An international study of Y chromosomal DNA shows that East Asian populations migrated out of Africa and suggests that little or no interbreeding of Homo erectus and Homo sapiens occurred after the migration.
Homo Erectus And Homo Sapiens Did Little Interbreeding
"Our goal was to test the hypothesis that the common origin of human populations is in Africa," said Dr. Mark D. Shriver, assistant professor of anthropology, Penn State. "We also wanted to see if there was evidence of archaic admixture of Homo erectus and Homo sapiens."
The researchers tested 12,127 male individuals from 163 East Asian populations. The Y chromosome was used because it remains the same when passed from father to son. The team reported their findings in Friday's (May 11) issue of the journal Science.
"The Y chromosome is nice because it does not recombine," says Shriver. "A lot more evolutionary information is available than is found in mitochondrial DNA." Mitochondrial DNA is non nuclear genetic material passed from mothers to their children and has been used for some previous African origin research.
Researchers from China, Indonesia, England and the U.S. collected samples, genotyped the Y chromosomes and analyzed the results. They looked for specific mutations at three locations on the Y chromosome and found that every one of the 12,127 samples typed, carried one of these three polymorphisms.
"These three markers can be used to test the completeness of the replacement of modern humans of African origin in East Asia," the researchers say. "An observation of a male individual not carrying one of the three polymorphisms would be indicative of a potential ancient origin and possibly leading to the rejection of such completeness."
This result indicates that modern humans of African origin completely replaced earlier populations in East Africa, according to the researchers.
Shriver warns, however, that there are some ways in which this result could be accurate even though interbreeding took place.
If all of the Y chromosomes inherited from Homo erectus were eliminated from the population because those with Homo erectus ancestors were swept from the population due to a disease to which they were especially susceptible, they would not appear in the sample.
Also, if only Homo erectus women mated with Homo sapiens men, but no Homo sapiens women mated with Homo erectus men, then there would be no Y chromosomal evidence of the admixture.
While these possibilities must be considered, one of the strongest components of this study is its size. The 163 population samples came from populations in Central Asia, Central Siberia, Okhotsk/Amur, Kamchatka/Chukotka, Northern East Asia, Northern Han Chinese, Southern Han Chinese, Taiwanese Aborigines, Southeast Asia, Indonesia/Malaysia, Poly/Micronesia and Northeast India, covering a broad geographic area.
The large number of populations also eliminates the possibility that genetic drift is the cause of the researcher's results. Genetic drift is a tendency for small populations to gradually alter their genetic make up over time. The researchers find it hard to imagine that all the 163 populations should drift in the same direction.
The size of the sample, 12,000 individuals, also provides strong statistical evidence that the researcher's findings are correct. The possibility of the study missing one case of admixture in one thousand subjects is so small the number is truly minute. Even if one investigated the likelihood of one Homo erectus admixture in a million individuals, the number remains far below that which validates the result.
The research team included Yuehai Ke, Xiufeng Song, Daru Lu, Lifeng Chen, Hongyu Li, Chunjian Qi and Jiangzhong Jin, Fudan University and Morgan-Tan International Center for Life Science, Shanghai, China; Bing Su, Kunming Institute of Zoology, the Chinese Academy of Sciences, Kunming, Fudan University and University of Texas-Houston; Sangkot Marzuki, Eijkman Institue for Molecular Biology, Jakarta, Indonesia; Ranjan Deka, University of Cincinnati; Peter Underhill and Peter Oefner, Stanford University; Chunjie Xiao,Yunnan University, Kunming; Mark Shriver, Penn State; Jeff Lell and Douglas Wallace, Emory University School of Medicine; R Spencer Wells, University of Oxford, UK; Mark Seielstad, Harvard School of Public Health; Dingliang Zhu, Shanghai Second Medical University, Shanghai: Wei Huang and Zhu Chen, Shanghai Second Medical University and National Human Genome Center; Ranajit Chakraborty, University of Texas- Houston and Li Jin, Fudan Univeristy and Morgan-Tan International Center for Life Sciences, University of Texas-Houston and National Human Genome Center, Shanghai. - By A'ndrea Elyse Messer
[Contact: A'ndrea Elyse Messer]