Futurists love stories of unintended consequences. I don't think there are many disciplines whose practitioners traffic in anecdotes that appear to undermine their own credibility, but professional futurists are nothing if not modest about their abilities to predict the future, and tend instead to emphasize the contingency of any forecast, and the role that human agency and choice in shaping the future. (An alternate explanation is that we're just masochistic.) Stories about unintended consequences are warnings about the non-deterministic, unpredictable nature of the future.

So I was very interested to read about a recent study arguing that a mutation that made our (i.e., Homo sapiens') ancestors immune to certain retroviruses is partly responsible for our susceptibility to HIV:

The ability of a virus to cause disease in one host but not another is a poorly understood but common phenomenon. Scientists have recently found the remnants of a virus present in the genomes of chimpanzees and gorillas but absent in humans. Humans, it appears, evolved resistance to this virus millions of years ago. Paradoxically, however, the evolution of resistance to that now extinct virus may have made us more susceptible to HIV today.

Scientists have been studying a retrovirus called PtERV, remnants of which can be found in the genome of chimps. From what they can tell, there were several PtERV epidemics about three million years ago, and the genomes of chimps and other great apes still cary fragments of PtERV. Humans, however, don't have it.

Why? It appears that our ancestors developed a resistance to it. "The PtERV virus," the Guardian explains, "can only successfully infect an animal if it avoids destruction by a protein called TRIM5alpha… [which] acts a kind of gatekeeper for the cell." In apes, that protein does a poor job of blocking PtERV; the human version, in contrast, does quite well.

However, there's a catch.

HIV belongs to the same family of retroviruses as PtERV, so the scientists were also interested in finding out whether the gatekeeping activity of the protein would keep out the Aids virus. What they found was that although the modern human TRIM5alpha protein was very effective at blocking the ape virus, it was very poor at blocking HIV infection. Conversely, the ape form of the gatekeeping protein blocked HIV infection but allowed in the PtERV virus. It seems that when one door was closed to infection, another one was opened.

In other words,

That mutation came at a hidden cost. It may have saved our ancestors from extinction, but it cast a shadow into the 20th century when a new virus, HIV, emerged. The gene that saved our ancestors from the ancient viral scourge provided little protection against the new virus.