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WASHINGTON – Africans have more genetic variation than anyone else on Earth, according to a new study that helps narrow the location where humans first evolved, probably near the South Africa-Namibia border.

The largest study of African genetics ever undertaken also found that nearly three-fourths of African-Americans can trace their ancestry to West Africa. The new analysis published Thursday in the online edition of the journal Science.

"Given the fact that modern humans arose in Africa, they have had time to accumulate dramatic changes" in their genes, explained lead researcher Sarah Tishkoff, a geneticist at the University of Pennsylvania.

People have been adapting to very diverse environmental niches in Africa, she explained in a briefing.

Over 10 years, Tishkoff and an international team of researchers trekked across Africa collecting samples to compare the genes of various peoples. Often working in primitive conditions, the researchers sometimes had to resort to using a car battery to power their equipment, Tishkoff explained.

The reason for their work? Very little was known about the genetic variation in Africans, knowledge that is vital to understanding why diseases have a greater impact in some groups than others and in designing ways to counter those illnesses.

Scott M. Williams of Vanderbilt University noted that constructing patterns of disease variations can help determine which genes predispose a group to a particular illness.

This study "provides a critical piece in the puzzle," he said. For example, there are clear differences in prevalence of diseases such as hypertension and prostate cancer across populations, Williams said.

"The human genome describes the complexity of our species," added Muntaser Ibrahim of the department of molecular biology at the University of Khartoum, Sudan. "Now we have spectacular insight into the history of the African population ... the oldest history of mankind.

"Everybody's history is part of African history because everybody came out of Africa," Ibrahim said.

Christopher Ehret of the department of history at the University of California, Los Angeles, compared genetic variation among people to variations in language.

There are an estimated 2,000 distinct language groups in Africa broken into a few broad categories, often but not always following gene flow.

Movement of a language usually involves arrival of new people, Ehret noted, bringing along their genes. But sometimes language is brought by a small "but advantaged" group which can impose their language without significant gene flow.

Overall, the researchers were able to study and compare the genetics of 121 African groups, 60 non-African populations and four African-American groups.

The so-called "Cape-colored" population of South Africa has highest levels of mixed ancestry on the globe, a blend of African, European, East Asian and South Indian, Tishkoff said.

"This will be a great population for study of diseases" that are more common in one group than another, she said.

The study also found that about 71 percent of African-Americans can trace their ancestry to western African origins. They also have between 13 percent and 15 percent European ancestry and a smaller amount of other African origins. There was "very little" evidence for American Indian genes among African-Americans, Tishkoff said.

Ehret added that only about 20 percent of the Africans brought to North America made the trip directly, while most of the rest went first to the West Indies.  

And, he added, some local African-American populations, such as the residents of the sea islands off Georgia and South Carolina, can trace their origins to specific regions such as Sierra Leone and Guinea.  

The study was funded by the National Cancer Institute, the National Institutes of Health, the Advanced Computing Center for Research and Education at Vanderbilt University, the L.S.B. Leakey and Wenner Gren Foundation, the National Science Foundation, the David and Lucile Packard and Burroughs Wellcome foundations.  

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