When once-formed tissues are preserved in a cold, dry environment, fragments of DNA can survive for hundreds of thousands of years. In fact, DNA does not have to remain in tissues. We have been able to obtain DNA from the soil of previously inhabited environments. DNA gets damaged and breaks into tiny bits, but it’s enough to allow DNA sequencing, to tell us what species lived there before.
In a stunning demonstration of just how well this works, the researchers obtained DNA from sediments that had been preserved in Greenland for about two million years. However, the sediments date from a relatively warm period in Greenland’s past and reveal an entire ecosystem that once populated the country’s northern coast.
Greenland is different
Over the past million years or so, Earth’s glacial cycles have had relatively short warm periods that do not reach sufficient temperatures to wipe out the major ice sheets in the polar regions. But before this time, cycles were shorter, warm periods longer, and there were times when ice sheets experienced significant retreats. It is estimated that around this time, minimum temperatures in northern Greenland were about 10 degrees Celsius warmer than they are now.
During this period, a group of sediments called the Cap Copenhaven Formation was laid down in what was likely an estuary environment. Some of the layers of this sediment are likely sediments that washed into the area from a terrestrial environment, and the other layers are sandy and likely deposited by saltwater.
Studies of these sediments have found pollen from various plant species and a handful of animal fossils. These indicate that there were more species in this former ecosystem than are currently found in northern Greenland, but it is unclear how representative the finds are. Pollen can travel long distances, for example, and only a small fraction of the animals are likely to be preserved.
So, a large international team decided to find out if they could learn more about the ecosystem using environmental DNA. While Greenland remained warm for some time after these deposits, it was only relatively warm; The winter lows were still below freezing. And for hundreds of thousands of years, the region was generally as cold as you’d expect an area near the boundary between the Atlantic and Arctic oceans to be.
The researchers then tried to find out the age of these deposits. Based on the magnetic field reversal that occurred during the placement of the Cap Copenhaven Formation, they conclude that it was deposited between 1.9 or 2.1 million years ago—reasonably close to the previous estimate of 2.4 million years. Then they plugged that age and local climatic conditions into a program that estimated how much damage their DNA should accumulate. This suggests that there must be only a fraction of the damage that would have been done to DNA in the warmer climate – the damage would likely have been reduced by more than 700-fold.
The researchers argue that the minerals in the sediment interact with the DNA, pulling it out of solution and protecting it from any environmental enzymes.
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