Repeated analysis of proteins from a fossilized Tyrannosaurus rex reveal new evidence of a link between dinosaurs and birds: Of the seven reconstructed protein sequences, three were closely related to chickens.
It was once thought impossible to obtain actual soft tissue, such as proteins, from fossils, but the impossible has happened and now, two research teams who published reports in this week’s Science describe their findings: the closest relative to the fearsome Tyrannosaurus rex is .. a chicken.
Scientists recovered and identified several proteins within a well-preserved thighbone from T. rex that was fossilized 68 million years ago. Fossilization occurs when minerals replace the constituents of bone, turning them to stone. In this case, the proteins deep within the bone were protected from both degradation and fossilization by the outer layers of fossilized bone. As a result, exploration of the molecular-level relationships of ancient, extinct animals, is now possible using advanced research techniques, instead of relying solely on skeletal remains.
“It has always been assumed that preservation of [dinosaur bones] does not extend to the cellular and molecular level,” said co-author Mary Schweitzer, from North Carolina State University in Raleigh.
The proteins discovered in the T. rex bones are collagens, which are tough but elastic connective fibers that connect muscles and other tissues within the body. Collagen comprises most of the organic material in bone, which consists of both minerals and protein. When the minerals are removed from bone, a flexible collagen matrix remains behind.
“The pathways of cellular decay are well known for modern organisms. And extrapolations predict that all organics are going to be gone completely in 100,000 years, maximum,” explained Schweitzer.
Previously, the oldest ancient proteins identified were from mammoths that died about 300,000 years ago. The researchers have not found any DNA, which is more fragile than proteins, and is necessary for genetic studies and for cloning experiments.
“But proteins are coded from the DNA, they’re kind of like first cousins,” Schweitzer added.
The oldest confirmed fragments of DNA have been found in Neanderthals that lived 30,000 to 50,000 years ago.
“The goal of obtaining sequences either from proteins or DNA for extinct [organisms] has been a long-standing goal to test evolutionary links and processes, or even functional information,” stated Brooks Hanson, an editor at Science.
This work builds on an earlier discovery of soft tissue — including blood vessels — made by Schweitzer’s team in the same, incredibly well-preserved T. rex fossils. Repeated analysis of the preserved T. rex proteins from these bones revealed yet more evidence for a link between dinosaurs and birds, a widely-held but sometimes contentious hypothesis based on similarities in bone structure.
“Most people believe that birds evolved from dinosaurs, but that’s all based on the architecture of the bones,” said John Asara of the Harvard Medical School and Beth Israel Deaconess Medical Center in Boston and leader of the second team. “This allows you to get the chance to say, ‘Wait, they really are related because their [protein] sequences are related.’ We didn’t get enough sequences to definitively say that, but what sequences we got support that idea.”
Three of the seven reconstructed T. rex collagen protein sequences were most similar to chicken collagens (collagen, pictured below)
Collagen proteins [Bigger and more readable image]
The first research team, headed by Schweitzer, reconstructed fragments of several dinosaur proteins found deep within the T. rex femur, or thighbone (pictured below). For example, collagen-1, the main organic (protein) component of bone, was found. Peptide fragments of the complete protein were pieced together into strands comprising the seven protein sequences. Three of these reconstructed sequences reacted with antibodies to chicken collagen. Two other reconstructed proteins appeared to be related to a frog and a newt.
The second research team, led by Asara, independently confirmed the presence of protein in the bone. To identify any proteins that could be present, this research team subjected samples of the material extracted by Schweitzer to examination by mass spectroscopy, which breaks down material into its basic components to determine its mass and chemical composition. This technology is used in medical research to precisely identify the nature of disease-causing agents.
Co-author on the second study, Lewis Cantley of Harvard Medical School, noted that this work is still in its infancy, and when it is improved he expects to be able to isolate more proteins and seek more matches.
“Knowing how evolution occurred and how species evolved is a central question,” Cantley said.
The dinosaur fossils were unearthed from rock in the Hell Creek Formation of eastern Montana, by a team headed by Jack Horner, from the Museum of the Rockies in Bozeman, Montana and co-author on the first paper. They included a skull, two thigh bones and both tibiae — lower leg bones.
“To get specimens like that involves excavating enormous amounts of material, covered with tens of feet of rock,” Horner explained. “The T. rex was under a thousand cubic yards of rock and therefore in a position not to have been invaded by bacteria or groundwater.”
As a result of these research successes, dinosaur fossil hunters are planning nine expeditions for this summer to search for more well-preserved specimens for similar tests. Additionally, several large dinosaur bones already in laboratories will also be examined for surviving traces of organic matter.
“I think we’re learning an important lesson here — that if we do get specimens like this, we spend a lot of time getting as deep into the sediment as we can in places where there has been very little atmospheric or water contamination,” Horner observed.
Matt Lamanna, a curator at the Carnegie Museum of Natural History in Pittsburgh, said the finding was “another piece in the puzzle that shows beyond the shadow of a doubt that dinosaurs are related to birds.” Lamanna was not part of either research team.
All dinosaurs, excluding birds, disappeared 65 million years ago. They are thought to have died after an asteroid impact which struck off the present-day Yucatan peninsula in Mexico.
Schweitzer, M.H., Suo, Z., Avci, R., Asara, J.M., Allen, M.A., Arce, F.T., Horner, J.R. (2007). Analyses of Soft Tissue from Tyrannosaurus rex Suggest the Presence of Protein. Science, 316(5822), 277-280. DOI: 10.1126/science.1138709.
Asara, J.M., Schweitzer, M.H., Freimark, L.M., Phillips, M., Cantley, L.C. (2007). Protein Sequences from Mastodon and Tyrannosaurus Rex Revealed by Mass Spectrometry. Science, 316(5822), 280-285. DOI: 10.1126/science.1137614.
BBC News (quotes, image).
Daily Herald (quotes).