The third great lineage of living beings is the archaea. At first glance, they look like bacteria — and were initially presumed to be so. In fact, some scientists still classify them as bacteria; but most now consider that there are enough differences between archaea and bacteria for the archaea to count as a separate realm.
The most prominent of these differences lies in the structure of the ribosome — the piece of cellular machinery that is responsible for turning the information contained in DNA into proteins. Indeed, it was the discovery of the archaeal ribosome by the biologist Carl Woese in the 1970s that led to their being recognized as the third branch of the tree of life.
What else sets them apart? They sometimes come in peculiar shapes: Haloquadratum walsbyi is rectangular, for example.