Lentiviruses, the class of viruses to which HIV belongs, are estimated to have infected primates as far back as 16 million years ago, the Economic Times reports. Publishing their findings in PLOS Pathogens, researchers made this estimate by studying an antiviral gene known as TRIM5, which gives rise to a protein of the same name that specifically and uniquely combats lentiviruses.

SIV, HIV’s simian cousin, is also a lentivirus.

The researchers made a projected map of the TRIM5 gene’s evolution by comparing its DNA sequences as found in 22 African primate species. The investigators zeroed in a cluster of genetic shifts within a subset of these monkeys: Cercopithecianae, which include macaques (an animal often used in HIV research), mangabeys and baboons. This genetic shift ultimately suggested to the researchers that lentiviruses first began infecting African primates 11 million to 16 million years ago.