Eske Willerslev of the University of Copenhagen and his colleagues collected sediment samples from the Siberian tundra in search of ancient bacterial DNA. To their surprise, they instead discovered DNA from 28 different families of trees, shrubs, herbs and mosses. What is more, the soil samples also contained DNA from eight types of mammals, both of species that survive today and ones that disappeared long ago, such as the wooly mammoth. In a paper published online today by the journal Science, the authors report that the oldest animal DNA was 30,000 years old; the plant DNA dated to between 300,000 to 400,000 years ago. The DNA analyses, which were conducted independently in Copenhagen and Oxford, indicate that the plant DNA is "the oldest reproducible and authenticated ancient DNA to date," the authors write.
Hundred-thousand-year-old samples of DNA have been recovered from fossilized bone, but contamination with modern DNA is rampant in many ancient DNA studies and previous claims of DNA that was millions of years old could not be reproduced. To assess the likelihood of contamination, Willerslev's team introduced a tagged strain of bacteria around the drilling apparatus. No evidence of DNA from the bacteria was detected in any of the samples analyzed in either laboratory, so the scientists are confident the DNA they detected truly is ancient.
To further establish the viability of collecting ancient DNA from soil samples instead of the usual hard and soft tissue remains, the scientists gathered additional sediments from caves in New Zealand. They discovered DNA from two species of extinct moa birds and 29 plants known to have existed prior to human arrival to the region. The researchers conclude that their findings demonstrate "that sedimentary genetic signals of plant and animal communities can be preserved for considerable periods, in both permafrost and temperate conditions."