Figure 3. Arguably the most exciting evolutionary idea to come along in many a day is the Serial Endosymbiosis Theory (SET) championed by Margulis and her assocaites. According to SET, eukaryotes were products of symbiogenesis from the get-go, with undulipodia (what used to be called cilia and flagellae), mitochondria and chloroplasts all being the incorporated remnants of once free-living prokaryotes. Here is one version of this theory laid out in the context of a taxonomic system called The Five Kingdoms. This scheme superceded the Linnean practice of dividing the world into plants and animals (with fungi going somewhere) and precedes the Three Domain classification followed in your text. Notice that, if SET is correct, the three domain theory becomes exceedingly awkward - i.e., Eukarya would derive from the fusion of Archaea and Bacteria. For a review of systematic practices over the years, go here. For a more recent version of SET that accounts for the origin of the eukaryotic nucleus, go here. (From Margulis, L. and M. D. Dolan. 2002. Early Life. Jones and Bartlett Publ. Boston. MA.)