Bacteria and Archaea are highly diverse groups that survive in almost every imaginable habitat on Earth. Many prokaryotes can survive and even thrive in habitats where no eukaryotes can live, including extremely hot, acidic, or saline conditions. Eukaryotes are most closely related to some lineages of prokaryotic archaea, although endosymbioses of bacteria within eukaryotic cells contributed to the evolution of eukaryotic organelles.
learning outcomes
You should be able to:
Interpret a phylogenetic tree of major lineages of Bacteria and Archaea and use it to explain (a) the difference between a prokaryote and a eukaryote, and (b) how prokaryotic archaea and bacteria both contributed to the evolution of eukaryotes.
Consider the difference between prokaryotes and eukaryotes shown in Table 25.1. Why might eukaryotes be more like prokaryotic archaea in some features and more like bacteria in others?
Similarities between any two groups shown in the table can occur for three reasons: (1) the similar feature between two groups is ancestral to life, and the third group has a derived character; (2) the similar feature is a derived character, indicative of a shared ancestry (such as between prokaryotic archaea and their eukaryotic relatives), and the third group (bacteria) has the ancestral condition; or (3) the two endosymbioses of bacteria within eukaryotes may have led to similarities between bacteria and eukaryotes. An example of the first category is that the two prokaryotic groups lack a nucleus, whereas eukaryotes have a nucleus (a derived character). The similarity of the RNA polymerases of prokaryotic archaea and eukaryotes is likely an example of the second category (a derived feature that indicates an evolutionary relationship between these two groups). A possible example of the third category is the ester-
Given that all species of life have evolved for the same amount of time since their common origin, how would you respond to someone who characterizes prokaryotes as “primitive”? Include at least two examples of major groups of prokaryotes to support your answer.
All organisms that are alive today are descendants of a common ancestor of life. Many changes have occurred in all these different species, including prokaryotes as well as eukaryotes. Many prokaryotes have undergone radical changes that allow them to live in extreme environments or in unusual ways. Numerous examples could be used, such as the following: One group of euryarchaeotes, the extreme halophiles (salt lovers), live exclusively in very salty environments, and some can live in lakes with pH values as high as 11.5. They have many adaptations that allow them to live in these environments, and so they are more “derived” in this respect than is any eukaryote. As another example, several groups of bacteria, such as chlamydias, are highly derived intercellular parasites. They have many derived changes that allow them to have very small cells than can live within the cells of other species. Thus the terms “primitive” and “derived” make sense only with respect to a particular feature of the organism, and no living organisms can be considered to be primitive overall (i.e., the common ancestor of life was very different from all organisms that are alive today).
Prokaryotes are found almost everywhere on Earth and live in a wide variety of ecosystems. In the next section we will examine the contributions of prokaryotes to the functioning of those ecosystems.