Let's hear it for the bugs--not your creepy-crawlies, but bacteria, the be-all (and possible end-all) of life on Earth, according to Margulis. Here she describes the once radical theory that cells have incorporated bacteria to mutual advantage and uses that as a springboard to summarize a still more radical theory of how species evolve. She calls it serial endosymbiosis theory (SET). It is now conventional wisdom that the energy-producing mitochondria in animal cells were once free-living bacteria. Indeed, they have their own genes--different from nuclear DNA. Margulis provides many examples of fruitful symbioses, including sexual union itself as the merger of sperm and egg cells. According to SET, there are successive steps or mergers that led to multicellular life forms: In steps one and two the oldest bacterial forms--the non-oxygen breathing ""archaebacteria"" found in deep ocean vents--merged with swimming bacteria two billion years ago to form the nuclear heart of animal, plant, and fungal cells and provide the cilia for swimming. Later steps introduced a third partner able to breathe oxygen and added the ability to engulf and digest food (phagocytosis). The last step involved engulfing yet another bacterium--but one these various new forms of life could not digest: bright green photosynthetic bacteria. The bone of contention here is the origin of ciliated cells--critical to evolution for their vital role as sperm tails, among other things. Margulis has a theory about their origin, but as they say, more research is needed. Margulis's theory also dictates a change in taxonomy to five kingdoms: bacteria at the base, then ""protoctists"" (algae, slime molds, ciliates) next, and then animals, plants, and fungi. Finally, she defends Lovelock's Gala theory, which she interprets to mean that enormous interacting ecosystems on Earth achieve homeostasis rather than that the planet is in the hands of some benign Mother Earth. This is vintage Margulis--personal, autobiographical, passionate, argumentative, at times over the top, but full of ideas--at least some of which, in the past, have proved to be right.