The oldest DNA ever recovered has revealed a remarkable two-million-year-old ecosystem in Greenland, including the presence of an unlikely explorer: the mastodon.
The DNA, found locked in sediments in an area called Peary Land in the far north of Greenland, shows what life was like during a much warmer time in Earth’s history. The landscape, which is now a harsh polar wasteland, was once home to trees, caribou and behemoths. Some of the plants and animals that once thrived there are now found in arctic environments, while others are now only found in more temperate boreal forests. “What we see is an ecosystem with no modern analogue,” says Eske Willerslev, an evolutionary geneticist at the University of Cambridge and lead author of the study, which was published in Nature.
So far, the oldest DNA ever recovered came from a million-year-old mammoth tooth. The oldest DNA ever found in the environment – rather than in a fossil specimen – was also a million years old and came from marine sediments in Antarctica. The recently analyzed ancient DNA comes from a fossil-rich rock formation in Peary Land called Kap København, which preserves sediments from land and a shallow ocean-side estuary. The formation, which geologists had previously dated to be around two million years old, has already yielded a treasure trove of plant and insect fossils, but almost no signs of mammals. DNA analysis now reveals 102 different plant genera, including 24 that have never been found fossilized in the formation, and nine animals, including horseshoe crabs, hares, geese and mastodons. It was “mind blowing,” says Willerslev, because no one thought the behemoths got that far north.
“It paints a picture of everything that was present in this ecosystem, and it’s really amazing,” says Drew Christ, postdoctoral researcher at the Gund Institute for Environment at the University of Vermont, who studies the history of the polar regions of Earth but was not involved. in research.
Researchers have reconstructed ancient Peary Earth using disembodied DNA fragments. Every time a leaf falls off a tree or a person sheds a bit of skin or a rabbit dies and decomposes in a meadow, fragments of DNA can enter the environment. Most of these fragments, called environmental DNA (eDNA), degrade rapidly. But under the right chemical conditions, DNA molecules can bind to sediment. This prevents them from being eaten away by the enzymes, says study co-author Karina Sand, a molecular geobiologist at the University of Copenhagen’s Globe Institute.
Researchers began collecting sediment from Peary Land in 2006, but it took years for technology to catch up with their ambitions. “Every time we’ve had improvements in terms of DNA extraction and sequencing technology, we’ve tried to revisit those samples — and we’ve failed, and we’ve failed,” Willerslev says. For years, the team was unable to extract usable DNA from the samples.
Finally, a few years ago, researchers finally succeeded in extracting highly damaged DNA. They were then able to compare the DNA fragments with the genomes of modern species. Similarities in sequences revealed that some of the species that left DNA were among the ancestors of modern species.
Two million years ago, the site of Kap København would have been a forested shoreline where a river flowed into an estuary, Willerslev explains. The river carried DNA fragments from the land to the marine environment, where they were preserved. That’s why researchers found evidence of horseshoe crabs – a family that lives much further south today – alongside caribou DNA. They also found evidence of corals, ants, fleas and lemmings.
Plant life dominating this landscape included willow and birch, which are found today in southern Greenland. However, there were also trees that are now only found in more temperate forests, such as poplar and cedar, says study co-author Mikkel Pedersen, a physical geographer at the University of Copenhagen. . Temperatures would have been on average 11 to 19 degrees Celsius higher than today. But Greenland was at the same latitude as it is today, meaning this ancient landscape was shrouded in darkness 24/7 for almost half the year. The fact that plant life can survive long periods without sunlight speaks to the power of evolutionary adaptation, Willerslev says.
Groups of organisms living in Greenland two million years ago were also able to survive and produce offspring, such as modern caribou, which now live in much colder arctic conditions. Studying the genetic sequences of these ancient animals could reveal adaptations that could help arctic species survive today’s human-caused climate change, Willerslev says.
Researchers do not know how long environmental DNA can remain intact in sediments. Willerslev says he wouldn’t be surprised to find fragments four million years old. There could be other places on Earth where ancient DNA can help uncover how ecosystems changed as the climate oscillated, says Linda Armbrecht, a researcher at the University’s Institute for Marine and Antarctic Studies. from Tasmania in Australia, who led the study that discovered million-year-old DNA in Antarctic sediments and was not involved in the new paper by Willerslev and colleagues.*
“Both of our studies looked for DNA in cold environments: Greenland and Antarctica,” says Armbrecht. “Searching for DNA in environments and sediments with favorable properties for DNA conservation (including, for example, cold temperatures, specific mineralogy) appears to be the key to determining how far in time this DNA can be stored and detected.”
*Editor’s Note (12/7/22): This sentence was edited after publication to correct Linda Arbrecht’s current affiliation.
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