Notes

4 Until Woese came along, scientists divided all life on Earth into two “domains”: eukaryotes and prokaryotes. Eukaryotes (you-CARRY-otes) are organisms whose cells have a nucleus—a sac inside each cell that holds its DNA.

5 some need to be kept in a very special environment (one without oxygen, for example). And, because they can transfer their genes easily from one to another, that bacterium you think you are studying might have already changed. From the outside, all bacteria are either rod-shaped or spherical, with the occasional spiral thrown in. Without stripes, beaks, or spots, they can’t be classified as easily as zebras, toucans, and ocelots can. To complicate things more, no one could see inside bacteria, so they couldn’t be classified based on how they looked internally either.

6Fortunately, a new method of comparing the RNA or DNA of different organisms had just been developed. It used enzymes to snip strands of the genetic material at precise points into short sequences. Strands from one organism could then be compared to another to find similarities and differences. Results showed up as fuzzy spots in distinctive patterns on X-ray film. But, while the technique worked, no one had yet used it on bacteria.

8 10 years after he began this bacterial family tree, Woese’s buddy Ralph Wolfe introduced him to a new creature that changed everything. Wolfe liked to study outlandish prokaryotes that live in very hot, oxygen-free environments and produce methane gas. Wolfe finally succeeded in both growing these so-called methanogens in the lab and labeling them with radioactivity so that their RNA could be sequenced. When Woese began studying Wolfe’s methanogen, he expected it to fit nicely into the prokaryote side of the two-domain system. “The big surprise was when we finally did do one of these methanogens … uh-oh, it didn’t fit into the prokaryote signature,” he recalls. The methanogen didn’t fit with the eukaryotic signature, either. But it had certain spots in its RNA common to the prokaryotes, and certain ones common to eukaryotes. “We were clearly dealing with recognizably the same sequence, and it came in three different flavors instead of two,” says Woese. “It was this amazing walk through a garden of new plants. There’s something new here,” he remembers thinking. “We’ve got a fish on the hook.” A fish on the hook indeed: it was a new life form. Woese named it archaea

9 This finding was huge, thrilling, and completely unexpected: it was as groundbreaking as Copernicus’s discovery that the earth orbited the sun. And Woese didn’t stop there. He looked at more methanogens to see if they fit this new pattern. The more methanogens Woese studied, the more convinced he became that they represented a different form of life; the sequences that made up their ribosomal RNA were of a third form. And, because they share some characteristics with both eukaryotes and prokaryotes, Woese determined that the archaea are the most primitive cells, and the closest we have come so far to finding the common ancestor of all life on Earth. Once he was completely confident of his results, he and Wolfe published their findings. There are three forms of life—archaea, bacteria, and eukaryotes—rather than two. broken through an intellectual roadblock that had held people back in their thinking about evolution without their even realizing it.