Dinosaur Soft Tissue Sequenced; Similar to Chicken Proteins
for National Geographic News
|April 12, 2007|
Ancient collagen—the main protein component of bone—has been extracted from the remains of a 68-million-year-old Tyrannosaurus rex, according to two new reports.
The new studies provide strong support for the hotly debated claims that organic material previously extracted from the T. rex's leg bone is original dinosaur soft tissue that somehow escaped fossilization.
Now, for the first time, scientists have obtained partial protein sequences from the soft tissue remains.
"The sequences are clearly from T. rex," said John Asara of Harvard Medical School in Cambridge, Massachusetts, who led one of the studies.
In addition, both studies found similarities between the dino sample and the bone collagen of chickens, providing molecular support for the hypothesis that modern birds are descended from dinosaurs.
Until now the dino-bird connection has been entirely based on physical similarities in fossils' body structures (related: "Earliest Bird Had Feet Like Dinosaur, Fossil Shows" [December 1, 2005]).
In a related study, a team led by Mary Higby Schweitzer of North Carolina State University conducted tests that also revealed the presence of collagen in the T. rex remains.
In one experiment, antibodies that normally react in the presence of chicken collagen reacted strongly to the dinosaur protein, suggesting a similar molecular identity.
For the protein sequencing study, Asara's team isolated seven fragmentary chains of amino acids—the building blocks of proteins—from the T. rex specimen.
The results are by far the oldest such data ever recovered. Previously, the earliest protein sequence data came from a 300,000-year-old mammoth specimen.
Asara's team extracted the amino acids using a highly refined version of the analytical technique known as mass spectrometry.
They also used the technique to isolate more than 70 amino acid sequences from a mastodon thought to be between 160,000 and 600,000 years old.
Comparing the dino and mastodon samples to data from modern animals allowed the team to identify sequences that link the ancient amino acids to modern collagen.
Schweitzer and colleagues independently used a variety of chemical and molecular tests to identify the preserved collagen.
Both of the new studies, which will appear in tomorrow's edition of the journal Science, were conducted using the same unusual T. rex remains Schweitzer and others first described in 2005.
In that report the researchers described the seemingly inexplicable preservation of soft tissues—including branching blood vessels and bone matrix—in a T. rex fossil from Wyoming.
Some experts were immediately skeptical, saying that preservation of organic material over such a vast period of time should not be possible.
"The accepted viewpoint is that collagen, like other organic molecules, will degrade relatively rapidly, so that after a maximum of about a hundred thousand years nothing will remain," Schweitzer acknowledged.
But when conditions for preservation are just right, she said, "degradation rates may differ from predictions. Data from both [new] papers suggest that original protein may be preserved."
Burden of Proof
Hendrik Poinar is an expert in fossil proteins and DNA at McMaster University in Ontario, Canada.
Like others in the field, he had questioned whether Schweitzer's 2005 report made a sufficiently strong case that the preserved tissues came from a T. rex and were not the result of more recent contamination.
The new studies have him more convinced.
"I'd have to say, I'm more optimistic about it than I was previously," Poinar said. "Now the burden of proof is on the skeptics."
One self-proclaimed skeptic is Christina Nielsen-Marsh, an expert on ancient bone proteins at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
"I would love it to be true," she said of the new T. rex findings. "But I do have serious doubts. I know of no other research group that has been able to extract—let alone sequence—indigenous proteins from fossils older than a million years.
"Based on what we presently understand, these T. rex sequences make no sense at all," Nielsen-Marsh said.
"That doesn't mean they are wrong. But if they are right, then we all need to rethink how molecules survive in the geological environment."
Schweitzer and her collaborators, including paleontologist John Horner of Montana State University, agree that their discovery should prompt such a rethinking, which could lead to changes in how fieldwork is conducted.
In a Wednesday teleconference, the researchers said several factors may help explain the unusual protein preservation in the T. rex fossil.
The size and density of some dinosaur bones, they said, may help shield internal structures from decay. And bones preserved in dry sandstone may resist degradation better than those trapped in moist soil layers.
Horner said that a central lesson is that paleontologists need to dig deeper to find exceptionally well-preserved fossils.
"If we spend time getting as deep into the sediment as we can, I think we're going to find that many specimens are like this," Horner said.
"This summer we're sending out a major expedition, going worldwide looking for exquisite preservation."
On the laboratory side, Harvard's Asara said, researchers should expect further improvements in analytical techniques, facilitating the recovery of protein sequences from very old remains.
Previous beliefs that proteins rarely if ever survive beyond a few hundred thousand years have now been proven false, he said.
"The mastodon [analysis] revealed a lot of protein," Asara said. "We can now get extensive sequences from species half a million years old, if they are very well preserved."
The researchers said that obtaining more ancient sequences should lead to a powerful new synthesis of paleontology and molecular biology.
"We can now begin [to study] evolutionary relationships between modern and extinct organisms at the molecular level," Asara said.
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