Scientists researching the human genome have discovered nineteen bits of DNA that may not belong to our species at all. According to a report from Phys.org, a study published in the Proceedings of the National Academy of Sciences reveals that the foreign DNA was likely left behind by viruses that infected our early ancestors hundreds of thousands of years ago.

So how did ancient virus DNA remain in our genome after all this time? Researchers aren’t positive as to how the DNA kept getting passed on throughout the generations, but they say it’s possible for the ancient viruses to affect people carrying the DNA today.

Scientists also confirmed that 17 other bits of DNA previously discovered did in fact belong to ancient viruses. The study examined the human genome from people around the world, including a large sample from the African continent, where humanity first arose.

The study shines new light on human endogenous retroviruses, or HERVs. This refers to ancient viruses that infected people by inserting their own RNA genetic material into early humans’ genomes. HERVs are similar to the modern human immunodeficiency virus, which leads to AIDS.

DNA generated by viruses continued to copy and transfer between generations; scientists believe this is how so much ancient DNA wound up in the modern human genome. Researchers believe that roughly 8 percent of human DNA actually came from viruses hundreds of thousands of years ago.

There have been some instances where HERV sequences actually adapted to serve a useful purpose in the human body – one in particular helps pregnant women’s bodies protect a developing fetus by building a cell membrane that blocks dangerous compounds found in the blood.

According to the study’s lead author, virologist John Coffin from the Tufts University School of Medicine, “This one looks like it is capable of making infectious virus, which would be very exciting if true, as it would allow us to study a viral epidemic that took place long ago. This research provides important information necessary for understanding how retroviruses and humans have evolved together in relatively recent times.”

A news release from Tufts University describing the details of the study can be found here.