Polygenism postulated the separate origins and independent evolution of the races along different, isolated lines. The Origin of Races,'' published in 1962. The chapters of historical background, replete with fascinating accounts of past wars in anthropology, are a book within a book. One sympathizes with the authors' conscientious effort to put distance between their theory and anthropology's racist past, but wishes there had been more streamlining here. The authors seek to rebut the DNA evidence for a recent origin only in Africa, stressing flaws in data analysis cited by other scientists.
Gene flow is the answer.
Fossils preserve early traces of regional variations and what is interpreted as a gradual evolution toward modern humans. But a certain amount of gene exchange insured that human evolution followed a common pattern and kept the variations from leading to separate regional species. One source of misunderstanding about the theory, the authors believe, is its characterization as parallel evolution from a single common base long ago.