Your DNA Contains Functional Code from Ancient Viruses

About 8 percent of human DNA consists of sequences left behind by retroviruses that infected our ancestors millions of years ago. These viral insertions, called human endogenous retroviruses (HERVs), are essentially fossilized remnants of ancient infections that became permanently woven into our genome. Most of these sequences are now inactive, but some have been repurposed by our bodies for actual functions. One HERV protein helps form the placenta during pregnancy, a critical structure for fetal development. Another viral sequence produces a protein involved in regulating the immune system. Scientists have discovered that certain HERVs activate during specific illnesses or developmental stages, suggesting they may play roles we're still uncovering. The retroviruses that created these insertions no longer pose a threat to us, but they left their mark so thoroughly that we carry their code in nearly every cell. This discovery fundamentally changed how biologists understand evolution, revealing that our genome is a layered archive of ancient parasites that became permanent residents, some transforming into useful tools rather than remaining purely genetic debris.