Many, many millions of years ago, an HIV-like virus wriggled its way into the genome of a floofy, bulgy-eyed lemur, and got permanently stuck.
Trapped in a cage of primate DNA, the virus could no longer properly copy itself or cause life-threatening disease. It became a tame captive, passed down by the lemur to its offspring, and by them down to theirs. Today, the benign remains of that microbe are still wedged among a fleet of lemur genes—all that is left of a virus that may have once been as deadly as HIV is today.
Lentiviruses, the viral group that includes HIV, are an undeniable scourge. The viruses set up chronic, slow-brewing infections in mammals, typically crippling a subset of immune cells essential to keeping dangerous pathogens at bay. And as far as scientists know, these viruses are pretty uniformly devastating to their hosts—or at least, that’s true of “all the lentiviruses that we know of,” says Aris Katzourakis, an evolutionary virologist at the University of Oxford. Which means, a long time ago, that lemur lentivirus was likely devastating too. But somewhere along the way, the strife between lemur and lentivirus dissipated enough that their genomes were able to mix. It’s proof, says Andrea Kirmaier, an evolutionary virologist at Boston College, that lentivirus and host “can coexist, that peace can be made.”
Détentes such as these have been a fixture of mammals’ genomic history for countless millennia. Scientists have stumbled across lentiviruses embedded in the DNA of not just lemurs, but rabbits, ferrets, gliding mammals called colugos, and most recently, rodents—all of them ancient, all of them quiescent, all of them seemingly stripped of their most onerous traits. The infectious versions of those viruses are now extinct. But the fact that they posed an infectious threat in the past can inform the strategies we take against wild lentiviruses now. Finding these defunct lentiviruses tells us which animals once harbored, or might still harbor, active ones and could potentially pass them to us. Their existence also suggests that, in the tussle between lentivirus and host, the mammal can gain the upper hand. Lemurs, rabbits, ferrets, colugos, and rodents, after all, are still here; the ancient lentiviruses are not. Perhaps humans could leverage these strange genetic alliances to negotiate similar terms with HIV—or even extinguish the modern virus for good.