新視野大學(xué)英語讀寫教程第四冊(cè)u(píng)nit7-a Section A Research into Population Genetics

Research into Population Genetics

While not exactly a top selling book, The History and Geography of Human Genes is a remarkable collection of more than 50 years of research in population genetics. It stands as the most extensive survey to date on how humans vary at the level of their genes. The book's firm conclusion: once the genes for surface features such as skin color and height are discounted, the "races" are remarkably alike under the skin. The variation among individuals is much greater than the differences among groups. In fact, there is no scientific basis for theories pushing the genetic superiority of any one population over another.

The book, however, is much more than an argument against the latest racially biased theory. The prime mover behind the project, Luca Cavalli-Sforza, a Stanford professor, labored with his colleagues for 16 years to create nothing less than the first genetic map of the world. The book features more than 500 maps that show areas of genetic similarity — much as places of equal altitude are shown by the same color on other maps. By measuring how closely current populations are related, the authors trace the routes by which early humans migrated around the earth. Result: the closest thing we have to a global family tree.

The information needed to draw that tree is found in human blood: various proteins that serve as markers to reveal a person's genetic makeup. Using data collected by scientists over decades, the authors assembled profiles of hundreds of thousands of individuals from almost 2,000 groups. And to ensure the populations were "pure", the study was confined to groups that were in their present locations as of 1492, before the first major movements from Europe began — in effect, a genetic photo of the world when Columbus sailed for America.

Collecting blood, particularly from ancient populations in remote areas, was not always easy; potential donors were often afraid to cooperate, or raised religious concerns. On one occasion, when Cavalli-Sforza was taking blood samples from children in a rural region of Africa, he was confronted by an angry farmer waving an axe. Recalls the scientist: "I remember him saying, ’If you take the blood of the children, I'll take yours.’ He was worried that we might want to do some magic with the blood."

Despite the difficulties, the scientists made some remarkable discoveries. One of them jumps right off the book's cover: a color map of the world's genetic variation has Africa at one end of the range and Australia at the other. Because Australia's native people and black Africans share such superficial characteristics as skin color and body shape, they were widely assumed to be closely related. But their genes tell a different story. Of all humans, Australians are most distant from the Africans and most closely resemble their neighbors, the Southeast Asians. What the eye sees as racial differences — between Europeans and Africans, for example — are mainly a way to adapt to climate as humans move from one continent to another.

The same map, in combination with ancient human bones, confirms that Africa was the birthplace of humanity and thus the starting point of the original human movements. Those findings, plus the great genetic distance between present-day Africans and non-Africans, indicate that the split from the African branch is the oldest on the human family tree.

The genetic maps also shed new light on the origins of populations that have long puzzled scientists. Example: the Khoisan people of southern Africa. Many scientists consider the Khoisan a distinct race of very ancient origin. The unique character of the clicking sounds in their language has persuaded some researchers that the Khoisan people are directly descended from the most primitive human ancestors. But their genes beg to differ. They show that the Khoisan may be a very ancient mix of west Asians and black Africans. A genetic trail visible on the maps shows that the breeding ground for this mixed population probably lies in Ethiopia or the Middle East.

The most distinctive members of the European branch of the human tree are the Basques of France and Spain. They show unusual patterns for several genes, including the highest rate of a rare blood type. Their language is of unknown origin and cannot be placed within any standard classification. And the fact that they live in a region next to famous caves which contain vivid paintings from Europe's early humans, leads Cavalli-Sforza to the following conclusion: "The Basques are extremely likely to be the most direct relatives of the Cro-Magnon people, among the first modern humans in Europe." All Europeans are thought to be a mixed population, with 65% Asian and 35% African genes.

In addition to telling us about our origins, genetic information is also the latest raw material of the medical industry, which hopes to use human DNA to build specialized proteins that may have some value as disease-fighting drugs. Activists for native populations fear that the scientists could exploit these peoples: genetic material taken from blood samples could be used for commercial purposes without adequate payment made to the groups that provide the DNA.

Cavalli-Sforza stresses that his mission is not just scientific but social as well. The study's ultimate aim, he says, is to "weaken conventional notions of race" that cause racial prejudice. It is a goal that he hopes will be welcomed among native peoples who have long struggled for the same end.

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