By studying chimpanzee droppings in remote African jungles, scientists reported yesterday, they have found direct evidence of a missing link between a chimpanzee virus and the one that causes human AIDS. Scientists have long suspected that chimpanzees are the source of the human AIDS pandemic because at least one subspecies carries a simian immune deficiency virus closely related to H.I.V., the virus that causes AIDS. But because the simian virus, known as S.I.V.cpz, was identified in chimpanzees in captivity, researchers could not be sure that the same simian virus existed among these apes in the wild. It does, the team...