The human genome proved surprisingly skimpy, encoding somewhere in the range of a mere 20,000 proteins, on the same order as the fruit fly genome—a psychologically humbling number. However, humans provide a scaffold upon which microbes build elaborate ecosystems and, by young adulthood, each of us carries many more microbial genes than our own. This endogenous microbial life includes 100 trillion bacteria in the distal gut alone. “Perhaps the genes supplied by our microbes are part of what make us human,” says microbiome researcher Peter Turnbaugh from the Center for Genome Sciences at Washington University School of Medicine in St. Louis, Mo. Together, the genomes of our resident microbes, collectively defined as the microbiome, provide traits that human genomes do not carry or were, perhaps, discarded as our species evolved, he suggests.