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University of Utah researchers today published the strongest genetic evidence yet that modern humans originated in Africa and spread around the world rather than arising in several regions. By analyzing DNA from Asians, Europeans and Africans, "we found greater [genetic] diversity in African populations," said Lynn Jorde, the U.'s associate chairman of human genetics. "That's consistent with Africa containing the source population from which modern humans arose 100,000 or so years ago."
The study by Jorde, five other Utahns and researchers from Pennsylvania and Finland was published in the new issue of the journal Proceedings of the National Academy of Sciences. Jorde said the study doesn't prove the Out-of-Africa theory of the origin of Homo sapiens. But it provides "the first statistically significant findings that Africans have greater nuclear DNA diversity than do Asians or Europeans," said a journal news release. "The [study's] authors achieve significant results where previous researchers have failed." Kristen Hawkes, who chairs the U. anthropology department, had not read the study but said its conclusion ''seems very reasonable." "A lot of people would argue the combination of accumulated genetic evidence and archaeology make the preponderance of evidence very clearly on the side of the Out-of-Africa hypothesis,'' she said. ''It's the one that makes the most evolutionary sense."
The research dealt with the origin of Homo sapiens - early people who looked like us - not more primitive hominids that evolved some 5 million years ago. The "African replacement theory" holds that Homo sapiens emerged from earlier hominids in Africa 100,000 to 200,000 years ago, then migrated to Asia and Europe and replaced more primitive hominids. Archaeological evidence indicates humans from Africa replaced Neanderthals in Europe 32,000 to 38,000 years ago, about the time Neanderthal artifacts and fossil skeletons disappear, said Frank Brown, dean of the U.'s College of Mines and Earth Sciences. "Out-of-Africa is probably still the best guess,'' said Brown, who has studied early human sites in Africa. "What seems to be the case is there were several waves of emigration from Africa."
The alternative "multiregional hypothesis" says independent groups of Homo sapiens evolved from prehuman hominids in Africa, Europe and Asia. If that theory is correct, modern Africans, Asians and Europeans should have equal genetic diversity because they all would have emerged about the same time and their DNA would have had the same amount of time to mutate and become more varied, Jorde said.
The new study found Africans have 20 percent more diversity in DNA in their cell nuclei than Europeans and Asians. That bolsters the idea European and Asian populations have existed for shorter periods of time because they evolved from a pre-existing population of Africans. The study doesn't prove the Out-of-Africa theory because greater DNA diversity in modern Africans also could occur if Africa had a larger population of early Homo sapiens than other regions, Jorde said. Mutations - and thus diversity - will appear more rapidly in a larger group than a smaller group. Nevertheless, any study that measures genetic diversity accurately is "a valuable step," said Hawkes.
Earlier studies didn't provide convincing evidence of greater African diversity because they analyzed DNA in mitochondria - parts of cells that convert food into energy and are located outside the cell nucleus. The Utah study examined harder-to-measure, faster-mutating DNA in cell nuclei. The DNA came from blood or other cells from 72 Africans, 63 Asians and 120 Europeans. Mitochondrial DNA provides only a partial picture of human evolution because it is inherited from women and not from men, Jorde said. DNA from the cell nucleus contains much more information and thus a more detailed view of genetic evolution. Horde said genetic variation in humans represents "an indelible record of our evolutionary past" and helps researchers "understand why diseases vary in frequency among populations."
Analysis of DNA and how much it varies among people also plays an increasing role in criminal trials, said Jorde, whose testimony on DNA evidence helped convict a former Midvale firefighter in the rapes of two 14-year-old girls. University of Utah co-authors of the study include anthropologist Alan Rogers, pediatric geneticist Michael Bamshad, technician W. Scott Watkins and graduate student Patrycia Krakowiak. Other co-authors are Utah State University graduate student Sandy Sung, University of Helsinki researcher Juha Kere and Pennsylvania State University scientist Henry Harpending, who will move to Utah later this year to work in Hawkes' department.