Scientists decode the genome of million-year-old mammoths
Pushing the limits of a technique that has revolutionised paelontologyFeb 17th 2021IN THE 1966 science-fiction movie “One Million Years B.C.”, Raquel Welch and John Richardson traversed 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.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.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 proven 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.The trouble with DNA is that it breaks down post mortem. The more broken-down 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.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 time-worn mammoth samples yielded strands mere dozens of letters long. This is close to the limit of what is scientifically useable, says Ludovic Orlando, a biologist at the Centre for Anthropobiology and Genomics of Toulouse.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 Chuckochya, 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.