When once-living tissue is preserved in a cold, dry environment, fragments of its DNA can survive for hundreds of thousands of years. In fact, DNA doesn't even have to remain in tissue; we've managed to obtain DNA from the soil of previously inhabited environments. The DNA is damaged and broken into small fragments, but it's sufficient to allow DNA sequencing, telling us about the species that once lived there.
In an astonishing demonstration of how well this can work, researchers have obtained DNA from deposits that were preserved in Greenland for roughly 2 million years. The deposits, however, date from a relatively warm period in Greenland's past and reveal the presence of an entire ecosystem that once inhabited the country's north coast.
A different Greenland
Over the last million years or so, the Earth's glacial cycles have had relatively short warm periods that don't reach temperatures sufficient to eliminate the major ice sheets in polar regions. But before this time, the cycles were shorter, the warm periods longer, and there were times the ice sheets underwent major retreats. Estimates are that, around this time, the minimum temperatures in northern Greenland were roughly 10° C higher than they are now.
During this period, a set of deposits called the Kap København Formation was put in place in what was likely to be an estuary environment. Some of the layers of this deposit are likely to be sediments that washed into the area from a land-based environment, and other layers are sandy and were likely laid down by salt water.
Studies of these deposits have found a handful of animal fossils, as well as pollen from various plant species. These indicate that more species were present in this past ecosystem than are presently found in northern Greenland, but it's unclear how representative the finds are. Pollen can travel long distances, for example, and only a fraction of the animals are likely to be preserved.
So, a large international team decided to find out whether 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; winter lows were still well below freezing. And, for hundreds of thousands of years, the area has generally been about as cold as you would expect an area near the border between the Atlantic and Arctic Oceans would be.
The researchers then attempted to figure out just how old these deposits are. Based on a magnetic field reversal that occurred as the Kap København Formation was being laid down, they concluded that it was deposited either 1.9 or 2.1 million years ago—reasonably close to past estimates of 2.4 million years. They then plugged that age and the local climate conditions into software that estimates the amount of damage the DNA should accumulate. This suggested that there should only be a tiny fraction of the damage the DNA would have picked up in a warmer climate—damage was likely down by more than 700-fold.
The researchers argue that the minerals in the deposit interact with DNA, pulling it out of solution and protecting it from any environmental enzymes.
An ecosystem long gone
The process worked. The researchers obtained nearly 3 billion individual pieces of sequence, most in the neighborhood of 50 base pairs long.
Presently, we know of 175 plant genera living in Greenland; 70 of these showed up in the ancient DNA. More than 100 additional genera were detected, about 40 of them are found in North American boreal forests, including spruce and poplars. A number of other woody shrubs from these environments were also found. A lot of the plants present had been seen in the pollen and other fossilized plant material. Many of these had a pattern of mutations that suggest they are ancestral to their current relatives.
Strikingly, the list of species includes a number that can't grow on permafrost. This puts some limits on the sort of environment that was present in Greenland back then.
As far as animals go, there are a handful of species, some of which still can be found in Greenland, like a black goose. An animal related to deer and another within the hares and rabbits were also present. But, most significantly, there's a member of the family that includes elephants and mammoths, although this sequence groups best with mastodons. Again, all of these sequences appear to be ancestral to any of the creatures we know from previous ancient DNA attempts, so it's difficult to say exactly what species was present.
Because the site includes deposits that formed while the area was underwater, there's also DNA from reef-building coral, horseshoe crabs, and a variety of plankton. Oddly, there's little sign of insect DNA, even though they're well represented in the fossils found in the deposit.
The past, but not the future
It's clear that in the right environment, DNA can last much longer than we've seen before. So there are lots of opportunities to rethink where we've been trying to get ancient DNA from.
But it also tells us a great deal about an ecosystem that existed when the Arctic was much warmer but saw limited daylight and cold temperatures over the winter. And the answer is that it seems to have developed an ecosystem with no modern analogues, containing a mix of Arctic species and those from northern forests.
But, even as our world is edging toward a warmer Arctic, it's clear that we probably shouldn't expect history to repeat if it gets warm enough. In the time relevant to this ecosystem, cold periods were shorter, which limited the spread of ice. That probably allowed some of the species that are now missing from Greenland to survive closer to this habitat and repopulate it as things warmed up again. That's no longer the case. The periods with the most extensive indications of woodlands are also when the mastodon appears to be present, and all the species that could play a similar ecosystem role are now extinct from the Arctic and subarctic regions. All of which makes a repeat of this sort of habitat very unlikely.
Nature, 2022. DOI: 10.1038/s41586-022-05453-y (About DOIs).
