Ancient history – new findings on preservation of ancient DNA in tree resin – The Year in Science: Evolution 1997 – Brief Article
When Mary Schweitzer, a paleontologist at Montana State University, speaks at scientific conferences, she sometimes shows her audience how to find DNA from a dinosaur: “Keep your eyes open for one of these,” she jokes as she flashes a slide of the ideal specimen — a toy dinosaur embedded in amber.
But it now appears that even if Schweitzer were handed a Velociraptor trapped in a ton of amber, she wouldn’t be able to get any DNA. For the past few years, astounding reports in scientific journals (and magazines such as this one) have trumpeted the discovery of ancient genetic material in insects caught in amber millions of years old. But for many researchers, the notion is now pretty much dead. It appears that the fossilized tree resin isn’t such a foolproof preservative as once thought. Pretty, yes; but an embalmer of ancient DNA, no. “A lot of the DNA you see in amber is some kind of highly modified ghost,” says Richard Thomas, a molecular systematist at the Natural History Museum in London.
Although there were several reports of discoveries of ancient DNA — particularly from amber — in the early 1990s, most subsequent attempts came up with nothing. No one expected getting DNA would be easy, but many researchers would have been more comfortable if these successes had been replicated even once. Recently Thomas and his postdoctoral researcher Jeremy Austin decided they would try to study the evolution of flies by examining specimens trapped in amber. They used a number of insect samples dating back 25 to 40 million years, including some from the same Dominican amber that have been the source of the reports of successful gene recovery. They never got to study fly evolution — because, as they reported this past year, they could not find any DNA. Trying out many methods for isolating DNA on 15 samples, Thomas and Austin found nothing.
For many researchers the results of this notably rigorous and thorough research was the last straw for ancient DNA. It is such a fragile molecule, they argue, that it can’t hold up for more than 100,000 years, even in amber. (The Neanderthal DNA discovered this past year was only between 100,000 and 30,000 years old.) “Either we’re all incompetent or it’s extremely difficult to make it work,” says Thomas. To him, the supposed successes of the past may have been the result of stray DNA from living organisms that drifted through laboratories. Since the common technique for finding ancient DNA involves replicating numerous copies of gene fragments — through a process called the polymerase chain reaction — even a tiny bit of contamination might fool a researcher.
Not everyone agrees with Thomas’s gloomy conclusions. “I don’t think the book’s closed,” says Rob DeSalle, a molecular systematist at the American Museum of Natural History who reported finding DNA from a termite trapped in amber in 1992 — and who still stands by his claim. “The fact that they’re not replicated doesn’t invalidate these results.” Nevertheless, many labs that were in hot pursuit of ancient DNA, including DeSalle’s own team, have dropped the research; DeSalle says the payoff isn’t worth the enormous effort. Mary Schweitzer herself tried and failed to get DNA out of a well-preserved fossil of Tyrannosaurus rex. This year, however, she reported her success in isolating blood proteins, which are far sturdier. It’s always possible that some similarly encouraging research will emerge in the field of ancient DNA, but for the moment it seems on its way to becoming ancient history.
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