Fertile Ground: Snippets of DNA persist in soil for millennia
Minuscule samples of sediment from New Zealand and Siberia have yielded bits of DNA from dozens of animals and plants, some long extinct. This genetic material, which includes the oldest DNA sequences yet found that can be traced to a specific organism, could help scientists reconstruct ancient ecosystems in those regions.
Nearly every cell of an organism carries DNA, the genetic information that researchers can use to identify species. Scientists usually study DNA extracted from living tissue or from preserved remains, says Eske Willerslev, a molecular biologist at the University of Copenhagen. However, his new research suggests that some soils may hold stockpiles of ancient DNA even if they don’t include identifiable fossils.
For part of the project, Willerslev and his colleagues analyzed samples of permafrost drilled from several sites along a 1.2-kilometer stretch of Siberia’s Arctic coast. The sediment cores, up to 31 meters long, included material dating from modern times to about 2 million years ago. The cores contained ice, soil, pollen, and plant rootlets, as well as small groups of unidentifiable cells.
Two-gram samples of sediment up to 30,000 years old included DNA from eight living and extinct animal species, including lemmings, hares, horses, reindeer, bison, musk oxen, and woolly mammoths. DNA extracted from sediment as old as 400,000 years matched the genetic signatures of at least 28 modern and ancient species of trees, shrubs, herbs, and mosses. Researchers didn’t find DNA in the older sediment samples, says Willerslev.
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Although the scientists don’t know how animal DNA ended up locked in permafrost, Willerslev speculates that the genetic material came from cells that creatures shed in their feces.
The scientists also looked at samples of silt taken from a cave in New Zealand and sand taken from within and around an ancient bird’s bone unearthed from a coastal dune there. From these 600-to-3,000-year-old sediments, the team identified at least 29 plant species, three types of extinct flightless birds called moas, and an extinct parakeet. They report their results in an upcoming issue of Science.
“This is a startling finding, if it’s true,” says David M. Lambert, a molecular biologist at Massey University in Palmerston North, New Zealand. Similar but preliminary efforts by his group haven’t yielded penguin DNA from Antarctic soils or moa DNA from New Zealand sediments, he notes.
By extracting and analyzing the DNA in small amounts of sediment, scientists might determine what animals have been present in a particular area, contends Hendrik N. Poinar of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. However, at some sites, the mixing of soil layers over time could complicate attempts to reconstruct ancient ecosystems, he adds.
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