A Fruitful Scoop for Ancient DNA

16 July 1998 6:30 pm

In the movie Jurassic Park, a collector snapped up hundreds of thousands of mosquitoes preserved in amber for DNA they had sucked from dinosaurs. In the real world, however, amber has yielded no reproducible traces of ancient genetic material. Now researchers report in tomorrow's Science that the treasure of ancient DNA can instead be gleaned from a less glamorous material: fossil feces. Such droppings may be able to provide a wealth of clues about the ecology and relationships of extinct animals--and perhaps even about early humans.

Last year geneticist Svante Pääbo of the University of Munich and his colleagues made headlines when they extracted DNA from a Neandertal bone (Science, 11 July 1997, p. 176). But the team hadn't been having any luck with the well-preserved samples of fossilized dung, called coprolites, left by an extinct ground sloth about 20,000 years ago in Gypsum Cave near Las Vegas, Nevada. The problem seemed to be Maillard products--sugar-rich tangles of proteins and nucleic acids that prevent DNA amplification.

Then the team heard about a chemical called N-phenacylthiazolium bromide (PTB), which cleaves the same kind of bonds that may entangle DNA in the Maillard products. Extracts from the sloth coprolite treated with PTB yielded sequences of mitochondrial DNA, presumably from intestinal cells shed into the feces. The team also snagged a wide variety of plant DNA from the coprolite--clues to the vegetarian sloth's diet. They identified sequences from eight plant families, including grasses, yucca, grapes, and mint. DNA analysis may help identify plants chewed beyond recognition, says molecular biologist Hendrik Poinar.

Still, some paleontologists caution that DNA from dung may not reveal everything its proponents hope for. Changes in coprolite contents could simply reflect seasonal shifts rather than pointing to causes of extinction, says Russ Graham, a paleontologist at the Denver Museum of Natural History. The technique may not work on coprolites found in warmer or wetter conditions, or on very ancient samples, as most DNA is thought to degrade within 100,000 years, says Poinar. Despite such caveats, "I'm gathering as much poop as I can," Poinar says. "There's going to be a run on feces."