In the preceding section, we reviewed the basic features of eukaryotic cells. The distinctive cellular organization of Eukarya differs markedly from the simpler cells of Bacteria and Archaea. How did the complex cells of eukaryotes come to exist? In Case 5: The Human Microbiome, we saw that symbioses are common, even in humans. Is the relationship between humans and bacteria unusual, or are symbioses so fundamental that they played key roles in the evolution of the eukaryotic cell?