Fossil DNA illuminates the life and climate of the past

Ancient Greenland was green. New Danish research has shown that it was covered in conifer forest and had a relatively mild climate. Professor Eske Willerslev has analysed the world’s oldest DNA, preserved under the kilometre-thick icecap. The DNA is likely close to half a million years old, and the research results are overturning all previous assumptions about biological life and the climate in Greenland. The results have just been published in Science.

The environment of the past

Ten percent of the Earth’s surface has been covered with ice for thousands of years. The ice has annual layers and is a frozen archive of the world’s climate.

“I wonder, if there could also be DNA down there”, thought Eske Willerslev, who is the world’s leading expert in extracting DNA from organisms buried in permafrost. His thinking was that perhaps he could reconstruct the environment of the past.

The research

The icecap itself is comprised of pure ice, but the lower sections are mixed with mud from the bottom, and it was this mud that Eske Willerslev wanted to research.

He got base layer samples from three drillings; one in the southern part of Greenland, one in the middle of the Greenlandic ice sheet, and for the third sample he used the John Evans glacier in Canada. The Canadian glacier is only a few thousand years old, and samples from it were used to test the method.

In the base layer sample from the middle of the Greenlandic ice sheet, there were no DNA remains at all. “The explanation,” he says, “is that the ice here is very thick- over three kilometres, and the greater pressure produces a higher temperature at the base, and so the DNA material, which cannot tolerate warmth, disintegrates”.

Ancient flora and fauna

At the southern Greenland drilling-site, the ice is ‘only’ two kilometres thick, and here the DNA-material was so well preserved that Eske Willerslev could extract genetic traces of a long list of plants and insects and thereby reconstruct ancient plant and animal life.

“This genetic material presents a biological environment, which is completely different to what we see today.” he says. “We have found grain, pine, yew and alder. These correspond to the landscapes we find in Eastern Canada and in the Swedish forests today. The trees provide a backdrop from which we can also ascertain the climate since each species has its own temperature requirements. The yew trees reveal that the temperature during the winter could not have been lower than minus 17 degrees Celsius, and the presence of other trees shows that summer temperatures were at least 10 degrees”.

Climate theories over-turned

The research results are the first direct proof that there was forest in southern Greenland. Furthermore Willerslev found genetic traces of insects such as butterflies, moths, flies and beetles. But when was that? According to most scientific theories to date, all of southern Greenland and most of the northern part were ice-free during the last interglacial period 125,000 years ago, when the climate was 5 degrees warmer than the interglacial period we currently live in.

This theory however, was not confirmed by Willerslev and co-workers subsequent datings. He analysed the insects’ mitochondria, which are special genomes that change with time and like a clock can be used to date the DNA. He also analysed their amino acids which also change over time. Both datings showed that the insects were at least 450,000 years old.

Furthermore, the ice-core researchers could date the fine dust which blows onto the ice and is preserved year by year. “We can fix when the ice was last in contact with the atmosphere,” says Jørgen Peder Steffensen who is a researcher in the Ice and Climate group at the Niels Bohr Institute.

The dating of dust particles also showed that it has been at least 450,000 years ago since the area of the southern part of Greenland, was ice-free.

That signifies that there was ice there during the Eemian interglacial period 125,000 years ago. It means that although we are now confronted with global warming, the whole ice sheet will not melt and bring about the tremendous sea-level rises which have been the subject of so much discussion.

Attention:

The wording of the last sentence is not precise and can give rise to misunderstanding. Please notice the additional description:

The scientists do not want to put into question the rise in sea level during a global warming. During the last interglacial period 125.000 years ago, temperatures in Greenland were 5 degrees higher and global sea level was 4-5 meters higher than it is today. However, since the new scientific results show that the ice sheet also covered southern Greenland, the melting of the Greenlandic ice cap can only have caused a sea level rise of about 2 meters. Therefore some of the ice contributing to the sea level rise must have come from other sources, for instance the Antarctic. Furthermore, thermal warming of the oceans will cause expansion of the sea water and result in a sea level rise of half a meter, and the melting of small glaciers around the globe will result in an additional half meter rise.


Posted on 5 July 2007