DNA from the animals and plants that populated Siberia and Alaska up to 395,000 years ago has been recovered from specks of permafrost.
The discovery of these genetic fragments 鈥� by far the oldest DNA sequences yet authenticated 鈥� will give scientists unprecedented power to reconstruct ancient ecosystems and track them through time. The work has already shed new light on the mysterious extinction of the region鈥檚 large mammals.
Eske Willerslev and his team at the University of Copenhagen in Denmark sampled permafrost along 1200 kilometres of Siberia鈥檚 Arctic coast. They drilled to depths of 31 metres, removed cores of soil, and then extracted any DNA remaining in the frozen earth.
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The researchers looked for segments of several vertebrate mitochondrial genes and a plant chloroplast gene, which vary slightly between different species. What they found greatly exceeded their expectations.
Mammoths and bison
The permafrost contained DNA from eight species of mammals including woolly mammoth, steppe bison and musk ox, dating back 30,000 years, as well as 28 families of trees, shrubs, mosses and herbs, some of which lived 300,000 to 400,000 years ago.
By drilling at widely spaced locations, and at different depths, the team could recreate ancient landscapes and watch them evolve. 鈥淔rom just two grams of soil, you can obtain a meaningful sample of the ecosystem,鈥� says Willerslev.
That is in sharp contrast to studies based on fossils, which only reveal information about a particular petrified animal or plant. 鈥淎ny time you can place a species in its ecological web, we all win,鈥� notes geneticist Rebecca Cann at the University of Hawaii.
Willerslev was particularly struck by the dramatic shift in vegetation revealed in Siberia during the past 400,000 years. Herbs dominated the oldest landscapes but steadily declined until shrubs strongly dominated by about 10,000 years ago. This shift helps explain the mysterious extinction of most of the region鈥檚 large mammals about 11,000 years ago, Willerslev thinks.
The team also studied sediments from a temperate New Zealand cave. They found DNA from 29 kinds of plants and two species of the huge, flightless moa bird. Willerslev hopes the 鈥渟edimentary DNA鈥� method will let researchers reconstruct ecosystems up to a million years old.
Avoiding contamination
Previous claims of ancient DNA have been dogged by the problem of contamination. Modern DNA is ubiquitous, and readily sneaks into samples, with the drilling process itself being one possible source of contamination.
So Willerslev鈥檚 team spiked their drills with a specific strain of bacteria. Since none of the bacterial DNA showed up in their samples, they could be sure they were not injecting modern DNA into ancient deposits.
They also sent samples to the Wellcome Ancient Biomolecules Centre in Oxford, UK, for independent testing. 鈥淲e extracted the DNA ourselves, did the PCRs, and got the same results,鈥� says Thomas Gilbert, a molecular biologist at the centre. 鈥淲e both found mammoth DNA, for example, but neither lab has worked with mammoth DNA before. I think it鈥檚 clear that it鈥檚 real.鈥�
Even a vocal critic of previous ancient-DNA claims, Hendrik Poinar at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, applauds the new findings, while voicing some caution about dating issues.
Journal reference: Science (DOI:10.1126/science.1084114)