Human-microbiota coevolution The bacteria that make their home in the intestines of modern apes and humans arose from ancient bacteria that colonized the guts of our common ancestors. Moeller et al. have developed a method to compare rapidly evolving gyrB gene sequences in fecal samples from humans and wild chimpanzees, bonobos, and gorillas (see the Perspective by Segre and Salafsky). Comparison of the gyrB phylogenies of major bacterial lineages reveals that they mostly match the apehominid phylogeny, except for some rare symbiont transfers between primate species. Gut bacteria therefore are not simply acquired from the environment, but have coevolved for millions of years with hominids to help shape our immune systems and development. Science , this issue p. 380 ; see also p. 350