Unit Four. The Evolution and Diversity of Life


15. How We Name Living Things


15.7. Domain Bacteria


The domain Bacteria contains one kingdom of the same name, Bacteria. The bacteria are the most abundant organisms on earth. There are more living bacteria in your mouth than there are mammals living on earth. Although too tiny to see with the unaided eye, bacteria play critical roles throughout the biosphere. They extract from the air all the nitrogen used by organisms, and they play key roles in cycling carbon and sulfur.

There are many different kinds of bacteria, and the evolutionary links between them are not well understood. Although there is considerable disagreement among taxonomists about the details of bacterial classification, most recognize 12 to 15 major groups of bacteria. Comparisons of the nucleotide sequences of rRNA molecules are beginning to reveal how these groups are related to each other and to the other two domains. The archaea and eukaryotes are more closely related to each other than to bacteria and are on a separate evolutionary branch of the tree (as seen in figure 15.10), even though archaea and bacteria are both prokaryotes.


Key Learning Outcome 15.7. Bacteria are as different from archaea as they are from eukaryotes.