Scientists have sequenced tiny pieces of protein from a 68 million-year-old Tyrannosaurus rex. The fragments seem to closely match those found in today’s chickens. This supports a recent theory that birds are descended from dinosaurs.
Scientists at Harvard Medical School, Beth Israel Deaconess Medical Center and North Carolina State University did the research. They report it in the April 13 issue of Science.
It is widely believed that birds evolved from dinosaurs, said John Asara. He is director of the mass spectrometer facility at Beth Israel Deaconess Medical School. “But that’s all based on the architecture of the bones.”
Asara sequenced the protein fragments – seven of them – over the course of a year and a half. His team used sensitive mass spectrometry methods. “This allows you to get the chance to say, ‘Wait, they really are related because their sequences are related.’”
The researchers did not get enough sequences to definitely say that, he added. “But what sequences we got support that idea.”
In another study in the same issue of Science, researchers report that extracts of T. rex bone reacted with antibodies to chicken collagen. This further suggests the presence of birdlike protein in the dinosaur bones.
This work was done by Mary Schweitzer and colleagues at North Carolina State University.
The mere existence of such ancient protein contradicts a long-held belief. When an animal dies, protein begins to degrade immediately. It is slowly replaced by mineral when fossils form.
This process was thought to be complete in one million years.
For centuries it has been believed that fossilisation destroyed any original material, said Schweitzer. She is also at the North Carolina Museum of Natural Sciences and is a co-author on the Asara study.
“Consequently no one looked carefully at really old bones.”
That may now change, said Lewis Cantley. He is HMS professor of Systems Biology and BIDMC chief of Signal Transduction. He also took part in the study.
“Basically, this is the breakthrough that says it’s possible to get sequences beyond one million years. At 68 million years, it’s still possible to get sequences.”
In addition to the seven dinosaur sequences, Asara and colleagues isolated and sequenced more than 70 protein fragments from a mastodon. This was 160,000-600,000 years old. That provides further evidence of the staying power of ancient protein.
There are important implications for handling newly discovered fossils, Asara said. Even with really old fossils people should avoid contamination. They should also “ensure that whatever is there gets well preserved because it can be interrogated.”
The scraps of dinosaur protein were obtained from a T Rex fossil femur. This had been discovered by John Horner, of the Museum of the Rockies, and colleagues in 2003. The location was Hell Creek Formation, a barren fossil-rich stretch of land that spans several states.
The scientists faced two challenges. The first was to gather enough protein to sequence. The bone extract was in the form of a gritty brown powder that had to be rid of contaminants. Asara purified the protein – identified as collagen – and broke it down into fragments, 10 to 20 amino acids long. He used techniques developed while working on the mastodon sample.
The peptides were passed over a liquid chromatography (LC) column. This separated them from one another. They were then sprayed at extremely low flow rates, for best sensitivity, into a mass spectrometer.
This measures the mass – specifically the mass-to-charge ratio – of peptides as they come off the LC column. To get the greatest possible yield, Asara used an ion-trap mass spectrometer. This captures and holds peptides.
The collected peptides were measured for mass. In a second step, they were isolated and fragmented to reveal their amino acid sequence. Using this two-step procedure, Asara gained seven separate strings of amino acid.
He now faced his second challenge. This was to make sense of the amino acid sequences. Normally, when a sequence comes out of a mass spectrometer, it is compared to a database of existing amino acid sequences. Collagen is a protein that does not vary much with evolution.
So it was highly likely that some of the dinosaur peptide sequences would match those of an existing species. Of the seven T. rex peptides, five were for a particular class of collagen protein – collagen alpha I. These were found to be matches to amino acid sequences in chicken collagen alpha I.
For extinct species, the real goal is to find sequences unique to that organism. Asara generated a set of theoretical collagen protein sequences. These were some of the kinds that might have been present around the time of T. rex.
None of his dinosaur peptides matched the theoretical set. But this is not surprising. “If you’re only finding seven sequences in T. rex, you’re not going to find novel ones,” said Asara.
He also tested mastodon bone, sent by Schweitzer, against a database of existing amino acid sequences and against mastodon theoretical sequences. He identified a total of 78 peptides, including four that were unique.
The team will get better at doing the extractions, Cantley said. And their instruments and techniques will improve. That means they will be able to get “much more extensive sequences” from ancient fossils. They should also get “novel sequences unique to that species, which will give us ideas about the relationship between species.”
Still, it may be very rare fossils that are as well preserved as the T. rex and mastodon specimens analysed in the current study.
“Nature has to give you the opportunity to do this first,” Asara said.
Links to free teaching resources for this story
Proteins are about half the dry weight of most cells, and are the most complex macromolecules known. Each protein has a unique structure and function. Includes nice amino acids interactive.
Narrated animation of protein synthesis, starting with DNA, which is transcribed into messenger RNA, which leaves the nucleus and binds to a ribosome. Transfer RNAs then attach and build a polypeptide chain that folds into a protein.
Learn how modern scientists can use a tool not available in Darwin’s time: molecular evidence. Molecular biologists have uncovered genetic links between diverse animal species, providing new evidence of common ancestry.
Companion website to the NOVA program Curse of T. Rex, which follows the trail of legal and illegal fossil-dealing as the FBI tries to protect the best Tyrannosaurus rex specimen ever found from winding up on the shelves of a souvenir shop.
When dinosaurs ruled the Earth from Discovery School. Students will understand: The age of the dinosaurs is called the Mesozoic age. It is divided into three periods, the Triassic, the Jurassic and the Cretaceous. Plants and animals on Earth differed during these three periods…
“Building a free-standing mount of a forty-foot skeleton wasn’t easy.”
“The more we learn about dinosaurs the more fascinating they become. But how do we know they really existed? And how do we know so precisely when they lived? The fossilised dinosaur bones and tracks in this still collage produced for Teachers’ Domain offer physical evidence – the foundation of all science – of the dinosaurs’ existence.”
Lesson from a teacher designed to give students a different way of looking at genetics and biotechnology. “After reading the novel “Jurassic Park” by Michael Crichton, I decided to incorporate the ideas of the novel into my genetics unit. Crichton is a graduate of Harvard Medical School and does an exceptional job of blending science and suspense; a perfect blend for any high school science student.”
Dinosaur Detectives from Discovery School. Students will: Understand that discoveries about dinosaurs have a long history and that each palaeontologist adds his or her work to a body of fossil evidence used to support theories about dinosaurs. Find out that palaeontologists often support one theory over another until additional fossil evidence …”
“The study of dinosaurs stretches our imaginations, gives us new perspectives on time and space, and invites us to discover worlds very different from our modern Earth.” Questions and answers from US Geological Survey.
Links to more links
Archaeopteryx, ancient birds, and dinosaur-bird relationships.
Lesson plans and activities for teachers based on DNA series from PBS.