A virus found in lemurs—squirrel-sized primates from the island of Madagascar—have given scientists new evidence about HIV’s origins and age, reports IRN/Plus News.

Researchers from Stanford University in Palo Alto, California, uncovered a virus in the animals that is genetically related to HIV. This newly found virus questions the widely believed theory that lentiviruses—the two HIV strains found in humans—were passed to humans from African primates who harbored the virus for about a million years.

“Our discovery means that primate lentiviruses have been present in Madagascar historically and may still be circulating there,” Robert Gifford, PhD, an infectious disease researcher at the Stanford School of Medicine and lead author of the research, told IRIN. “Since Madagascar has been very isolated throughout evolutionary history, it’s not clear how we could have these viruses present both there and in Africa—unless they are in fact many millions of years old.”

It’s believed that Madagascar split from the continent of Africa about 160 million years ago.