Viral DNA that infected human ancestors 50 million years ago could help protect us from diseases, according to a new study. Around eight percent of the DNA in modern humans derives from these ancient microscopic invaders, researchers report.

These endogenous viruses were not believed to play a major role in humans today, but this idea may need to be rethought in light of this new finding.

Over time, the human body has learned to alter the purpose of this ancient viral DNA, turning them into tools to fight contemporary disease. The innate immune system within people was found to be crippled when fragments of viral DNA within our genomes is disabled.

"We show that some of these endogenous viruses have shaped our biology. Within mammalian genomes are reservoirs of viral DNA that have fueled innovation of the innate immune system," said Cédric Feschotte, human geneticist and co-senior author of a paper announcing the results of the study.

When the human innate immune system recognizes the presence of a foreign invader, it releases interferons, which trigger cells to release a barrage of genes to battle the pathogen. Researchers soon discovered thousands of endogenous retroviruses were activated in the presence of the molecular signal. In addition, detailed investigation revealed these ancient genetic codes are stored near genes utilized by the immune system to fight disease, further suggesting a connection.

Using the gene-editing tool Crispr/Cas9, researchers were able to show that cells without the ancient viral DNA were unable to properly respond when interferon was released by the body. Responses of cells to the presence of viral invaders was also negatively affected when DNA from the ancient invaders was removed from human cells.

As time goes on, viruses mutate, finding new ways to attack the body systems of humans and other mammals. In a similar fashion, the immune systems of mammals re-purposed the ancient genetic codes into components of well-coordinated attacks against microscopic pathogens. Additional endogenous retroviruses may have, similarly, hard-wired the immune systems of other mammals, assisting non-human species in their battles against modern disease.

Investigation of how these ancient viral invaders have been re-purposed into tools to fight disease was published in the journal Science.