Ancient DNA enters humanity's heritage. (Stone Age Genetics).
Fossils of two anatomically modern H. sapiens found in a southern Italian cave yielded mitochondrial DNA, which is inherited from the mother, say Giorgio Bertorelle of the University of Ferrara in Italy and his colleagues. The DNA contains chemical sequences that resemble those of people today but differ substantially from those previously isolated from four Neandertal specimens, the scientists report.
One of the Italian Cro-Magnons dates to 25,000 years ago; the other, to 23,000 years ago. Neandertal fossils that have yielded mitochondrial DNA range from about 29,000 to 42,000 years old (SN: 4/1/00, p. 213).
"These results are at odds with the view [that] Neandertals were genetically related with the anatomically modern ancestors of current Europeans or contributed to the present-day human gene pool," Bertorelle's group concludes.
Contamination of ancient DNA can occur easily. However, the mitochondrial DNA obtained from the Cro-Magnon bones exhibits no trace of genetic material from other animals unearthed in the Italian cave or from people who have handled the bones, the scientists assert in an upcoming Proceedings of the National Academy of Sciences.
The researchers compared Cro-Magnon genetic sequences from an especially variable stretch of mitochondrial DNA with correspending sequences from Neandertal fossils and from 80 people now living in Europe or western Asia.
Cro-Magnon sequences fall within a genetic category shared by people today but not by Neandertals, the scientists report. This result aligns with the theory that modern H. sapiens originated in Africa around 150,000 years ago and then replaced Neandertals in Europe rather than interbred with them, Bertorelle and his coworkers say.
Mark Stoneking of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, an advocate of this single-origin model of human evolution, nonetheless regards the new evidence with caution. He hasn't seen the report but worries that the Cro-Magnon DNA is contaminated. However, mitochondrial DNA analyses of living people align with the single-origin, or out-of-Africa, scenario, Stoneking says.
Adherents of the contrasting multiregional-origin theory of evolution view the Cro-Magnon findings even more skeptically. They argue that anatomically variable H. sapiens in Europe, Africa, and Asia interbred enough over the past 1 million years or more to evolve as a single species.
The reported genetic differences between Cro-Magnons and Neandertals may be consistent with interbreeding of small Neandertal and large H. sapiens populations, comments John H. Relethford of the State University of New York at Oneonta.
Moreover, if mitochondrial-DNA alterations spread quickly by providing survival advantages instead of gradually by chance, as is usually assumed (SN: 2/6/99, p. 88), then such evidence can't be used to reconstruct ancient human evolution, he notes.
Statistical analyses of worldwide living populations' nuclear DNA--the DNA that holds most of a person's genes--indicate that interbreeding of H. sapiens and other Stone Age Asian or European groups, if not Neandertals, contributed to modern humanity's evolution, remarks Alan R. Templeton of Washington University in St. Louis.
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|Date:||May 17, 2003|
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