The human genome is riddled with endogenous retroviruses – little pieces of degraded and generally harmless retrovirus DNA passed down through the generations, along with our own genetic information. Because most endogenous retroviruses have been part of our DNA for millions of years, scientists can’t explain how they went from their virulent, disease-causing forms to the inert bits of “junk DNA” most of them are today. A new study published today in the journal PNAS looks to koalas for an answer.