Genetic Mutations Offer Insights on Human Diversity

By David Brown
Washington Post Staff Writer
Friday, February 22, 2008

We're all pretty much the same except, of course, for the little things that make us different.

Those are the conclusions of three studies published this week that looked at human diversity through the keyhole of the genetic mutations we all carry.

The findings -- the latest dividend from the world's investment in the Human Genome Project in the 1990s -- confirm a broad narrative of human history known from previous biological, archaeological and linguistic studies. But the new research adds an astonishing level of detail and a few new insights that were not previously available.

All three studies support the idea that modern human beings left East Africa, walked into Central Asia and then fanned out east and west to people the entire planet. The studies also confirm earlier research showing that as a group, Africans have more diverse genes than people of other continents. But the new research further shows that genetic diversity declines steadily the farther one's ancestors traveled from Addis Ababa, Ethiopia, which is roughly the site of the exit turnstile for the "out-of-Africa" migration.

The studies also show that many seemingly "purebred" ethnic groups have ancestry traceable to more than one continent.

For example, the Arabian Peninsula's Bedouin -- a culturally distinct group -- are descended not only from longtime Middle Eastern peoples, but also from Europeans and peoples originating from around modern Pakistan. The Yakut people of eastern Siberia share blood with East Asians, Europeans and American Indians, but very little with Central Asians, who are geographically closer to them than two of those populations.

The research may also shed light on the genetic underpinnings of human disease. One study found that Americans of European descent carry a larger number of damaging gene variants than African Americans do -- a byproduct of Caucasians' arduous march westward to the shores of the Atlantic.

The biggest message, though, is that these differences are the details, not the main message, of human diversity.

About 90 percent of the full catalogue of human genetic diversity exists in every human population. Individuals are likely to have almost as many differences with people we consider to be "like us" as with strangers on the other side of the world.

"What this says is that we are all extremely related to each other," said Richard M. Myers, a geneticist at Stanford University School of Medicine, who helped lead one of the studies being published today in the journal Science.

"Most genetic variation is shared worldwide. It is only a small part of human genetic variation that is private to particular continents," said Noah A. Rosenberg, of the University of Michigan. His group's findings were published yesterday in the journal Nature.

All three studies examined single "letter" changes in the 3-billion-letter transcript that makes up each person's genome. Every individual carries tens of thousands of these variations. Some do not change the "words" that are the genes; some change a word but not its meaning; and some change the meaning in a way that can be beneficial or harmful.

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