Human cells and microbial cells are incredibly interdependent, because we have evolved together. We provide their habitats; they provide their labor. Actually, Yong writes, “they are more management than labor.” The human genome consists of about 25,000 genes. But the combined genomes of all of our fellow travelers (some microbiologists call them our “old friends”) are about 500 times larger. At the biochemical level they are much more nimble and versatile than we are. Our bodies, Yong writes, “are continuously built and reshaped by the bacteria inside us.” This kind of profound symbiosis has probably been going on since the evolution of the first multicellular animals. “Perhaps,” Yong writes, “it is less that I contain multitudes and more that I am multitudes.” - http://www.nytimes.com/2016/08/21/books/review/i-contain-multitudes-ed-yong.html