This revised edition introduces evidence that symbiogenesis is a major source of evolutionary innovation leading to the origins of new species. The author offers insights into the genetic and metabolic interactions of the bacterial comunities that became protocists. Among these diverse organisms, the earliest eukaryotes, including some that are fossilized in the Proterozoic record, are those that then evolved to become animals, plants and fungi. The book presents a perspective on evolution during the Archaen and Proterozoic eons of pre-Phanerozoic time, with consequences for taxonomy. A single dipartite phylogenetic tree includes all major groups of organisms.