孙吉芯：List of Headings题型全面解析_雅思英语阅读

孙吉芯：List of Headings题型全面解析

　　C An important project, led by the biological anthropologist Robert Williams, focused on the variants of one particular protein - immunoglobin G- found in the fluid portion of human blood. All proteins drift, or produce variants, over the generations, and members of an interbreeding human population will share a set of such variants. Thus, by comparing the Gmallotypes of two different populations , one can establish their genetic distance, which itself can be calibrated to give an indication of the length of time since these populations last interbred.

　　D Williams and his colleagues sampled the blood of over 5,000 American Indians in western North America during a twenty- year period. They found that their Gmallotypes could be divided into two groups, one of which also corresponded to the genetic typing of Central and South American Indians. Other tests showed that the Inuit and Aleut formed a third group. From this evidence it was deduced that there had been three major waves of migration across the Bering Strait. The first, Paleo-lndian, wave more than 15,000 years ago was ancestral to all Central and South American Indians. The second wave, about 14,000-12,000 years ago, brought Na-Dene hunters, ancestors of the Navajo and Apache . The third wave, perhaps 10,000 or 9,000 years ago, saw the migration from North-east Asia of groups ancestral to the modern Eskimo and Aleut.

　　E How far does other research support these conclusions Geneticist Douglas Wallace has studied mitochondrial DNA in blood samples from three widely separated Native American groups: Pima- Papago Indians in Arizona, Maya Indians on the Yucatan peninsula, Mexico, and Ticuna Indians in the Upper Amazon region of Brazil. As would have been predicted by Robert Williamss work, all three groups appear to be descended from the same ancestral population.