A large international group of scientists that has been studying the genes of nearly 200 African, African-American and non-Africa populations for ten years, examining more than 4 million genotypes, have published their results which they hope will create a rich library of genetic information for future research in medical, scientific and other fields such as anthropology and history.
The study was led by Sarah Tishkoff of the University of Pennsylvania, in Philadelphia, USA and appeared on 30 April as an early online issue in the journal Science.
Although geneticists know that Africa is where humans came from originally, there is no single African population that represents the genetic diversity of Africans alive today.
This study traced the genetic origins of Africans and African-Americans and found a startling diversity and shared ancestry among geographically dispersed groups.
Tishkoff said the study highlighted the need to include many ethnically diverse African populations in research where it is important to take genetic diversity into account, such as when looking as susceptibility to disease and response to drugs.
For the study, Tishkoff and colleagues in the US, Europe and Africa, studied 121 African, 4 African American, and 60 non-African populations for patterns of variation in over 1,300 DNA sites.
They traced the genetic structure of Africans to14 ancestral population clusters in Africa and found remarkable correspondence between cultural, linguistic and genetic diversity in Africa.
They also found high levels of mixed ancestry in most populations, reflecting the history of human migration across the continent.
The data showed that geographically diverse hunter-gatherers had ancestors in common, for instance Khoesan-speakers from southern and eastern Africa and Pygmies from central Africa.
They researchers said there was more genetic diversity in Africa than anywhere else in the world.
The data shows that African-Americans are descended primarily from Niger-Kordofanian ancestors (about 71 per cent), with some European (about 13 per cent), and other African (about 8 per cent) groups, although the mix varies considerably individual by individual.
These findings could have important implications for research that looks at genetic and environmental risk factors for diseases common among African-Americans, including prostate cancer, hypertension and diabetes.
The researchers found that the ancestral origin of humans was probably somewhere in southern Africa, near the South Africa-Namibia border, and by extrapolating the data, they determined that the migration out of Africa left from a point near the middle of the Red Sea in East Africa.
Tishkoff and colleagues concluded that:
“This study helps tease apart the complex evolutionary history of Africans and African Americans, aiding both anthropological and genetic epidemiologic studies.”
An audio visual presentation of the fieldword is available from the University of Pennsylvania.
Tishkoff, who is has joint appointments in the School of Arts and Sciences and the School of Medicine at Penn, said:
“This is the largest study to date of African genetic diversity in the nuclear genome.”
“This long term collaboration, involving an international team of researchers and years of research expeditions to collect samples from populations living in remote regions of Africa, has resulted in novel insights about levels and patterns of genetic diversity in Africa, a region that has been underrepresented in human genetic studies,” she added.
She said their goal was to:
“Do research that will benefit Africans, both by learning more about their population history and by setting the stage for future genetic studies, including studies of genetic and environmental risk factors for disease and drug response.”
As well as being a rich resource for medical and health research, as a result of this work, anthropologists, historians and linguists can also use the data to test theories of human migration, cultural evolution and population history in Africa.
The researchers wished to thank all the people who donated DNA samples for the study.
“The Genetic Structure and History of Africans and African Americans.”
Sarah A. Tishkoff, Floyd A. Reed, Françoise R. Friedlaender, Christopher Ehret, Alessia Ranciaro, Alain Froment, Jibril B. Hirbo, Agnes A. Awomoyi, Jean-Marie Bodo, Ogobara Doumbo, Muntaser Ibrahim, Abdalla T. Juma, Maritha J. Kotze, Godfrey Lema, Jason H. Moore, Holly Mortensen, Thomas B. Nyambo, Sabah A. Omar, Kweli Powell, Gideon S. Pretorius, Michael W. Smith, Mahamadou A. Thera, Charles Wambebe, James L. Weber, and Scott M. Williams.
Science, Published online 30 April 2009.
Other sources: University of Pennsylvania.
Written by: Catharine Paddock, PhD