Ancient DNA research: growing pains....

When a promising, fast-moving field threatens to spin out of control, what do you do? Meet for a reality check.

That's exactly what scientists in the young area of ancient DNA research did recently. Instead of announcing success after success -- the usual stuff of scientific meetings -- the researchers pondered the many ways in which ancient specimens can lure experimenters down the wrong path.

In response to criticism voiced earlier by colleagues, Noreen Tuross called for "more analytical rigor to make this a credible field." Tuross is a biochemist with the Smithsonian Institution in Washington, D.C.

Ancient DNA comes from dead of extinct organisms 100 to millions of years old. If well preserved, such DNA can be recovered from ancient bone, animals frozen in Arctic soil, or creatures trapped in amber (SN: 10/24/92, p.280). Using a copying technique called polymerase chain reaction (PCR), scientists amplify -- reproduce in large quantities -- traces of such DNA and then decipher its genetic code.

The advantages of PCR -- its simplicity and low cost -- also present dangers, contents Tuross, since it attracts many people to the field. some of whom fail to scrutinize the condition of their samples with sensitive chemical methods. Results are sometimes hard to reproduce, the researchers agreed, particularly when one extracts DNA out of tiny, precious samples entombed in amber.

Ancient bone contains far less DNA than people originally thought, Tuross says. A chemical called fulvic acid caused false positive results in tests measuring DNA content and led to incorrect reports in the literature, she says, adding that the presence of fulvic acid makes it necessary to purify ancient DNA carefully. Over time, water and oxygen damage DNA. When scientists try to amplify old DNA, that damage can either yield erroneous findings or thwart PCr altogether, the researchers warned. PCR can also amplify minute contamination of DNA, such as an excavator's fingerprints, and delude scientists. Bryan Sykes of the University of Oxford in England cited a case in which amplified DNA, supposedly from a mammoth tusk, turned out to be human.

Not all is lost, though. Sykes says that much DNA contamination can be removed by tretaing the bones with bleach or by sandblasting them. Tuross is "very optimistic" that ancient DNA reserach will overcome its early troubles.