Little-Explored African Genetic Diversity May Hold Key To Human Origins, Medical Questions

"Africa has been greatly underrepresented in studies of genetic diversity compared with European and Asian populations," says Dr. Sarah Tishkoff, postdoctoral research fellow in biology. "This is not only important in determining where, when and how modern humans evolved, but also in understanding genetic diseases of Africans and African Americans and in identifying potential treatments for diseases like malaria and HIV."

Tishkoff; Andrew Clark, Penn State; Kenneth Kidd, Yale University; Giovanni Destro-Bisol, University "La Sapienza," Rome, and Himla Soodyall and Trefor Jenkins, WITS University, South Africa, looked at three locations on DNA samples from 13 to 18 populations in Africa and 30 to 45 populations in the remainder of the world.

"We found an enormous amount of diversity within and between the African populations, and we found much less diversity in non-African populations," Tishkoff told attendees today (Jan. 22) at the annual meeting of the American Association for the Advancement of Science in Anaheim. "Only a small subset of the diversity in Africa is found in Europe and the Middle East, and an even narrower set is found in American Indians."

The researchers were looking at genetic information that is inherited from both the mother and the father, and exists on a strand of DNA close enough together so that the markers are transferred intact.

"Using these markers to trace lineages, we find that modern humans appear to have emerged from Africa between 100,000 and 150,000 years ago and the population that left Africa was rather small," says Tishkoff.

The researchers' data agrees with earlier studies using mitochondrial DNA -- inherited only from the mother -- and the Y chromosome -- inherited only from the father -- that indicated an African origin for modern humans. Some paleoanthropologists disagree and suggest an alternative model of Homo erectus migrating out of Africa and then evolving, in parallel, into Homo sapiens -- modern humans -- throughout Africa, Europe and the Middle East. The team's research does not support this model.

Tishkoff also suggests that the group that migrated from Africa came from northern East Africa.

"The diversity of groups in Ethiopia and Somalia is intermediate between that of the rest of Africa and the rest of the world," says Tishkoff. "Perhaps this group was isolated from the rest of the African continent before they migrated into the Middle East and Europe."

From a medical viewpoint, African genetic diversity is important in understanding genetic diseases of African Americans and for finding treatment methods for contagious diseases that originated in Africa.

"When HIV, malaria and Ebola, for example, evolved, humans also evolved mechanisms for protecting themselves against these diseases," says Tishkoff. "If we could identify the genetic changes that provided this protection, then we might find treatment methods for the diseases."

The Penn State researcher is currently looking at genes that protect against malaria. Knowledge of African diversity can also help in understanding and treating genetic diseases that affect African Americans.

"Most African Americans in the U.S. came from West Africa and are undoubtedly as diverse, genetically, as those individuals who remained in Africa," says Tishkoff. "The causes of genetically based diseases in African Americans may be different from the causes of the same diseases in Americans whose roots are in Europe."

The incidence of hypertension in African Americans is quite high and probably not caused by the same mechanism as in Europeans. Understanding the underlying genetic cause, could lead to more effective treatments.

This research was partially supported by a National Science Foundation Sloan Postdoctoral Fellowship and the Burroughs Wellcome Fund.