DNA studies trace migration from Ethiopia
Research dates origins up to 100,000 years
 

Scrutinizing the DNA of 938 people from 51 distinct populations around the world, geneticists have created a detailed map of how humans spread from their home base in sub-Saharan Africa to populate the farthest reaches of the globe over the last 100,000 years.

The pattern of genetic mutations, to be published Friday in the journal Science, offers striking evidence that an ancient band of explorers left what is now Ethiopia and -- along with their descendants -- went on to colonize North Africa, the Middle East, Europe, southern and central Asia, Australia and its surrounding islands, the Americas and East Asia. A second analysis based on some of the same DNA samples corroborated the results. Those findings, published Thursday in the journal Nature, demonstrated that the greater the geographic distance between a population and its African ancestors, the more changes had accumulated in its genes.

The story of human migration revealed by DNA "compliments what's known through history, linguistics or anthropology," said Jun Li, the University of Michigan human geneticist who led the Science study.

Both research groups relied on DNA from blood samples collected by anthropologists around the world as part of the Human Genome Diversity Project, a controversial effort from the mid-1990s to gather genetic specimens from thousands of populations, including many indigenous tribes.

Previous studies have relied on data from the International HapMap Consortium, which cataloged DNA from 269 people of Nigerian, Japanese, Chinese and European descent.

"Instead of saying a particular person's genome is from Africa, this kind of data allows us to say which part of Africa they were from," said Andrew Singleton, chief of the molecular genetics section at the National Institute on Aging in Bethesda, Md., and senior author of the Nature report. The studies were funded by the NIH, the National Science Foundation and private foundations.

In both studies, the researchers analyzed more than a half-million single-letter changes among the approximately 3 billion As, Cs, Ts and Gs that make up the human genome. Those changes -- called "single nucleotide polymorphisms," or SNPs -- begin as random mutations and accumulate over time as they are passed from one generation to the next.

Each time a small group left its home territory to found a new population, the migration ultimately led to a unique pattern of SNPs. Comparing those patterns, the researchers were able to show that humans spread around the globe through a series of migrations that originated from a single location near Addis Ababa, Ethiopia.

With the expanded DNA data set, Li's group was able to make finer distinctions among groups that were previously treated as homogeneous populations. In Europe, for example, the researchers were able to distinguish between Orcadians from present-day Scotland, the French, Tuscans, and Northern Italians from what is now Bergamo.

In the Far East, population geneticists previously had surmised that northern and southern Han Chinese were distinct populations, and that the Japanese islands were populated by northern Han.

"Now we have direct evidence that that's true," Li said.