The story of where modern humans came from has never been cut-and-dried, but two theories occupy the forefront of the debate. According to the Out of Africa model, Homo sapiens arose as a new species approximately 150,000 to 200,000 years ago in Africa and went on to replace archaic humans such as the Neandertals. The multiregional evolution model, in contrast, holds that archaic populations, the Neandertals among them, contributed to the modern human gene pool. New findings published online this week by the Proceedings of the National Academy of Sciences lend support to the former model, suggesting that moderns replaced Neandertals without interbreeding.

David Caramelli of the University of Florence and his colleagues extracted and analyzed mitrochondrial DNA (mtDNA) from the skeletons of two early modern Europeans (sometimes referred to as Cro-Magnons) collected from Paglicci Cave in southern Italy. The remains of the teenaged boy and young woman date to 23,000 and 25,000 years ago. The scientists compared the mtDNA from the two individuals to mtDNA from contemporary Europeans as well as to published mtDNA results from three Neandertal individuals between 29,000 and 42,000 years old. (The image above shows a cast of a skull of an early modern European on the left and one of a Neandertal specimen on the right.) They found that the Paglicci samples fit well within the genetic variation exhibited by today's Europeans, but differed significantly from the Neandertal specimens. "This discontinuity," the team writes, "is difficult to reconcile with the hypothesis that both Neandertals and early anatomically modern humans contributed to the current European gene pool."