DNA studies trace human migration from Africa
Friday, February 22, 2008
Separate teams of researchers from Stanford and the University of Michigan have found convincing evidence locked inside the human genome that much of the world today is populated by descendants of a small band of migrants who left Africa for the Middle East some 100,000 years ago.
The tale told by DNA - the chain of chemicals that carries information in our genes - was uncovered by new machines that can rapidly scan for subtle differences in the genetic makeup of people living in far-flung parts of the globe.
Earlier genetic studies and paleontologic evidence supported the "Out of Africa" theory of global colonization. The latest genome studies are, by far, the largest and most conclusive on that topic to date.
"There is lots and lots of confirmation that the Middle East was the gateway for migration out of Africa," said Richard Myers, chairman of the department of genetics at the Stanford University School of Medicine and senior author of a study appearing in today's issue of the journal Science.
The Stanford study built on the work of senior colleagues Luca Cavalli-Sforza and Marcus Feldman, co-authors of the latest paper. For decades they have carefully collected blood samples from diverse groups of people around the world. Using the new rapid gene scans, the Stanford scientists screened 938 stored blood samples drawn from 51 different ethnic groups.
"There are thousands of stories in these data," Myers said.
It used to take a genetics researcher an entire career to explore differences of just a few genes within different ethnic groups. The Human Genome Project, however, employed new computerized tools that allowed researchers to record the sequence of all 32,000 genes that provide the instructions for making a human being.
Similar computerized technologies are now giving scientists the opportunity to compare and contrast within a few months the genes of diverse people. The similarities and differences they find may help explain susceptibilities to various diseases. In this case, they are revealing secrets of our common ancestry.
The power of these technologies is astounding. "We've had a millionfold increase in efficiency and a millionfold decrease in cost," Myers said.
Information from the study helped Myers and colleagues build a human family tree, rooted in Africa, that branches out through the Middle East, Europe, South Asia, East Asia, Australia and ultimately to the Americas.
Researchers describe their new gene-scanning tools as something akin to a telescope, which brings to observers a higher-resolution image of what they are exploring. In this case, they are exploring the history of human migration, etched into our own DNA.
Those higher-resolution images have already yielded interesting insights. For example, previous studies of the Han people of China have not been able to distinguish any genetic differences between the Han of northern China and those of the south. Using the tools of modern genomics, however, Myers' team at Stanford has spotted gene variants that allow a laboratory scientist to identify whether a blood sample from a Han Chinese came from the north or south.
Myers stressed that the differences spotted by these genetic tests are merely markers that seldom correlate to any observable differences between ethnic groups. The markers are like prominent rocks on a landscape, which help researchers navigate around a genome, and seldom represent mutations that might affect health or physical traits.
The longer a population has stayed in one geographic area, the more time it has to accumulate these genetic signposts. Each individual may have only a portion of those markers common to his or her ethnic group, but in the group as a whole, distinctive patterns emerge.
The Stanford and University of Michigan studies found that African groups, such as the Yoruba, Bantu and Mbuti Pygmies, had the most variants of the 51 ethnic groups tested around the world.
Those living farthest from Africa, such as Siberians and American Indians, had the fewest number of these variants.
That supports the notion that it was a relatively small group of Africans who migrated to the Middle East and went on to populate much of the rest of the planet.
Had the original break from Africa been a mass migration, those early colonizers would have carried the full library of gene variants among Africans with them. Instead, a relatively small band of migrants must have made the journey, bringing a smaller number of those genetic markers to pass on to their descendants.
In fact, Middle Easterners have fewer markers than Africans, Europeans and South Asians have fewer markers than Middle Easterners, and East Asians have fewer markers than South Asians.
"Diversity has been eroded through the migration process," explained Noah Rosenberg, a University of Michigan researcher who published in the journal Nature on Thursday findings similar to those of the Stanford team.
Fewer variants in the non-African populations may also be evidence of the hardships faced by these migrating human groups. Disease, disaster and isolation are played out in the genome - researchers call these events "bottlenecks," where the number of people who are able to survive and pass on their genes is limited. A smaller group of surviving ancestors has fewer genetic markers to pass on to their descendants.
"Think how harsh it must have been for the people who moved to Europe," Myers said. "Every 10,000 years, there was an Ice Age. There must have been huge die-offs."
E-mail Sabin Russell at email@example.com.