The traditional tree of life (shown above) presents the three domains as distinct, monophyletic lineages. However, other hypotheses propose different views on the relationships among the Archaea, Bacteria, and Eukarya. In particular, the two-domain hypothesis—or eocyte hypothesis—is emerging as a well-supported alternative to the three-domain hypothesis. The eocyte hypothesis, illustrated below, suggests that eukaryotes evolved from eocytes (also known as the Crenarchaeota—a major lineage of the Archaea). Resolving the relationships among these ancient lineages is difficult, but it has profound implications on our understanding of the origin of eukaryotic cells.
Why are Archaea considered a monophyletic group according to the three-domain hypothesis?
a. Because this group includes all organisms except eukaryotes.
b. Because this group includes an ancestral population and all of its descendants.
c. Because all members of this group lack membrane-bound organelles.
d. Because this group evolved after the origin of bacteria.