Ancient DNA has been used to describe a two million-year-old ecosystem that has undergone extreme climate change, allowing scientists to look directly at the DNA of a past ecosystem going back in time.

Partial samples, a few millionths of a millimeter long, were taken from the Kap København Formation, a nearly 100-meter-thick sediment deposit spanning 20,000 years located at the mouth of a fjord in the Arctic Ocean in north Greenland. These sediments were preserved in ice or permafrost and, crucially, were undisturbed by human activity for two million years. At that time, Greenland's climate ranged from Arctic to temperate, and was 10 to 17°C warmer than present-day Greenland.

Scientists found DNA traces of animals, plants and microorganisms, including reindeer, hares, lemmings, birch trees and poplars. The researchers even discovered that the Mastodon, an Ice Age mammal, lived as far away as Greenland before becoming extinct.

The detective work of 40 scientists from Denmark, the United Kingdom, France, Sweden, Norway, the United States and Germany has unlocked the secrets of the DNA fragments. Each DNA fragment was matched against large libraries of DNA collected from present-day animals, plants and microorganisms. A picture of the DNA of trees, bushes, birds, animals and microorganisms began to emerge. Some DNA fragments were easy to classify as predecessors of present-day species, while others could only be related at the genus level; some came from species impossible to place in the DNA libraries of animals, plants, and microorganisms still living in the 21st century.

The scientific team considered a range of substances that might have helped DNA fight degradation and time, and ultimately considered clay minerals as the culprit. Clay minerals are versatile, found all over the Earth's surface, and their unique structure allows them to absorb DNA like a sponge. It is possible that, protected in clay minerals, DNA can survive extreme conditions such as high humidity, high temperatures or constant attacks by bacteria that use DNA as food.

The Kap København ecosystem, which has no current equivalent, existed at temperatures considerably higher than those we experience in this region today, and similar to those we expect on our planet in the future due to global warming. A key factor here is the extent to which species will be able to adapt to the changing conditions resulting from a significant increase in temperature. The data suggest that many species can evolve and adapt to widely varying temperatures, but these results show that they need time to do so. The rapidity of current global warming means that organisms and species do not have that time, so the climate emergency remains a huge threat to biodiversity.

In examining the ancient DNA from the Kap København formation, scientists also found DNA from a wide range of microorganisms, including bacteria and fungi, which they are still mapping. The hope is that some of the "tricks" of discovered plant DNA can be used to help make some endangered species today more resilient to global warming. It is possible that genetic engineering can mimic the strategy developed by plants and trees two million years ago to survive in a climate characterized by rising temperatures and prevent the extinction of certain species, plants and trees.

The preservation of DNA is generally optimal in cold and dry conditions. However, the preservation of DNA in contact with clay opens new perspectives for the search for environmental DNA in the hot and humid conditions encountered in Africa, for example, and thus for the origin of the species that developed there, including the first humans.