A DNA double helix is seen in an undated artist's illustration released by the National Human Genome Research Institute to Reuters. REUTERS/National Human Genome Research Institute/Handout

The number of viral DNA in the human genome just went up with the discovery of 19 new pieces left by viruses in our ancestors thousand years ago. Already 17 pieces of viral DNA have been detected by scientists.

Researchers from Tufts University and the University of Michigan Medical School also found one stretch of DNA with the full genetic code for an entire virus in 50 of the 2,500 people they studied. The study confirmed the 17 pieces identified earlier.

The new viruses are part of a family of human endogenous retroviruses (HERV) called HERV-K. Its intact genome, the second such whole viral genome in human DNA, was found on the X chromosome and is called Xq21.

"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," says senior author and virologist John Coffin, Ph.D. of the Tufts University School of Medicine.

The work has been published in the Proceedings of the National Academy of Sciences.

In the co-evolution of humans and viruses, the virus-generated DNA was copied and passed along down generations leading to almost 8 percent of human DNA coming from viruses.

The HERVs are infectious viruses that inserted a DNA-based copy of their own RNA genetic material into our ancestors' genomes. The HIV belongs to this type of virus.

"Many studies have tried to link these endogenous viral elements to cancer and other diseases, but a major difficulty has been that we haven't actually found all of them yet," says co-first author Zachary H. Williams, a Ph.D. student at the Sackler School of Graduate Biomedical Sciences at Tufts University in Boston. "A lot of the most interesting elements are only found in a small percentage of people, which means you have to screen a large number of people to find them."

The study sample contained large numbers from Africa from where the ancestors of modern humans originated before migrating around the world. Using sophisticated techniques the team compared key areas of each person's genome to the "reference" human genome.

Many of the genomes they examined were from the 1000 Genomes Project while another set of genomes came from work done at Stanford University as part of the Human Genome Diversity Project. These latter samples with more African genomes showed more signs of HERVs, in line with the high level of genetic diversity expected in them.

While whole viruses lurking in our DNA may be rare, the impact of many HERV sequences on human health or disease is probably not. The study is a step in the direction, having shown that genomic data from multiple individuals can be compared to the reference human genome to detect new HERVs. Some people however carry insertions that could not be mapped back to the reference.

Unlike what we think, the human DNA has borrowed genes from many other species, owing to common ancestors dating back to million years. Last year, biologists from the University of Texas at Austin showed that hundreds of genes from a common ancestor live on nearly unchanged in yeast and humans. The human genome and the chimp's match to around 96 percent.

Genetic studies have also suggested a diverse kind of evolution where horizontal gene transfer could have, and also continue to, move genes from bacteria, algae, viruses or fruitflies into the human genome.