 Major transitions have been a focal topic in philosophy of biology since John Maynard Smith and Eörs Szathmáry’s book, The Major Transitions in Evolution (1997). They focused on key transitions in the history of life on Earth ranging from the origin of cells to the origin of sex to the origin of language. Philosophical discussion of the major transitions in evolution largely revolves around efforts to find a unifying theme to bind major transitions into a single theory (the same aspiration that drives many species and tree of life discussions). A common choice for that theme has been individuality, meaning any major transition in evolution is understood to involve most centrally a transition in evolutionary individuality (e.g., Ratcliff et al. 2017; Michod 2011; however, cf. O’Malley and Powell 2016). But irrespective of any particular theoretic commitment to the nature of evolutionary transitions, microbial evolution is crucial for understanding the various transformations life on Earth made on the way to its current state. This is very much the case even if theorists want to understand only the evolution of eukaryotes, or even if they care exclusively about the origin of metazoa—the “kingdom” within eukaryotes to which humans  belong.[5]