2-million-year-old DNA reveals a lost Arctic world

Two million years ago, on the fringe of some of the northernmost land on Earth just 500 miles from the North Pole, the landscape couldn’t have looked more different from today’s polar desert. Instead, a new study finds, it was richly forested—so productive, it was home to a menagerie of reindeer, rodents, and surprisingly even majestic mastodons.

Using ancient fragments of DNA—likely the oldest ever successfully analyzed—scientists have carefully reconstructed an “unprecedented” glimpse into an ancient, forested landscape at the roof of the world, says Mikkel Pedersen, a geographer at the University of Copenhagen and an author of the new analysis, published Wednesday in Nature.

This Arctic landscape has no modern equivalent, the team says; it is neither boreal forest, as is found in modern Scandinavia, nor temperate forest, but instead a unique mix. They found traces of over a hundred different plant species; insects; marine species; and most excitingly, nine vertebrate species whose fossils had never been seen in Greenland, from small rodents to springy rabbit ancestors to a single trace of a mastodon.

Together, the DNA paints the picture of a complex, fully integrated ecosystem—one that existed and developed during an era when the polar region was somewhere between 11 to 17 degrees Celsius (about 20 to 30 Fahrenheit) warmer than today, and an era that provides one of the closest possible analogs to our future with climate change.

The new study’s findings are “not just a window into an old world, but into a different climate reality,” says Brian Buma, an ecologist at the Environmental Defense Fund who was not involved in the study.

There is a thread of hope, he says—it’s remarkable that such a rich habitat existed in a hot world. But a crucial difference is that ecosystem likely had a long time to adapt and evolve, compared with the planet’s current rate of warming.

A landscape of abundance

The Arctic is warming by at least 0.5 degrees Celsius (about 1 degree Fahrenheit) each decade; winter temperatures have risen by 6C (about 10F) since the 1980s. Under worst-case climate change scenarios regional temperatures could eventually spike nearly as high as those seen in the era represented by the samples.

In the face of such change, understanding the Arctic’s future requires seeing its past clearly, says study co-author Ekse Willerslev, an evolutionary geneticist at the U.K.’s University of Cambridge. It is one of the only ways to “give us some kind of idea of how nature can respond to increasing temperatures.”

The study’s era, about two million years ago, is not a perfect analogy for the future. Earth had been hot for several million years but was cooling toward a more familiar climate state, shifting into a period of oscillating icy and warm stages.

But “there was a legacy of incredible biodiversity,” says Natalia Rybczynski, a paleobiologist at the Canadian Museum of Nature, who was not involved in the study. The warmth had supported forests across the Arctic, populated by camels, bears, beavers, and many other vertebrates.

“It's amazing to see that retained” even as the climate has begun to cool, she says.

Trees species like red cedar, today found much farther south in places like Vancouver Island, dotted the landscape. Fragments of DNA from horseshoe crabs, which today thrive in the warm Gulf Stream waters of the eastern U.S., suggest coastal waters were much toastier. The presence of reindeer and mastodon, big animals that eat a lot, means the landscape was highly productive.

The team didn’t find any genetic traces of carnivores, a vital piece of a complete ecosystem, but it’s likely they were present—just rare, leaving behind only smidgens of genetic material, says Pedersen.

“If we continued sequencing, taking samples and looking deeper, my prediction is that we would have at some point captured some of the carnivores,” he says.

Remarkable scientific breakthroughs

The detailed reconstruction was nearly 20 years in the making.

In 2006, some of the team members visited the Kap Kobenhavn site in northern Greenland, prospecting for frozen dirt that might contain preserved DNA.

At the site, discovered about 40 years ago, scientists searching for ancient Arctic sediments had already uncovered small fragments of ancient birch branches and cones, a piece of a rabbit tooth, and more evidence of a forested landscape.

Those previously discovered fossils and pollen grains told much of the story. But the team wondered if there were even more secrets hidden in the soil. In 2006 and during subsequent field seasons, they collected more frozen dirt and brought it back to the University of Copenhagen lab. But the tools for analyzing DNA—especially such old material—weren’t yet sensitive enough to get any robust results.

So they waited. Every time a new technique emerged, they’d try again. Someone learned DNA fragments attached to clay particles preserved particularly well, so they targeted clays. Another team discovered they could “shotgun” sequence all DNA fragments, some as small as 30 base pairs—just 1/1000 the size of the average vertebrate gene. Slowly but surely these advances allowed the team to analyze the Greenland DNA, which is probably a million years older than any previously analyzed samples.

“It feels almost magical to be able to infer such a complete picture of an ancient ecosystem from tiny fragments of preserved DNA,” Beth Shapiro, an evolutionary biologist at the University of California, Santa Cruz who was not involved in the study, wrote by email.

“We keep setting maximum age limits for ourselves and then breaking these limits as the tech improves.”