Confirming that the virus is very old also helps explain why it infects almost all African monkeys but does not sicken them. Over many generations, as any disease kills off vulnerable victims, the host adapts to it.

The new research, published Thursday in Science magazine, was relatively simple. Scientists tested 79 monkeys from Bioko, a volcanic island 19 miles off the West African coast. Bioko used to be the end of a peninsula attached to the mainland in what is now Cameroon, but it was cut off when sea levels rose 10,000 years ago at the end of the last ice age.

Since then, six monkey species have developed in isolation on the island, and scientists from the National Primate Research Center at Tulane University in Louisiana and other American and African universities found that four of them  drills, red-eared guenons, Preuss’s guenons and black colobuses  had members that were infected with S.I.V.

The four strains in the four species were genetically very different from one another  meaning they presumably did not come from monkeys carried over to the island by humans in the last few centuries. But each was close to the strain infecting members of the same four genuses on the mainland, meaning they must have existed before Bioko was cut off.

Knowing that all four strains were at least 10,000 years old, scientists recalculated the virus’s “molecular clock,” measuring how fast it mutates. They now believe that all the S.I.V. strains infecting monkeys and apes across Africa diverged from a common ancestor between 32,000 and 78,000 years ago.