gene sequence comparisons became possible. Carl Woese, Norm Pace, and others compared sequences of conserved genes such as ribosomal RNAs and discovered that prokaryotes fall into two distinct molecular classes that are as different from each other as either is from the eukaryotes.
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