Despite over 36 million people worldwide living with HIV, and over 2 million new infections occurring every single year, surprisingly little is known about its deep evolutionary origin. Understandably, much interest since its discovery in 1981 has focused on how the disease infects people and evades their immune system, but a team of researchers decided to see how far back they could trace its origin.
They found that while previously HIV's oldest-known lineage dated back to at most 12 million years ago, they uncovered evidence that this was far short of its actual evolutionary origin, and that the class of virus to which HIV belongs is far more ancient than anyone had postulated, dating back as early as 60 million years ago. The evidence of this long history has been hiding in the genome of a little known creature called a colugo, also known as a flying lemur, although it neither flies nor is a lemur.
HIV is a type of virus known as a lentivirus, which is subdivided five times depending on the type of animal they infect: primates, sheep and goats, horses, cats, and cattle. Lentiviruses are a genus of retroviruses, which means the virus contains RNA that when infecting a cell is transcribed into DNA and integrated into the host cell's own genome. From this, the cell then churns out the viral proteins as it would any other, which makes it tricky for the immune system to identify as an infected cell. When the virus has done its thing, the cell will eventually burst, killing itself and releasing many more homemade viruses to infect more cells.
But it also means that bits of the viral DNA can get trapped and remain within the host animal's DNA, which can subsequently get passed on from generation to generation. In fact, it is thought that large portions of our DNA is made up of remnants of old viral DNA, in what is known as “endogenous retroviruses." Within these, researchers from the Czech Academy of Sciences looked at lentiviruses found in the Malayan flying lemur.
By using genetic analysis, they found that the bits of lentiviruses still present in the colugo DNA probably found their way in at least 21-40 million years ago, but that because the same bits of viral DNA were present in all living colugo species – the last common ancestor of which lived 60 million years ago – the viral DNA was likely incorporated by at least that point. Their work is published in Molecular Biology and Evolution.
This pushes the history of the virus much further back than anyone had thought. The researchers next want to widen their search, and look at other groups of animals to see if they can find any other evidence of the disease hidden within them.