You know, of course, that there have been several different species of humans (hominids) which co-existed in ancient evolutionary periods: from the australopithes through and including neanderthals and the cro-magnons. Many paleontologists argue that it was the Cro-mags ("modern" humans) who eventually dispatched, or at least hastened, the extinction of neanderthals circa 40,000 years ago.

You are right that, "No evidence of more than one human race has ever been found" but only in the sense that "race" is a sociological, cultural and political construction. But there certainly have been many different human species. There is only one human species today -- Homo sapiens sapiens--us.