A leukaemia-causing virus has been found in ancient DNA traces of the bent-winged bat, a breakthrough that could lead to new treatments.
The discovery provides a "missing link" in the fossil record of retroviruses - a family of viruses that date back almost half a billion years.
Known as a 'deltaretrovirus', human T-lymphotropic virus 1 (HTLV-1) infects 15 to 20 million people worldwide and can cause leukaemia.
It has long been believed deltaretroviruses have infected humans since prehistoric times.
But because these viruses had no 'fossil record' their deeper origins have until now remained a mystery.
Dr Robert Gifford, of Glasgow University's centre for virus research, said: "The discovery of this viral sequence fills the last major gap in the fossil record of retroviruses.
"It provides a means of calibrating the timeline of interaction between deltaretroviruses and their hosts."
The study published in Proceedings of the National Academy of Sciences offers conclusive evidence these viruses are between 20 and 45 million years old.
It also offers key insights into their characteristics and will allow scientists to better understand them in the future.
Dr Gifford said: "Importantly this finding could also be used as a tool for understanding the mechanisms mammals have evolved specifically to counter the threat from these viruses.
"Understanding the history of these viruses will help scientists to better understand how they affect people and animals now and in the future."
The deltaretrovirus group which includes HTLV-1 can cause a rare blood cancer called ATLL (Adult T-Cell Leukaemia/Lymphoma (ATLL).
Infection is very rare in the UK and most people who carry the virus will not develop the disease.
The study led by Dr Daniel Elleder at the Czech Academy of Sciences in Prague identified the remnants of a deltaretrovirus in the genome of 'bent-winged bats'.
They are members of a group of bats called the Minopterids and the virus sequence was found to be integrated in a range of distantly related species - showing it originated 20 to 45 million years ago.
Dr Gifford said: "Deltaviruses infect humans. The genome of the bent-winged bat became publicly available recently and we found the sequence was in all the species before they diverged 20 to 45 million years ago.
"In the long-term this could be important clinically because now we know it's such an old virus it could hep us identify human genes that target it."
The researchers also found an unusual - and as yet unexplained - feature of the virus which is also present in contemporary deltaretroviruses.
The discovery that this characteristic has defined deltaretroviruses for millions of years indicates it is somehow key to their biology - and could help scientists study them in the future.
The fossil record is comprised of DNA sequences that are derived from ancient retroviruses - and have been 'preserved' in animal genomes.
The study was funded by the Medical Research Council (MRC) and the Czech Ministry of Education, Youth and Sports.
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