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Palaeontology
古生物學
Very, very long in the tooth
牙齒很長很長
Million-year-old mammoth genomes push the limits of a revolutionary technique
百萬年前的猛犸象基因組突破了一項革命性技術的極限
In the 1966 science-fiction movie "One Million Years B.C.", Raquel Welch and John Richardson traverse a primitive landscape inhabited by dinosaurs and early humans. The film was low on science and high on fiction: by then dinosaurs were long dead and humans—at least, ones resembling Ms Welch and Mr Richardson— were hundreds of millennia away.
在1966年的科幻電影《公元前一百萬年》中，拉克爾·韋爾奇和約翰·理查森穿越了恐龍和早期人類居住的原始景觀。這部電影的科學含量低，但科幻含量高：那時恐龍早已滅絕，而人類——至少像韋爾奇和理查森這樣的人——數十萬年后才能出現。
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, also in Sweden, describe sequencing DNA samples from mammoths that lived and died in north-eastern Siberia around a million years ago.
一幅更精準的地球居民圖正徐徐展開。在《自然》雜志上發表的一項研究中，斯德哥爾摩大學的安德斯·哥斯特倫和同樣在瑞典的古遺傳學中心的洛夫·戴倫領導的一個科學家團隊描述了大約100萬年前在西伯利亞東北部生活和滅絕的猛犸象的測序DNA樣本。
The team's work represents a new record, for their mammoth DNA is, by some half a million years, the oldest ever successfully reconstituted. Extracted 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 gross physical features of extinct animals, they are silent about many crucial details that even an incomplete genome can help to fill in.
這個團隊的這一工作創造了新的紀錄，因為他們的猛犸象DNA是迄今為止成功重組的最古老的DNA，距今大約50萬年。這種古老的DNA從馬、熊，甚至是現代人的近親尼安德特人和丹尼索瓦人身上所提取，并被證明是研究過去的寶貴工具?；M管保存了滅絕動物的大體物理特征，但它們無法查證許多關鍵的細節，而就算是一個不完整的基因組也能幫助填補這些細節。
The trouble with DNA is that it breaks down post mortem. The more brokendown 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, the equivalent of trying to reconstruct a book from a heap of its constituent letters. Under the right conditions, however, such as the extreme cold of Arctic permafrost, this decay can be slowed.
DNA的問題在于死后會分解。越分解就越難排序?？茖W家們認為大約600萬年之后只會剩下單個的堿基對，這就相當于從一堆組成字母中重建一本書。然而，在適當的條件下，比如極寒的北極永久凍土層，分解的速度可以減緩。
Dr Dalen and his colleagues were interested in three mammoth molars 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 unduly contaminated by bacteria or the shaking hands of awe-struck palaeontologists, strands of DNA were extracted, sequenced, and dated. Whereas DNA samples from a living animal can run to several hundreds of thousands of letters, the timeworn mammoth samples yielded strands mere dozens of letters long. This is close to the limit of what is scientifically usable, says Ludovic Orlando, a biologist at the Centre for Anthropobiology and Genomics of Toulouse.
戴倫博士和他的同事們對三顆猛犸象的臼齒感興趣，這三顆臼齒是20世紀70年代從年代久遠的西伯利亞地質層中提取出來的。2017年，所有的樣本都被送到了戴倫博士的實驗室。經過檢查，它們沒有被細菌和充滿敬畏的古生物學家們的握手過度污染，DNA鏈被提取出來，測序，并確定了年代?；铙w動物的DNA樣本可以有幾十萬個字母，而年代久遠的猛犸象樣本中的DNA樣本只有幾十個字母長。圖盧茲人類生物學和基因組學中心的生物學家盧多維克·奧蘭多說，這接近科學可用的極限。
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