Along the way, the researchers tweaked their methods for extracting DNA from ancient samples and upgraded the machines they used to sequence it.
As they became better at fishing for genes, they would take out more of the Kap Kobenhavn samples for another shot. But for years they failed, again and again.
From time to time they were tantalized by what looked like short bits of DNA, which are called reads.
The researchers couldn't rule out the possibility that bits of young DNA in Greenland, or even in their lab, had contaminated the reads.
Finally, after a major upgrade in their technology, they found DNA in the samples in 2017. The permafrost turned out to be loaded with genetic material. Before long they had collected millions of DNA fragments.
"It was a breakthrough," Dr. Willerslev said. "It was going from nothing or very little that you don't know is real, to suddenly: It's there."
The researchers lined up the fragments with DNA sequences of living species to figure out where they belonged on the evolutionary tree.
They found 102 different kinds of plants--including 78 that had previously been identified from fossils and 24 new ones. The plant DNA painted a picture of forests dominated by poplar and birch trees.
Other sequences come from land animals, including caribou, hares, mastodons, geese, lemmings and ants. The researchers also found marine species, such as horseshoe crabs, corals and algae.
"It superseded everything we imagined," Dr. Kjaer said.
The researchers also searched the permafrost for new clues to the age of the fossils. They found layers in the sediment in which the minerals revealed that the Earth's magnetic field had flipped.
The age of those reversals helped the researchers determine that Kap Kobenhavn was at least two million years old, but they could not establish a clear upper limit. "My gut feeling as a geoscientist is that it's older," Dr. Kjaer said.
The researchers ruled out the possibility that the DNA came from younger species that contaminated the older permafrost.
The DNA of the birch trees lacked many of the mutations that living species have, indicating that they were ancient. The DNA also had a distinct pattern of damage that occurs only when the molecules have been sitting in sediment for geological stretches of time.
"It really helps show that this really is old DNA," said Tyler Murchie, a postdoctoral researcher at McMaster University who was not involved in the new study.