A new study from scientists at Oxford University, published in the journal Nature Communications, suggests that retroviruses - a family of viruses that includes HIV - are several hundred million years older than previously thought.
According to lead researcher Dr Aris Katzourakis, it was previously thought that these viruses - which can embed themselves in the genome and lie dormant - may have arisen only 100 million years ago, making them young whippersnappers in evolutionary terms.
Because viruses don’t fossilise, scientists have to figure out their age by looking at the viral DNA across many different species which separated a long time ago - these can be thought of as genetic fossils rather than physical ones.
By comparing the remnants of DNA of so-called "foamy" retroviruses across species as diverse as mammals, fish and amphibians, Katzourakis and his team conclude that retroviruses are almost half a billion years old, and evolved together in a kind of arms race with the evolution of immune responses in their hosts.
Probing how animal and viral genomes have evolved hand in hand over millions of years is enabling researchers can gain a deeper understanding of immune responses to these invaders and leading to more effective antiviral treatments in the future.