2 . In the 1966 science-fiction movie One Million Years B. C., the movie characters had a time travel and arrived in an ancient landscape inhabited by dinosaurs and early humans. The movie was low on science and high on fiction: by then dinosaurs were long dead and modern humans were millions of years away.
A more accurate picture of Earth’s inhabitants at the time is now being revealed. In research published in Nature, a team of scientists led by Anders Gotherstrom at the University of Stockholm, and Love Dalen at the Centre for Palaeogenetics (古遗传学), also in Sweden, describe sequencing (测序) DNA samples from mammoths (猛犸象) that lived and died in north-eastern Siberia around a million years ago.
The team’s work represents a new record, for their mammoth DNA is, by some half a million years, the oldest ever successfully reconstructed. Extracted (提取) from horses, bears and even Neanderthals and Denisovans, two close cousins of modern humans, such ancient DNA has proved an invaluable tool for investigating the past. Although fossils preserve the basic physical features of extinct animals, they are silent about many crucial details that even an incomplete genome (基因组) can help to fill in.
The trouble with DNA is that it breaks down after death. The more broken down it is, the harder it is to sequence. Scientists think that, after about 6m years, all that would be left would be individual base pairs (碱基对), the equivalent of trying to reconstruct a book from several letters. Under the right conditions, however, such as the extreme cold of Arctic permafrost (冻土层) this decay can be slowed.
Dr. Dalen and his colleagues were interested in three mammoth molars (臼齿) extracted in the 1970s from Siberian geological layers that suggested great age. Samples from each were sent to Dr. Dalen’s laboratory in 2017. Having checked they had not been contaminated by bacteria or the shaking hands of Paleontologists, the DNA were extracted, sequenced, and dated. Whereas DNA samples from a living animal can run to several hundreds of thousands of letters, the ancient mammoth samples yielded merely dozens of letter long. This is close to the limit of what is scientifically usable, says a biologist named Ludovic Orlando.
1. What does the underlined word “contaminated” probably mean?
| A．Protected. | B．Polluted. | C．Estimated. | D．Discovered. |
2. According to the passage, the challenges the research team face may include ________.
①the limited number of DNA in mammoth samples
②the break-down of mammoth’s DNA after death
③the wide spread of mammoth samples
④the damage done to the mammoth samples from external environment
⑤the difficulty in extraction of the mammoth’s DNA
3. Which of the following statements is true according to the passage?
| A．The fact that DNA can break down makes it easier to sequence. |
| B．The incomplete genome can’t give any details of the extinct animals. |
| C．Mammoths’ DNA samples are invaluable for their extremely long history. |
| D．The research team created a new record for reconstructing an ancient book. |
4. What is the passage mainly about?
| A．The movie One Million Years B. C revealed the early human civilization. |
| B．Scientists have uncovered the secrets of life by studying mammoths’ DNA. |
| C．The mammoths’ DNA may give a clearer picture of ancient inhabitants on earth. |
| D．Discoveries of mammoths’ DNA samples help the development of DNA reconstruction technology. |
