The human genome may have been sequenced back in 2004, but that was a far cry from documenting all the genes inside us. Our bodies are home to a dizzying number and variety of bacteria, and in a study published in this week's Nature, researchers have used metagenomic sequencing to catalog the genes that belong to the microbes living in our guts. The project, which sampled 124 European people, found that each individual had at least 160 species of bacteria living in his or her digestive tract, and there's a lot of overlap between our guts.