of the oldest branches on the tree of life. In this figure, for example, they compare versions of the RpoB gene in giant viruses (red), bacteria (purple), eukaryotes (blue), and archaea (green). Two branches that turned up in the global ocean survey are in cyan. Unknown 2 seems to be like the giant viruses, but Unknown 1 is just off the map. Eisen and his colleagues consider it possible that these genes come from a fourth domain. Analyzing the ancient history of genes is notoriously tricky, with lots of opportunities for spurious results to crop up, so this possibility might soon evaporate. But scientists have a number of tools at their disposal to test the results. In this case, the first order of business will be to find more of these exotic genes to see if they continue to fall into a distinct domain of their own. TIf they do, scientists will need to track down the actual organisms that carry them. Are they viruses, or are they true cells? That discovery might show how this possible fourth domain got its start. Did it start out as ordinary cellular life, and then some of its genes ended up in viruses? Or is the fourth domain another sign that life as we know it actually originated as viruses ? Answering those questions will be tough. And redrawing the tree of life will probably be even tougher.