Your DNA Contains Instructions Written by Ancient Viruses

About 8 percent of your DNA comes from retroviruses that infected your ancestors millions of years ago. These viral sequences, called Human Endogenous Retroviruses (HERVs), became permanently embedded in the human genome and have been passed down through every generation since. Most of this viral DNA is now inactive, like archaeological layers buried in your cells. But some fragments still function. Researchers have found that certain HERV genes help regulate pregnancy, playing a role in placental development. Others appear to influence immune responses. The virus that infected a distant ancestor could not be defeated by the immune system, so instead of being cleared away, its genetic material integrated into the egg or sperm cell and became hereditary. This process happened repeatedly over evolutionary time, which is why these viral sequences are so common in humans and other mammals. Scientists initially viewed HERVs as junk DNA, evolutionary leftovers with no purpose. More recent research suggests some of these ancient viral genes have been co-opted for essential human functions. It's a strange reminder that your body is partly the product of an ancient viral arms race, and that line between infection and inheritance is far blurrier than most people realize.