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第4册 - Unit 7, Section A - Research into Population Genetics

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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.

《人类基因的历史与地理分布》尽管不是严格意义上的畅销书,却是一本汇集了50多年人类遗传学方面研究成果的好书。

它对人类在基因层面上的差异作了迄今为止最为广泛的调查,

得出了明确的结论:如果不考虑影响肤色、身高等表面特征的基因,不同的"种族"在外表之下相似地令人吃惊。

个体之间的差异远远大于群体之间的差异。实际上,那种认为某一种群比另一种群的基因更优越的理论是毫无科学根据的。

然而,此书还不仅仅是对目前的种族偏见理论的反驳。

这一项目的主要倡导者,斯坦福大学教授路卡·卡瓦里-斯福尔扎,与同事一起经过16年的努力,绘制了这一世界上首幅人类基因分布图谱。

此书的一大特点是提供了500多幅图,显示了相同的遗传基因所处的区域,这很像其他地图上用同样的颜色标示同样海拔高度的地区。

通过测定当前人类种群间的亲缘关系,作者们勾绘了地球上早期人类迁徙的路线。他们的研究结果相当于一份全球家谱。

他们在人类血液中找到了绘制这一家谱所需的信息:不同的蛋白质就是显示一个人的基因构成的标志。

作者们利用几十年来科学家们收集的数据,汇编成了近2,000个群体中成千上万个个体的数据图。

为了确保种群的"纯正",这项研究将对象限定于自1492年起,即欧洲最初的大规模迁徙之前就一直居住在现生活区域的那些群体。这实际上就是一幅哥伦布驶向美洲时期的世界人口基因分布图。

收集血样,特别是到偏远地区的古老人群中收集血样,并非总是易事。

潜在的供血者通常不敢合作,或存在宗教上的担心。

有一次在非洲农村,正当卡瓦里·斯福尔扎要从儿童身上采血时,一个愤怒的村民手执斧头出现在他面前。

这位科学家回忆道:"我记得他说,'如果你从孩子们身上抽血,我就要放你的血。'那个人是担心我们可能用这些血来施魔法。"

尽管有困难,科学家们还是取得了一些引人注目的发现。

其中之一就醒目地印在此书封面上:人类基因变异彩图表明,非洲与澳洲分别位于变化范围的两端。

因为澳洲土著和非洲黑人之间有一些共同的外表特征,如肤色、体型等,所以被普遍认为有密切的亲缘关系。

但是他们的基因却表明并非如此。

在所有人种中,澳洲人与非洲人的关系最远,而与其邻居东南亚人非常接近。

我们所看到的人种差异,例如欧洲人与非洲人的差异,主要是人类从一个大陆向另一个大陆迁徙时为适应气候所产生的。

结合对远古人骨的研究,这一图谱证实了非洲是人类的诞生地,因而也是人类迁徙的始发地。

这些发现,再加上现代非洲人与非非洲人之间的巨大基因差异,说明了从非洲种群开始的分支是人类家谱上最早的分支。

这一基因分布图谱对长期以来困绕着科学家的人种起源问题也做出了新的解释。

南部非洲的科伊桑人就是一个例子。

很多科学家认为科伊桑人是一个独立的非常古老的人种。

他们语言中那种独特的短促而清脆的声音使得一些研究者认为科伊桑人是最原始的人类祖先的直系后裔。

然而他们的基因却表明情况并非如此。

基因研究表明科伊桑人可能是古代西亚人与非洲黑人的混血。

图谱上显示的遗传轨迹表明这一混血人种的发源地可能在埃塞俄比亚或中东地区。

法国和西班牙的巴斯克人是人类家谱图上欧洲人分支的最特殊的成员。

他们有几组少见的基因类型,包括一种罕见血型的发生率在巴斯克人中也是最高的。

他们的语言起源不明,也无法被归入任何标准的类别。

他们居住的地区紧挨着发现早期欧洲人壁画的几个著名的洞穴这一事实使卡瓦里·斯福尔扎得出这样的结论:"在欧洲最早的近代人中,巴斯克人极有可能与克罗马努人关系最直接。"

人们认为所有的欧洲人都是混合人种,有65%的亚洲人基因,35%的非洲人基因。

除了揭示人种的起源以外,基因信息对医学界来说也是最新资料。医学界希望能用人类脱氧核糖核酸(DNA)制成特别的蛋白质,这些蛋白质可能具有某种抗病药物的价值。

保护土著人权益的活动家们担心科学家可能会利用土著人:从当地人血样中提取的基因物质可被用于商业目的,而DNA 提供者却不会获得足够的报酬。

卡瓦里·斯福尔扎强调,他的工作不仅有科学意义,而且也有社会意义。

他说研究的最终目的是"削弱"造成种族偏见的"传统的种族观念"。

他希望这一目的会得到一直为同样目的进行抗争的土著民族的接受。

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