DNA breaks down over time, so the older the DNA, the smaller the pieces get—until there’s nothing left to detect. And the shorter the fragments, the harder it is to assign them to a particular group of plants or animals.
“The massive damage model shows very clearly that it is ancient DNA,” said Willerslev. He said he and his colleagues began working with the Greenland sample in 2006. “When it’s been 2 million years, there’s been too much evolutionary time, so whatever. [species] what you’re looking for doesn’t have to be very similar to what you see today.”
The Danish team says the DNA they found is preserved at freezing temperatures, and because it’s bound to clay and quartz, this also slows down decomposition.
Exactly how far back in time researchers can see is still an open question. “We’re probably close to the limit, but who knows,” said Tyler Murchie, a postdoctoral fellow at McMaster University who developed methods for studying ancient DNA. He notes that Dutch researchers have succeeded in combining several techniques to “create a powerful reconstruction of this ecosystem”.
Willerslev once predicted that it would be impossible to recover DNA from anything that lived more than a million years ago. Now that he’s broken the record, he’s reluctant to say where the limit lies. “I wouldn’t be surprised if…we could double back,” he said. “But I can’t guarantee that.”