关注微信领¥50

百门课程¥0元


50万+已关注

2018年雅思阅读模拟题及答案解析(20)

2018-07-11 10:52:00来源:网络

  以下是新东方在线雅思网给大家分享的2018年雅思阅读模拟题及答案解析(20)。希望对大家的雅思备考有所帮助,更多雅思备考资料欢迎大家随时关注新东方在线雅思网。

热门课程】【2018年雅思阅读模拟题及答案解析汇总】【备考资料免费下载

  Don’t wash those fossils!

  Standard museum practice can wash away DNA.

  1. Washing, brushing and varnishing fossils — all standard conservation treatments used by many fossil hunters and museum curators alike — vastly reduces the chances of recovering ancient DNA.

  2. Instead, excavators should be handling at least some of their bounty with gloves, and freezing samples as they are found, dirt and all, concludes a paper in the Proceedings of the National Academy of Sciences today.

  3. Although many palaeontologists know anecdotally that this is the best way to up the odds of extracting good DNA, Eva-Maria Geigl of the Jacques Monod Institute in Paris, France, and her colleagues have now shown just how important conservation practices can be. This information, they say, needs to be hammered home among the people who are actually out in the field digging up bones.

  4. Geigl and her colleagues looked at 3,200-year-old fossil bones belonging to a single individual of an extinct cattle species, called an aurochs. The fossils were dug up at a site in France at two different times — either in 1947, and stored in a museum collection, or in 2004, and conserved in sterile conditions at -20 oC.

  5. The team’s attempts to extract DNA from the 1947 bones all failed. The newly excavated fossils, however, all yielded DNA.

  6. Because the bones had been buried for the same amount of time, and in the same conditions, the conservation method had to be to blame says Geigl. “As much DNA was degraded in these 57 years as in the 3,200 years before,” she says.

  Wash in, wash out

  7. Because many palaeontologists base their work on the shape of fossils alone, their methods of conservation are not designed to preserve DNA, Geigl explains.

  8. The biggest problem is how they are cleaned. Fossils are often washed together on-site in a large bath, which can allow water — and contaminants in the form of contemporary DNA — to permeate into the porous bones. “Not only is the authentic DNA getting washed out, but contamination is getting washed in,” says Geigl.

  9. Most ancient DNA specialists know this already, says Hendrik Poinar, an evolutionary geneticist at McMaster University in Ontario, Canada. But that doesn’t mean that best practice has become widespread among those who actually find the fossils.

  10. Getting hold of fossils that have been preserved with their DNA in mind relies on close relationships between lab-based geneticists and the excavators, says palaeogeneticist Svante P bo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. And that only occurs in exceptional cases, he says.

  11. P bo’s team, which has been sequencing Neanderthal DNA, continually faces these problems. “When you want to study ancient human and Neanderthal remains, there’s a big issue of contamination with contemporary human DNA,” he says.

  12. This doesn’t mean that all museum specimens are fatally flawed, notes P bo. The Neanderthal fossils that were recently sequenced in his own lab, for example, had been part of a museum collection treated in the traditional way. But P bo is keen to see samples of fossils from every major find preserved in line with Geigl’s recommendations — just in case.

  Warm and wet

  13. Geigl herself believes that, with cooperation between bench and field researchers, preserving fossils properly could open up avenues of discovery that have long been assumed closed.

  14. Much human cultural development took place in temperate regions. DNA does not survive well in warm environments in the first place, and can vanish when fossils are washed and treated. For this reason, Geigl says, most ancient DNA studies have been done on permafrost samples, such as the woolly mammoth, or on remains sheltered from the elements in cold caves — including cave bear and Neanderthal fossils.

  15. Better conservation methods, and a focus on fresh fossils, could boost DNA extraction from more delicate specimens, says Geigl. And that could shed more light on the story of human evolution.

  (640 words nature )

  Glossary

  Palaeontologists 古生物学家

  Aurochs 欧洲野牛

  Neanderthal (人类学)尼安德特人,旧石器时代的古人类。

  Permafrost (地理)永冻层

本文关键字: 雅思阅读模拟题

为你特别匹配的雅思超值课程,快速提分先人一步!
  • 新东方雅思11月公益课(第二期)

    适合人群:不知道如何高效备考,想要在短期考到7及7以上的考生。

    课时:12

    查看详情
  • 雅思机经高频词汇套装

    适合人群:为背单词枯燥而抓狂,又急需提高词汇量。

    课时:20

    查看详情
    -->
  • 【知心雅思】全科提高冲 7分班

    适合人群:想要冲7分的考生

    课时:229

    查看详情
  • 【知心雅思】旗舰VIP直达 6.5 分班

    适合人群:想要冲刺6.5分的考生

    课时:344

    查看详情
  • 雅思核心1200词背诵

    适合人群:有背单词困难症的考生

    课时:44

    查看详情
更多>>
更多内容
更多>>
距结束 : 00 00 00 00

IELTS 5天搞定500个雅思听力核心词汇

专业外教团队录音班级群互动打卡班主任全程指导

雅思听力核心考点词,助你不再因听不懂写不对失分!

活动日期:2018.12.03-2018.12.07

价格 : ¥免费

限制人数:500

我要报名
更多>>
更多公开课>>
更多>>
更多课程>>