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.

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.

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.

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