In 1977 Carl Woese found life had three great branches, not two
Scientists long split living things into two camps: cells with a nucleus and cells without. Then Carl Woese compared sequences of a molecule found in every ribosome and discovered that the nucleus-free microbes were really two separate lineages. Archaea, once dismissed as odd bacteria, turned out to be profoundly different inside.
A domain is the highest rank in biological classification, sitting above kingdoms. The term arrived in 1990 with the three-domain system devised by Carl Woese, Otto Kandler and Mark Wheelis, who divided life into Archaea, Bacteria and Eukarya. The first two are prokaryotes, single cells without a membrane-bound nucleus. Every organism whose cells hold a nucleus and other enclosed compartments falls into Eukarya.
The evidence came from ribosomal RNA. In 1977 Woese compared nucleotide sequences of 16S ribosomal RNA and saw that each domain carries its own distinctive version. Archaea and bacteria can look alike under a microscope, which is why they were lumped together for so long, but their membranes are built differently. Bacterial membranes are phospholipid bilayers, whereas archaeal membranes use branched chains joined by ether linkages, a chemistry that helps some archaea endure extreme heat and acidity. Many still live in mild places, while others thrive in brine or scalding water.
Bacteria pose a special headache. They swap genes across distant lineages so readily that counting species, or even arranging them into a neat family tree, becomes nearly impossible; their history looks more like a network with cross-links. Eukaryotes, by contrast, are usually grouped into four kingdoms: plants, animals, fungi and protists.
Not everyone accepts three domains. The eocyte hypothesis, proposed by Lake and colleagues in 1984, argues that eukaryotes branched from within the archaea, leaving just two domains. Mayr's two-empire scheme keeps the older split between prokaryotes and eukaryotes. Viruses sit outside the whole framework, since they are not cells; in 2012 Stefan Luketa suggested adding separate groups for viruses and for prions, and in 1974 the mycologist Royall T. Moore had proposed a similar rank he called dominion.
Source: Domain (taxonomy)
