The dream of recovering DNA from preserved dinosaur tissue inspires serious research as well as science fiction, and in November a group of biologists reported that they had apparently hit pay dirt.
But a fresh assessment of their evidence by four independent scientific teams has concluded that hunters of dinosaur DNA have not yet captured their quarry and must go back to their cloning boards for another try.
The conclusion has tossed cold water on a paper published Nov. 18 in the journal Science by Dr. Scott R. Woodward, a microbiologist, and his colleagues at Brigham Young University in Provo, Utah.
Dr. Woodward's team reported that they had extracted genetic material from small, 80 million-year-old bone fragments embedded in a coal seam in eastern Utah. Dr. Woodward says the fragments are dinosaur bone, but they are too small for positive identification.
Four groups that reanalyzed Dr. Woodward's data, however, reached the conclusion that the DNA in question looks more like DNA from mammals than from birds.
Since birds and dinosaurs have many physical similarities, the genes of dinosaurs, if and when they are found, are expected to have more in common with the genes of birds than those of mammals.
Worse still for Dr. Woodward's thesis, three of the four teams concluded that the DNA fragments he recovered from the bone shards seemed more similar to human DNA than anything else, and that the putative dinosaur genes were nothing more than accidental contaminants of modern human origin.
The four unfavorable critiques of Dr. Woodward's claim were published as "technical comments" in the May 26 issue of Science.
"There can no longer be any question about it," said Dr. S. Blair Hedges of Pennsylvania State University, a co-author of one of the critiques. "The piece of DNA identified by Woodward is clearly human.
"In looking for dinosaur DNA we all sometimes find material that at first looks like dinosaur genes but later turns out to be human contamination, so we move on to other things. But this one was published."
Dr. Hedges said that he and a colleague, John R. Horner, a paleontologist at the Museum of the Rockies in Bozeman, Mont., nearly made the mistake of submitting for publication a possibly spurious discovery.
"We were working on an astonishingly well-preserved fossil of a Tyrannosaurus rex, in which the bones themselves have survived without becoming mineralized into stone," Dr. Hedges said.
"We found DNA sequences in the dinosaur bone that were in the right places and really looked like what we expected dinosaur DNA to be. We submitted a paper reporting our discovery to the journal Nature, but then we found that we couldn't replicate our results, so at the last minute, we pulled the paper out. Replication is one of the key essentials of scientific method."
In a response also published by Science, Dr. Woodward said that the supposed similarity in genes between extinct dinosaurs and modern birds was by no means certain, since no dinosaur DNA had ever been positively identified and analyzed.
"There's no reason to discard good data just because of the hypothesis that the genes of dinosaurs and birds ought to be similar," he said.