By Helen Briggs
BBC News Online science reporter
The oldest DNA ever discovered has been found preserved in ice in Siberia.
The record-breaking samples are from plants which lived there 400,000 years ago.
Mammoth DNA was recovered from the permafrost
The genetic material is probably three or four times older than any other ancient DNA found on the planet.
Soil frozen into the ice has also yielded fragments of DNA of large prehistoric animals, including the woolly mammoth, reindeer and musk ox.
The ice cores from which the DNA was extracted have been dated to between 300,000 and 400,000 years old.
Thomas Gilbert of the Ancient Biomolecules Centre at the University of Oxford, UK, helped check the samples.
He said: "We believe it is the oldest DNA to date. Other people have made similar claims but nobody else has been able to replicate the findings."
Snapshot of diversity
Various claims have been made for the oldest ever DNA including that extracted from ancient bacteria and even dinosaur bones.
The reports have proved controversial, however, because of the possibility samples could have been contaminated by traces of modern DNA.
The researchers, led by Eske Willerslev of the University of Copenhagen, Denmark, have tried to avoid any dispute in this case by getting researchers in other laboratories to verify their work.
Their analysis, published in the online edition of the journal Science, shows the DNA comes from at least 19 different plant families.
DNA sequences from large plant-eating mammals, such as the mammoth, bison and horse, were also found.
The East Siberia Sea coast was formed by ice wedges
The approach gives archaeologists a new window into the past, creating a vivid picture of plant and animal diversity at the time.
Mr Gilbert said: "[The DNA] is incredibly old, which shows that DNA can be preserved that far back.
"It's a whole new technique that gives you a snapshot of past plant and animal diversity in one sample rather than by sifting through hundreds of bones."
The DNA has been broken into tiny pieces, so there is little chance of bringing any of the species back from the dead.
"Cloning is in our view impossible at this stage. You'd need the whole DNA and you would have to construct a primitive cell to put the DNA in," added Mr Gilbert.