T. Rex Protein "Confirms" Bird-Dinosaur Link
for National Geographic News
|April 24, 2008|
A new study of ancient proteins retrieved from a Tyranosaurus rex fossil confirms the long-hypothesized evolutionary connection between dinosaurs and modern birds, experts say.
The finding is the first molecular evidence that birds, not lizards or other reptiles, are the closest living relatives of dinosaurs, the researchers note.
A close relationship between the two groups was already widely suspected, based on similarities in skeletal features.
The new research follows a breakthrough study last year in which scientists reported the recovery and partial molecular sequencing of T. rex and mastodon proteins.
Both dinosaur studies examined samples of collagen, the main protein component of bone.
In addition to cementing the dino-bird connection, the new study provides the first molecular evidence that mastodons and elephants are also closely related.
"This shows that if we can sequence even tiny pieces of fossil protein, we can establish evolutionary relationships," said co-author John Asara of Harvard Medical School, who also led the previous T. rex study.
Chris Organ of Harvard University is the lead author of the new report, which appears in tomorrow's issue of the journal Science.
From T. Rex to Chicken
The T. rex proteins were extracted from soft tissues preserved inside 68-million-year-old fossil remains first described in 2005.
The mastodon remains were much younger, dating to between 160,000 and 600,000 years ago.
Using a variety of techniques, the researchers compared the T. rex and mastodon protein sequences with those of 21 living animals, including ostriches, chickens, and alligators.
Such comparisons are commonly used by biologists to construct evolutionary "family trees," since similar protein structure is a sign of shared genetic makeup.
Until very recently, however, protein sequences have not been available for ancient organisms such as dinosaurs, since most fossils do not yield proteins or DNA.
The family trees of dinosaurs and other ancient vertebrates are instead known largely by comparing many fine details of skeletal anatomy.
If molecular data become more widely available for dinosaurs, Asara noted, researchers will be able to fill in gaps and overcome possible errors in existing classification based on physical features.
To illustrate his point, he noted that the shared ancestry of two present-day groups—elephants and shrew-like tenrecs—is known solely from DNA and protein comparisons.
"Nobody could make that connection based on bones," he noted.
"The amazing part of this study is that we could establish the dinosaur-bird connection using only 89 total amino acids [the building blocks of proteins]," Asara said.
With only a small amount of sequence data, he continued, "we can take an unidentified or fragmented fossil bone and not only identify the species but also help place it in evolution."
It remains to be seen whether even small sequences can be extracted from ancient fossils with any regularity, experts say.
Mary Schweitzer of North Carolina State University is a co-author of the new study and made the initial discovery of the T. rex soft tissue remains.
She has argued that such remains may be relatively common in well-preserved fossils but are often overlooked.
Others have said that protein preservation over tens of millions of years should not be possible. Some scientists have continued to question whether Asara's and Schweitzer's sequences really came from an ancient T. rex.
Proteins from some other biological source could have somehow contaminated the dinosaur remains, the skeptics note.
The new finding that the proteins are most similar to those of birds, Asara said, helps rule out the possibility of contamination from other sources such as mammals.
But doubts remain. Peggy Ostrom is a biologist at Michigan State University in East Lansing and an expert on fossil proteins.
Many have remained skeptical about the T. rex protein findings, she said, because of the small size of the sequences.
"They have a very tiny bit of data relative to the size of the collagen molecule," Ostrom said.
"What's going to be really convincing is to actually see some more sequences," she added.
"If [preservation of dinosaur proteins] is a ubiquitous occurrence, then that should be forthcoming."
Ostrom also noted that many recent findings, including the mastodon remains dated to nearly half a million years ago, have greatly pushed back previously accepted time limits for protein molecule preservation.
"In 2000, there probably wasn't one biochemist around who would tell you we'd find a protein over 40 thousand years old," she said.
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