Can the DNA of dinosaurs be extracted from their fossilized bones? Not long ago the idea would never even have been considered. But with the advent of sensitive new ways of detecting and amplifying minute traces of dna, a number of researchers in recent years have claimed to have isolated small bits of ancient genetic material from such sources as fossil leaves, dinosaur bones, and bugs trapped in amber. How reliable are those claims? Jeffrey Bada of the Scripps Institution of Oceanography in La Jolla, California, says that most of them are probably invalid. He and his colleagues have found a way to distinguish authentic ancient fragments of dna from dna that is more likely a contaminant from soil or groundwater that had been in contact with the fossil.
The method doesn’t look at dna directly but instead measures how amino acids--the subunits of proteins--break down over time. When an animal dies, amino acids in its cells--including bone cells--begin to degrade, twisting into mirror images of their original shapes. Three years ago Bada discovered that the breakdown rate of one particular amino acid, aspartic acid, closely matches the rate at which dna degrades. This discovery, Bada realized, might offer a way to cross-check whether the dna in a fossil was truly ancient. If the aspartic acid in a sample had largely degraded, it would be unlikely that the sample’s dna had somehow managed to remain intact.
Bada told his idea to molecular biologist Svante Pääbo of the University of Munich, who was then a visiting professor at the University of California at Berkeley. Bada and Pääbo proceeded to examine several samples, dating from 50 to 40,000 years old. Some of the samples were known to contain uncontaminated dna; others were known to be contaminated. They found that when about 10 percent of the aspartic acid in a sample had broken down, invariably the samples also contained no old dna.
The researchers next looked at fossils that had reportedly yielded ancient dna, including dinosaur bones recovered from a coal mine in Utah and a Tyrannosaurus rex bone. In all cases, says Bada, the aspartic acid had degraded to such an extent that it is extremely unlikely that the dna in any of the fossils they examined was indeed ancient.
Bada estimates that dna could survive intact in bone for at most a few thousand years in temperate climates, and perhaps 100,000 years in cold climates like Alaska or Antarctica. The exception to this rule, he says, is dna from insects preserved in amber. Bada and Pääbo found virtually no amino-acid degradation in any of the amber specimens they studied, even some that were 130 million years old. Amber is just a great medium for preserving stuff, Bada says. It’s waterproof, and when you get water out of a system, you have very little degradation.