Your DNA Contains Viral Code From Ancient Infections

About 8 percent of your DNA comes from viruses. Not metaphorically, but literally. Over millions of years, retroviruses have inserted their genetic material into human ancestors' genomes, and those sequences have been passed down through every generation since. These viral insertions are called human endogenous retroviruses, or HERVs. Most are now dormant and harmless, broken down by mutations accumulated over time. But some remain partially functional. Scientists have found that certain HERV sequences actually play roles in normal human development, including placenta formation during pregnancy. Without these ancient viral contributions, human reproduction as it exists today might not work. The discovery upended the idea that viral DNA in our genome is purely parasitic junk. Research suggests that our bodies have even co-opted some viral genes for immune defense, turning an old invader's toolkit into a protective mechanism. This isn't unique to humans. Virtually all complex organisms carry similar viral sequences in their DNA, evidence of an evolutionary arms race spanning hundreds of millions of years. Each insertion represents a moment when a virus successfully infiltrated a reproductive cell and permanently altered the genetic inheritance of an entire species. We are, in a very real sense, walking repositories of ancient viral history.