The ancestors of modern bacteria were single-celled microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago. For about 3 billion years, all organisms were microscopic, and bacteria and archaea were the dominant forms of life. Bacteria have a bad rap as agents of disease, but scientists are increasingly discovering their many benefits, such as maintaining a healthy gut. A new study now suggests that bacteria may also have helped kick off one of the key events in evolution: the leap from one-celled organisms to many-celled organisms, a development that eventually led to all larger multi-celled animals, including humans.