A glance at Table 25.1 will show you that there are major differences (most of which cannot be seen even under an electron microscope) between the two prokaryotic domains. Prokaryotic archaea share a number of features with their relatives the eukaryotes, but they also retain some ancestral similarities with bacteria. (Note that we use lowercase when referring to members of these domains and initial capitals when referring to the domains themselves.) The basic unit of an archaeon (the term for a single archaeal organism) or bacterium (a single bacterial organism) is the prokaryotic cell. Each single-celled prokaryote contains a full complement of genetic and protein-synthesizing systems, including DNA, RNA, and all the enzymes needed to transcribe and translate genetic information into proteins. The prokaryotic cell also contains at least one system for generating the ATP it needs.

Genetic studies clearly indicate that all forms of life share a single common ancestor. As we noted earlier, eukaryotes share a more recent common ancestor with certain groups of prokaryotic archaea than they do with bacteria (see Figure 25.1). However, the *mitochondria of eukaryotes and the chloroplasts of photosynthetic eukaryotes (such as plants) originated through endosymbiosis with bacteria. Some biologists prefer to view the origin of eukaryotes as a fusion of two equal partners (one ancestor that was related to modern prokaryotic archaea and another that was more closely related to modern bacteria). Others view the divergence of the early eukaryotes from specific groups of prokaryotic archaea as an event separate from and earlier than the later endosymbioses. In either case, most eukaryote genes are more closely related to those of specific groups of prokaryotic archaea, whereas other genes (especially genes related to mitochondria and chloroplasts) are most closely related to those of bacteria. The tree of life therefore contains some merging of lineages as well as the predominant divergence of lineages.

Biologists estimate that the last common ancestor of the three domains lived about 3 billion years ago. We can deduce that it had DNA as its genetic material, and that its machinery for transcription and translation produced RNAs and proteins, respectively. This ancestor likely had a circular chromosome. All living organisms are the products of billions of years of mutation, natural selection, and genetic drift, and they are all well adapted to present-day environments. The earliest prokaryote fossils, which date back at least 3.5 billion years, indicate that there was considerable diversity among the prokaryotes even during those earliest days of life.