Using a variety of techniques, the researchers were able to put an approximate date on the frozen mud: between 450,000 and 800,000 years.
"I'm not super-surprised that DNA could last that long" in Greenland, Willerslev said. "It's pretty much ideal conditions for DNA preservation."
Today it's a frigid -4°F (-20°C) at the bottom of the ice sheet at this spot, about 300 miles (500 kilometers) north of the southern tip of the island (see a map of Greenland).
Researchers have known that Greenland had some trees in the past, since they've found fossilized trees more than two million years old on the north coast of the island.
But it has been difficult to tell what the island's full ecosystem was actually like.
"Ten percent of Earth's surface is covered with ice," Willerslev said. "We have little information about the ecosystems that were there before [these places froze over]."
By comparing the DNA they found to that of today's plants and insects, the team identified the flora and fauna that used to live in this part of Greenland.
They found a variety of trees, including spruce and pine, similar to those found in northern boreal forests across Canada and northern Eurasia.
They also found signs of a number of creatures such as beetles, spiders, and butterflies.
All this suggests that before the area froze over, it had an open forest that supported a diverse ecosystem.
While these DNA discoveries help answer questions about Greenland's past environment, they may have created a new mystery related to global warming.
Based on computer models, some researchers had thought that most of southern Greenland was ice-free about 125,000 years ago, during the last interglacial period—a gap in time between ice ages when Earth was warmer.
During this period, sea levels rose about 16 to 19 feet (5 to 6 meters).
Many researchers thought that much of this water came from the melting of the southern part of Greenland's ice sheet during this time.
Several studies have found that Greenland's ice sheet is melting today, so scientists are working to predict how much the melt might contribute to sea level rise if the world continues warming.
(Related news: "Greenland Melt May Swamp LA, Other Cities, Study Says" [April 8, 2004].)
But "then we started to date [the ice] and we found that it wasn't that young," Willerslev said.
If the island had been free of ice during the last interglacial, it would likely have supported plants and animals.
In that case, DNA from those creatures should have been found instead of the DNA from the much older forest.
The new discoveries suggest that southern Greenland has been ice-covered for at least four times longer than previously thought.
"We have firm data to state that there was ice in central and northern Greenland," said Valerie Masson-Delmotte of the Laboratory of Climate and Environmental Sciences in Saclay, France.
"The main surprise [of the new study] is the persistency of ice in southern Greenland," said Masson-Delmotte, who was not involved in the new study.
And if the region was still frozen during the last interglacial period, lead author Willerslev said, then some of that water "must have come from some melting of ice somewhere else."
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