If we can believe that creatures of differing species derived from a common organism, why could we not believe?

that humans of differing races derived from a common human? If the similarities between a bird and a man can make us conclude that there must have been a common ancestor from which both evolved, does not the much stronger similarity between someone chinese and somone english (or humans of different cultures) lead us to the conclusion there must have been a common forefather, an "Adam"?