The teeming masses of the microbiome also contain a record of our evolutionary past. Howard Ochman found that the evolution of the gut microbes in great apes perfectly recaps that of their host. The bacteria from the two species of gorilla are more closely related to each other than they are to human gut bacteria. Geography, diet and disease aside, the main thing that influences the members of these bacterial cities is the species of the host. You could reconstruct the evolution of the apes, simply by comparing the bacteria in their bowels.