Protein Analysis of T. Rex Bone Finds Link to Chickens
Friday, April 13, 2007
Unleashing a new, highly sensitive medical analyzer on fossilized bone from Tyrannosaurus rex, scientists have for the first time determined the precise molecular code of a dinosaur protein.
The feat, long presumed impossible because so little protein is present in dinosaur remains, opens the door to a redrawing of the evolutionary tree -- one based on molecular evidence instead of the crude comparisons of bone shapes and sizes that experts rely on today.
The first results, described in today's issue of the journal Science, show that the collagen protein in T. rex bone is extraordinarily similar to that of the modern chicken, confirming current thinking that dinosaurs' nearest cousins are birds.
But the approach promises to settle far more contentious debates about the relationships among various extinct and surviving species, scientists said. Those links until now have been impossible to verify because DNA, the gold standard for determining evolutionary connectedness, is degraded beyond recognition in remains that are more than a few tens of thousands of years old.
By contrast, the new work unexpectedly shows that protein -- a passable substitute for DNA for determining evolutionary relatedness -- can survive reasonably intact for tens of millions of years. The T. rex thigh bone analyzed in the study is 68 million years old.
"This interplay between the fossil record and the molecular record will be more and more useful for understanding the evolution of life on this planet," said Mary H. Schweitzer of North Carolina State University in Raleigh, who led the studies with John M. Asara of Harvard Medical School and Beth Israel Deaconess Medical Center.
"It's terrifically interesting," said Eddy Rubin, director of the Energy Department's Joint Genome Institute in Walnut Creek, Calif., and an expert in old DNA. "It really does say that biological molecules survive way longer than anyone thought."
The marriage of high-tech molecular biology and rock-and-hammer paleontology will not be easy, scientists said. Fossils for protein analysis must be meticulously collected from deep rock beds, where the risk of contamination by other life forms is low.
|Extracted bone powder analyzed from a 68-million-year-old fossilized femur bone of a Tyrannosaurus rex helped scientists find DNA evidence that the dinosaurs' nearest cousins are birds.|
"Most curators of paleontology don't like me," Schweitzer said. "They like to keep their bones intact, and they don't like me to come in and dissolve them away."
The work comes from two seemingly unrelated advances. At Beth Israel, Asara was using an unprecedentedly sensitive method of mass spectrometry, which identifies the molecular makeup of a substance by analyzing its charged atoms.
He was using it to find subtle abnormalities in proteins, which are strings of amino acids. The order of those amino acids determines a protein's function and can make the difference between healthy and cancerous tissue.
At the same time, Schweitzer was finding to her surprise that some fossil bones had smidgens of collagen, the major protein in bone.
By applying the new device to old bones, Asara, Schweitzer and their colleagues were able to identify the precise order of amino acids in snippets of collagen retrieved from a T. rex thigh bone and from a mastodon bone hundreds of thousands of years old -- then compare the sequences with those of living animals.
For the analysis, minerals are gradually removed from the bone and collagen is extracted and purified. It takes a piece of T. rex bone about the weight of a nickel -- five grams -- to get the less than a billionth of a gram of collagen needed for accurate sequencing.
To make sure the collagen was not from a more modern organism that may have died nearby, the team used bones that had been isolated. The T. rex bone came from beneath 1,000 cubic yards of rock in Hell Creek Formation in eastern Montana.
John R. Horner, of Montana State University's Museum of the Rockies, who participated in the work, said teams will roam the world this summer in search of more such pristine specimens.
"If we spend a lot of time getting as deep into the sediment as we can, and to places where there's been very little atmospheric or water contamination, I think we're going to find there are many specimens like this," Horner said.
Some types of rock also seem to be better at preserving proteins than others -- in particular sandstone that was simply sand in the days of the dinosaurs.
"Think about a giant Tyrannosaurus rex drumstick rotting in the sand," Schweitzer said. "As it liquefies . . . the enzymes of decay and degradation drain away." On a less porous surface, she said, the corpse would "stew in its own juices."