Erik Trinkaus, a physical anthropologist at Washington University in St. Louis (who was not involved in the study), says most evolutionary biologists and anthropologists believe there were three major waves of migration from Africa to Europe: the first occurring about two million to 1.5 million years ago during the late Pliocene and early Pleistocene epochs; a second during the mid Pleistocene, roughly one million to 500,000 years ago; and ending with the spread of modern humans, 50,000 to 30,000 years in the past.
The new findings, published this week in Proceedings of the National Academy of Sciences USA, casts doubt on the second migration out of Africa. "[The researchers] are not denying that it happened," Trinkaus says, "just that it was less important than movement across Eurasia."
The study was led by Maria Martinón-Torres, a paleobiologist at the National Research Center on Human Evolution in Burgos, Spain. The research team analyzed the choppers of human ancestors from the Pleistocene and late Pliocene epochs.
"Teeth are the best genetic marker that we have in the fossil record itself," Trinkaus says, because "they are as close as we can get to a reflection of the individual's genetic makeup." The reason: Tooth crowns are genetically determined—and thus reflect an individual's genotype—and are not affected by environmental stress during development.
Scientists found that teeth from African specimens were a different shape or morphology than those from Eurasian samples. The researchers wrote that teeth toward the front of the mouth from Eurasians had more "morphological robusticity," such as a triangular, shovel shape. Their back teeth were smaller and had smoother chewing surfaces; the rear teeth from African samples were larger and the chewing surfaces on them more pointy and jagged.
"The continuity of the 'Eurasian dental pattern' from the early Pleistocene until the appearance of upper Pleistocene Neandertals suggests that the evolutionary courses of the Eurasian and the African continents were relatively independent for a long period and that the impact of Asia in the colonization of Europe was stronger than that of Africa," the researchers wrote in the new report. "This finding does not necessarily imply that there was no genetic flow between continents, but emphasizes that this interchange could have been both ways."