Your DNA Contains Instructions Written by Ancient Viruses

About 8 percent of human DNA comes from retroviruses that infected our ancestors millions of years ago. These viral sequences, called Human Endogenous Retroviruses (HERVs), have been embedded in our genome so long that most are now inactive. But they're still there, copied into nearly every cell in your body. What makes this stranger is that some of these ancient viral genes still get used. Your placenta, the organ that sustains a fetus, relies on a protein called syncytin that originally came from a retrovirus infection around 25 million years ago. Without this viral legacy, human pregnancy wouldn't work as it does. Scientists have found evidence that these dormant viral sequences may influence how our immune system develops and how certain genes are regulated. A few HERVs appear to activate during specific diseases, suggesting they're not entirely benign passengers. Some research hints they might play a role in conditions like multiple sclerosis and lupus, though the relationship is complex and still being studied. This isn't unique to humans. Other mammals carry their own endogenous retroviruses, and the proportion varies. The finding upended the old idea of DNA as a neatly organized instruction manual. Instead, it's more like a library where chapters were written by organisms that no longer exist, yet still shape who we are.