Viruses can provide answers to questions we have never even asked

Villarreal predicts that, without an effective AIDS vaccine, nearly the entire population of Africa will eventually perish. “We can also expect at least a few humans to survive,’’ he wrote. They would be people who have been infected with H.I.V. yet, for some reason, do not get sick. “These survivors would thus be left to repopulate the continent. However, the resulting human population would be distinct” from those whom H.I.V. makes sick. These people would have acquired some combination of genes that confers resistance to H.I.V. There are already examples of specific mutations that seem to protect people against the virus. (For H.I.V. to infect immune cells, for example, it must normally dock with a receptor that sits on the surface of those cells. There are people, though, whose genes instruct them to build defective receptors. Those with two copies of that defect, one from each parent, are resistant to H.I.V. infection no matter how often they are exposed to the virus.) The process might take tens, or even hundreds, of thousands of years, but Darwinian selection would ultimately favor such mutations, and provide the opportunity for the evolution of a fitter human population. “If this were to be the outcome,’’ Villarreal wrote, “we would see a new species of human, marked by its newly acquired endogenous viruses.” The difference between us and this new species would be much like the difference that we know exists between humans and chimpanzees.

For Villarreal, and a growing number of like-minded scientists, the conclusion is clear. “Viruses may well be the unseen creator that most likely did contribute to making us human.”

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