New parents can act pretty strangely—for example, they often have an obsessive interest in what comes out of their baby's bottoms. Now researchers at Stanford University join them in their diaper diving to explore when and how bacteria colonize the human gut. Adults harbor complex microbial ecosystems within their GI tracts. These bacteria help digest food and contribute to a healthy immune system. But when a baby is born, its intestine is devoid of bacterial inhabitants. Within days, however, a cavalcade of microbes set up shop, establishing a vibrant intestinal community whose residents soon outnumber the baby's own cells. The researchers found that the early months were chaotic...with established species disappearing and new ones taking their place. But by the time the babies were 12 months old, each harbored a dynamic but unique collection of intestinal fauna. A set of fraternal twins had the most similar bugs, which suggests that genetics may have something to do with which bacteria stick around.