Much of the baggage in the genome comes not from dead genes, however, but from invading viruses. Viruses repeatedly infected our distant ancestors, adding their DNA to the genetic material passed down from generation to generation. Once these viruses invaded our genomes, they sometimes made new copies of themselves, and the copies were pasted in other spots in the genome. Over many generations, they mutated and lost their ability to move. “Our genome is littered with the rotting carcasses of these little viruses that have made their home in our genome for millions of years,” Dr. Haussler said.