A significant portion of what we think of as our "human" DNA actually came from viruses, and a new discovery suggests that our DNA is even less human than scientists previously thought.
Nineteen new pieces of non-human DNA -- left by viruses that first infected our ancestors hundreds of thousands of years ago -- have just been found, lurking between our own genes.
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 the scientists.
Whether or not it can replicate, or reproduce, it is not yet known. But other studies of ancient virus DNA have shown it can affect the humans who carry it.
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.
"This research provides important information necessary for understanding how retroviruses and humans have evolved together in relatively recent times," said senior study author John Coffin from Tufts University School of Medicine in Boston, US.
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The study was published in the journal Proceedings of the National Academy of Sciences.
In fact, about eight percent of what we think of as our "human" DNA actually came from viruses, the study pointed out.
The research looked into 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.