经济学人下载:史前动物猛犸象有望复活(2)

To date a specimen, fragments of its DNA are compared to corresponding chunks from known descendants. Armed with a few evolutionary rules-of-thumb, scientists can calculate how long it would have taken for the observed mutations to arise. Analysis of this sort revealed that the youngest molar, found near a village called Chukochya, was between 500,000 and 800,000 years old. A tooth found near the Adycha river was from an animal that had died between 1m and 1.2m years ago. A third, found near another village called Krestovka, was dated at between 1.1m and 1.2m years. The previous record had been held by a set of horse DNA thought to be as much as 780,000 years old. 

确定一个标本的年代要将其DNA片段与其已知后代的相应片段进行比较。科学家们借助一些进化经验法则可以计算出观察到的突变产生所需的时间。此类分析表明，在一个名叫楚科奇的村庄附近发现的最年轻的臼齿年龄在50万至80万岁之间。在阿德恰河附近发现的一颗牙齿属于一种死于100万至120万年前的动物。第三个是在另一个叫做克列斯托夫卡的村庄附近发现的，它的年代在110万到120万年之间。此前的记录是由一组马的DNA保持的，这组DNA据有78万年的历史。

The teeth held other surprises. The Krestovka mammoth belongs to a previously unknown branch of the mammoth family tree, an ancestor of the Columbian mammoth which roamed North America 1.5m years ago. The Adycha mammoth was an ancestor of the iconic woolly mammoth. It appeared to possess many of its descendant's features half a million years earlier, suggesting the woolly mammoth's distinctive physiology evolved more slowly than had been thought. 

这些牙齿还包含着其他惊喜。克雷斯托夫卡猛犸象属于猛犸象家族的一个未知分支，该分支是150万年前生活在北美的哥伦比亚猛犸象的祖先。阿德恰猛犸象是长毛猛犸象的祖先。它拥有其后代在50万年前的许多特征，这表明长毛象独特的生理进化速度比人们想象的要慢。

These are the sorts of insights into the slow workings of evolution that very ancient DNA can offer. And, it seems, prospects for collecting more such ultra-old samples are good. Permafrost has existed on Earth for the past 2.6m years. That puts an upper limit on the age of sequenceable DNA, but one that still leaves a million years more headroom. "I'm sure that in the permafrost there are going to be samples that have survived longer," says Patricia Pecnerova of the Swedish Museum of Natural History, and a co-author on the study. Records, after all, are made to be broken. 

这些都是非常古老的DNA所能提供的关于缓慢进化过程的洞见。而且，收集更多这种超古老样本的前景似乎很好。多年冻土在地球上已经存在了260万年。这就给可测序DNA的年龄设定了上限，但仍留有100万年的空间。瑞典自然历史博物馆的帕特里夏·佩涅洛娃是这项研究的合著者之一，她说：“我敢肯定永久冻土层中会有存活时间更长的样本。”毕竟，记录就是用来被打破的。