2017届江苏省高考英语一轮复习：必修5 Unit 3课时提升作业(牛津译林版)_高考复习指南

2017届江苏省高考英语一轮复习：必修5 Unit 3课时提升作业(牛津译林版)

　　Researchers have agreed that today’s dog is the result of the domestication(驯化)of wolves thousands of years ago. Before this recent study, a common thought about the precise origin of North America’s domestic dog was that Natives domesticated local wolves, the descendants(后代)of which now live with people in Alaska, Canada, and the Lower 48.

　　Dog remains from a Fairbanks-area gold mine helped the scientists reach their conclusion. Leonard, an evolutionary biologist, collected DNA from 11 bones of ancient dogs that were locked in permafrost(永冻层)until Fairbanks miners uncovered them in the 1920s. The miners donated the preserved bones to the American Museum of Natural History in New York City, where they remained untouched for more than 70 years. After borrowing the bones from the museum, Leonard and her colleagues used radiocarbon techniques to find the age of the Alaska dogs. They found the dogs all lived between the years of 1450 and 1675 A. D. , before Vitus Bering and Aleksey Chirikov who were the first known Europeans to view Alaska in 1741. “The bones of dogs that wandered the Fairbanks area centuries ago should therefore be the remains of pure native American dogs, ”Leonard said. The DNA of the Fairbanks dogs would also expose whether they were the descendants of wolves from North America.

　　Along with the Fairbanks samples, the researchers collected DNA from bones of 37 dog specimens(标本)from Mexico, Peru, and Bolivia that existed before the arrival of Columbus. In the case of both the Alaska dogs and the dogs from Latin America, the researchers found that they shared the most genetic material with gray wolves of Europe and Asia. This supports the idea of domestic dogs entering the New World with the first human explorers who wandered east over the land bridge.