Anthropologists have long speculated on the origins of the native populations in the Americas. One of the more recent theories holds that three distinct waves of immigrants--corresponding to three proposed linguistic groups among Native Americans (Amerind, Na-Dene and Eskaleut)--crossed the Bering strait from Asia no earlier than 13,000 years ago. Molecular anthropologist Theodore Schurr’s research on genetic variation in the mitochondrial DNA of native populations in Asia and the Americas casts some doubt on this view. His research suggests that the first Americans may have come to the New World more than 30,000 years ago. Although there is concordance between the linguistic and genetic affinities of Na-Dene Indians and Eskimo-Aleuts, this type of linkage is less robust for the so-called Amerinds. According to Schurr the genetic evidence is, instead, more consistent with a complex migration pattern involving at least two ancient expansions of ancestral populations who may have come from widely separated parts of the Asian continent, as well as the re-expansion of Beringian populations into the New World following the last period of glaciation.