An estimated 5 million trillion trillion bacteria live on Earth (and they have a combined weight roughly equal to that of the top three feet of France). That bacterial census, the first of its kind, was taken by microbiologist William Whitman and his colleagues at the University of Georgia, who divided the planet into different habitats, such as ocean, soil, subsurface, air, and the insides of animals. After scanning the literature for bacterial counts in each zone, Whitman realized that "a lot of the habitats weren't very significant." Although bacteria live almost everywhere, from some 40 miles high in the atmosphere to deep-sea vents, 94 percent of them live in the top 1,300 feet of Earth's surface. The bacteria inside animals and us account for just a fraction of 1 percent. Whitman's estimate reemphasizes the enormous genetic diversity of bacterial life. Within the multitude of oceanic bacteria alone, he calculated, any given gene is struck by four mutations every 20 minutes. Though most mutations are detrimental for the bugs, he says, "this gives you a tremendous opportunity for change and adaptation to a new environment."