Do you know that the DNA in our own cells isn’t solely ours and is actually less human than previously thought? Yes, more than eight percent of the human genome is not human at all—it’s from viruses. According to a new study, scientists have found that 19 new pieces of non-human DNA — left by viruses that first infected our ancestors hundreds of thousands of years ago — lurking between our own genes.

After surveying 2,500 human genomes, a team discovered evidence of 36 different viruses that had accumulated in there over the course of our evolution, including 19 that had never been found before, and one that might still be infectious if it was turned on.

And one stretch of newfound DNA, found in about 50 of the 2,500 people studied, contains an intact, full genetic recipe for an entire virus, said researchers from the University of Michigan and Tufts University. Whether or not it can replicate, or reproduce, it isn’t yet known. But other studies of ancient virus DNA have shown it can affect the humans who carry it, they said.

In addition to finding these new stretches, the scientists also confirmed 17 other pieces of virus DNA found in human genomes by other scientists in recent years.

The study looked at the entire span of DNA, or genome, from people from around the world, including a large number from Africa – where the ancestors of modern humans originated before migrating around the world. The team used sophisticated techniques to compare key areas of each person’s genome to the “reference” human genome.

The findings add to what science already knows about human endogenous retroviruses (HERVs). HERVs is the name for the ancient infectious viruses that inserted a DNA-based copy of their own RNA genetic material into our ancestors’ genomes. They are part of the same type of virus that includes the modern human immunodeficiency virus, which causes AIDS.

Over generations, the virus-generated DNA kept getting copied and handed down when humans reproduced. That’s how it ended up in our DNA today. In fact, about 8 percent of what we think of as our “human” DNA actually came from viruses. In some cases, HERV sequences have been adopted by the human body to serve a useful purpose, such as the one that helps pregnant women’s bodies build a cell layer around a developing fetus to protect it from toxins in the mother’s blood.

“This research provides important information necessary for understanding how retroviruses and humans have evolved together in relatively recent times,” John Coffin from Tufts University said. The findings were published in the Proceedings of the National Academy of Sciences (PNAS).