For about 15 years, evolutionary biologists have been using ancient DNA to study how organisms change through time. But the field has gotten a bad rap because early attempts to extract DNA from dinosaurs or ancient insects embedded in amber instead proved to be studies of modern contamination. Now, in the 22 March issue of Science, two independent research groups show multiple samples of DNA from old bones preserved in cold environments can teach quite a bit about the past.
A team led by Alan Cooper, a molecular evolutionist at the University of Oxford, United Kingdom, reconstructed the migration patterns of subarctic brown bears living up to 60,000 years ago. Cooper's team analyzed DNA from 36 bears whose bones had been well preserved in the frozen soils of Alaska, the Yukon, and Siberia. The researchers sequenced two pieces of DNA from each bear and grouped the bears according to the degree of similarity in the sequences. They found that the distributions of these ancient genetic groups did not correspond to those of modern populations, as researchers have assumed. The findings also suggest that brown bears disappeared from much of Alaska and the Yukon 35,000 years ago, only to reappear 14,000 years later. Their return may have contributed to the disappearance of a competing species, the short-faced bear, Cooper suggests.
On the opposite side of the globe, a team led by David Lambert at Massey University in Palmerston North, New Zealand, examined frozen remains of Adélie penguins, whose large colonies have existed for thousands of years. The same birds and their descendents return to the sites year after year, leaving layer upon layer of old material. Lambert's team collected and dated 96 bones from various layers and gathered 300 blood samples from living birds. The researchers looked at a rapidly evolving sequence of mitochondrial DNA and catalogued changes between the ancient and modern samples. From that they calculated the rate of evolutionary change--which they found was about two to seven times faster than previous estimates for other species.
With these two studies, "the study of ancient DNA has now advanced to a point where one can study genetic variation within species over thousands of years and [obtain] data that can be trusted," says Svante Pääbo, an evolutionary biologist at the Max Planck Institute for Evolutionary Anthropology. Lambert's and Cooper's studies may thus push the use of ancient DNA further along the path to redemption.