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Palaeontology
古生物学
Very, very long in the tooth
牙齿很长很长
Million-year-old mammoth genomes push the limits of a revolutionary technique
百万年前的猛犸象基因组突破了一项革命性技术的极限
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年的科幻电影《公元前一百万年》中，拉克尔·韦尔奇和约翰·理查森穿越了恐龙和早期人类居住的原始景观。这部电影的科学含量低，但科幻含量高：那时恐龙早已灭绝，而人类——至少像韦尔奇和理查森这样的人——数十万年后才能出现。
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.
一幅更精准的地球居民图正徐徐展开。在《自然》杂志上发表的一项研究中，斯德哥尔摩大学的安德斯·哥斯特伦和同样在瑞典的古遗传学中心的洛夫·戴伦领导的一个科学家团队描述了大约100万年前在西伯利亚东北部生活和灭绝的猛犸象的测序DNA样本。
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.
这个团队的这一工作创造了新的纪录，因为他们的猛犸象DNA是迄今为止成功重组的最古老的DNA，距今大约50万年。这种古老的DNA从马、熊，甚至是现代人的近亲尼安德特人和丹尼索瓦人身上所提取，并被证明是研究过去的宝贵工具。化石尽管保存了灭绝动物的大体物理特征，但它们无法查证许多关键的细节，而就算是一个不完整的基因组也能帮助填补这些细节。
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的问题在于死后会分解。越分解就越难排序。科学家们认为大约600万年之后只会剩下单个的碱基对，这就相当于从一堆组成字母中重建一本书。然而，在适当的条件下，比如极寒的北极永久冻土层，分解的速度可以减缓。
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.
戴伦博士和他的同事们对三颗猛犸象的臼齿感兴趣，这三颗臼齿是20世纪70年代从年代久远的西伯利亚地质层中提取出来的。2017年，所有的样本都被送到了戴伦博士的实验室。经过检查，它们没有被细菌和充满敬畏的古生物学家们的握手过度污染，DNA链被提取出来，测序，并确定了年代。活体动物的DNA样本可以有几十万个字母，而年代久远的猛犸象样本中的DNA样本只有几十个字母长。图卢兹人类生物学和基因组学中心的生物学家卢多维克·奥兰多说，这接近科学可用的极限。
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