Chapter 5 outlined the two distinct ways that cells are organized internally. Eukaryotic cells, which include the cells that make up our bodies, have a membrane-bounded nucleus and organelles that form separate compartments for distinct cell functions. Prokaryotic cells have a simpler organization. No membrane surrounds the cell’s DNA, and there is little in the way of cell compartments. Prokaryotic cell organization is an ancestral character for life as a whole—that is, it is a feature that was present in the last common ancestor of all organisms alive today. The group defined traditionally as the prokaryotes, however, is paraphyletic in that it excludes some descendants of the last common ancestor of all living organisms, namely eukaryotes (Chapter 23).

Bacteria and Archaea are the two domains characterized by prokaryotic cell structure. These organisms are present almost everywhere on Earth. What they lack in complexity of cell structure, these tiny cells more than make up for in their dazzling metabolic diversity. As will become clear over the course of this chapter, Bacteria and Archaea underpin the efficient operation of ecosystems on our planet.