DNA from dinosaurs: impossible dream?

Molecular biologists have been tripping over each other in recent years in the race to retrieve samples of DNA from dinosaur bones and other fossils. Now, new research challenges these claims of finding ancient DNA.

Molecules of DNA fall apart after a few thousand years and are unlikely to have survived from the time of the dinosaurs, more than 65 million years ago, report Hendrik N. Poinar of the University of Munich and his colleagues. "The prospects of retrieving DNA sequences from dinosaur fossils seem bleak," the scientists report in the May 10 Science.

Poinar and his coworkers describe a quick, simple test that reveals whether DNA in old samples has degraded or been contaminated by modern genetic material. Their technique focuses on amino acids-the building blocks of proteins-which come in right-handed and left-handed forms. Most organisms build proteins using only left-handed amino acids, known as L-enantiomers. After death, a chemical reaction called racemization changes L-enantiomers into right-handed D-enantiomers until a balance is reached.

The racemization reaction for one amino acid-aspartic acid-proceeds at about the same rate as DNA degradation. Scientists can therefore use aspartic acid's ratio of D- to L-enantiomers as an independent means of authenticating DNA extracted from old samples, proposes Poinar's group. If extensive racemization has occurred, the original DNA will have deteriorated. In such cases, any DNA extracted from a sample must come from a modern contaminant, say the scientists.

To test their idea, Poinar and his colleagues examined 26 archaeological remains, including a 27,000-year-old horse from Alaska and a 4,500-year-old human leg from Egypt. In all specimens known to contain ancient DNA, less than 10 percent of the aspartic acid had shifted from the L form to the D form.

Researchers could not obtain reliable DNA samples from remains in which the D-enantiomer reached more than 10 percent.

Poinar's group then used the technique to assess fossils that had purportedly yielded ancient DNA. In the much-publicized case of a dinosaur from Utah, they found that D-enantiomers in aspartic acid had reached 21 percent, casting further doubt on the authenticity of DNA from this sample (SN: 12/2/95, p.373).

Laboratory tests suggest that DNA should break up within a few thousand years in warm climates and 100,000 years in cold regions. Poinar's group found one environment that may preserve DNA for millions of years, however. Insects encased in amber showed almost no racemization, apparently because amber keeps out water, which is necessary for this chemical reaction.

"That made us much more enthusiastic about retrieving DNA sequences [from amber]," says Svante Paabo, a coauthor of the Science paper. Poinar and others have previously reported DNA sequences from insects trapped in amber, but they have yet to replicate these findings. - R. Monastersky