What scientists once dismissed as junk DNA may actually be some of the most powerful code in our genome. A new international study reveals that ancient viral DNA buried in our genes plays an active role in controlling how other genes are turned on or off, especially during early human development. These sequences, originally from long-extinct viruses, have evolved to act like tiny genetic switches. Using new analysis tools and large-scale experiments, researchers discovered that certain viral DNA fragments are especially strong at activating genes and may even have helped shape what makes humans different from other primates.