Evolutionary genetics. Africans' deep genetic roots reveal their evolutionary story.

Africa is the birthplace of modern humans, and so our species has lived longest—200,000 years—on that continent. As a result, Africans have had more time to accumulate changes in their DNA than humans elsewhere. But until now, researchers have barely scratched the surface of the rich diversity in Africans' nuclear DNA. Most genetic studies of Africans and African-Americans are based on data from just a few gene lineages, or on genomewide scans of a handful of the diverse African groups. "Africans have been the most completely neglected and underrepresented genetically of any continental group, because the most diverse groups are often remote," says evolutionary geneticist Sarah Tishkoff of the University of Pennsylvania. "Hunter-gatherers don't usually get to clinics." Now, in the largest study es;er of African genetic diversity, an international team of researchers led by Tishkoff has analyzed nuclear DNA collected over a decade from 113 populations of Africans from across the continent. In a report published online in Science (www.sciencemag.org/cgi/content/abstract/ 1172251) this week, the team has found that Africans are descended from 14 ancestral populations, which often correlate with language and cultural groups. They found that all hunter-gatherers and pygmies in Africa today shared ancestors 35,000 years ago and that East Africa was the source of the great migration that populated the rest of the world. They also found that African-American individuals, on average, have mixed ancestry from all over western Africa, which will make it difficult to trace roots to specific ethnic groups. "Wow! This data gives us raw material for understanding human evolution that we have never had before," says geneticist Jeffrey Long of the University of Michigan (UM) Medical School in Ann Arbor. Adds geneticist Noah Rosenberg, also of UM, "It's a treasure trove of information," Tishkoff and a team of European and African collaborators, in particular postdoc Floyd Reed, now at the Max Planck Institute for Evolutionary Biology in Plon, Germany, began collecting blood from far-flung tribes in Africa more than a decade ago. They drove off-road to visit peoples as diverse as the pygmies of Cameroon and hunter-gatherers in Tanzania. They set up centrifuges wherever they could find generators, survived a car crash, and spent years negotiating permits to collect blood ethically. "It was no small feat to get these samples," says Tishkoff. They ended up with blood from 3194 Africans from 113 populations. Working with additional collaborators, they genotyped the samples for a panel of 1327 well-known markers used to map genetic diseases in diverse populations. They then used various statistical methods to sort the DNA into closely related clusters and to trace patterns of inheritance. They also compared markers in Africans with those from 98 AfricanAmericans, 21 Yemenites, and 952 individuals from around the world. In many cases, the team found that ethnic, cultural, and linguistic differences reflected real genetic differences, which is "reassuring," says paleoanthropologist Stanley Ambrose of the University of Illinois, Urbana-Champaign. For example, the hunter-gatherers spread throughout Africa, including the Sandawe and Hadza of Tanzania and the Khoisan speakers of southern Africa, shared common ancestors. All three of these groups speak click languages, which incorporate strong "click" conNEWS OF THE WEEK