Dark, iron-rich hematite particles may have preserved collagen protein fragments in this 195-million-year-old dinosaur rib, according to one of two independent studies of dinosaur proteins.
It’s not quite Jurassic Park: No one has revived long-extinct dinosaurs. But two new studies suggest that it is possible to isolate protein fragments from dinosaurs much further back in time than ever thought possible. One study, led by Mary Schweitzer, a paleontologist from North Carolina State University in Raleigh who has chased dinosaur proteins for decades, confirms her highly controversial claim to have recovered 80-million-year-old dinosaur collagen. The other paper suggests that protein may even have survived in a 195-million-year-old dino fossil.
The Schweitzer paper is a “milestone,” says ancient protein expert Enrico Cappellini of the University of Copenhagen’s Natural History Museum of Denmark, who was skeptical of some of Schweitzer’s earlier work. “I’m fully convinced beyond a reasonable doubt the evidence is authentic.” He calls the second study “a long shot that is suggestive.” But together, Cappellini and others argue, the papers have the potential to transform dinosaur paleontology into a molecular science, much as analyzing ancient DNA has revolutionized the study of human evolution.
Back in 2007 and 2009, Schweitzer reported in Science that she and her colleagues had isolated intact protein fragments from 65-million- and 80-million-year-old dinosaur fossils. But the claims were met with howls of skepticism from biochemists and paleontologists alike who saw no way that fragile organic molecules could survive for tens of millions of years, and wondered whether her samples were contaminated with modern proteins.
Then last year Cappellini, Matthew Collins, a paleoproteomics expert at the University of York in the United Kingdom, and colleagues succeeded in identifying protein fragments from 3.8-million-year-old ostrich egg shells, a claim that most of their colleagues found convincing. Now, the case for dramatically older proteins seems to be firming up, too. Last week in the Journal of Proteome Research, Schweitzer, her postdoctoral assistant Elena Schroeter, and colleagues report that they did a complete makeover of their 2009 experiment to rule out any possible contamination. They took new samples from the same 80-million-year-old fossil, of a duck-billed dinosaur called Brachylophosaurus canadensis. They reworked procedures for extracting would-be proteins from the bone, identified protein fragments with a more sensitive mass spectrometer, and compared the recovered protein sequences to those from many more living animals. Schroeter even went so far as to break down the mass spectrometer piece by piece, sterilize the whole thing in methanol to remove any possible contaminants, and reassemble the machine. “About the only thing that is the same [as the 2009 experiments] is the dinosaur,” Schweitzer says.
In their 2009 paper Schweitzer’s team had identified three fragments of a protein called collagen 1 from their fossil. Collagen is the main protein in connective tissue and is abundant in bone. Each fragment contained about 15 amino acids strung together, which the mass spectrometer was able to identify. In their current study, Schweitzer’s team identified eight protein fragments, two of which matched those identified originally. “If [both sets] are from contamination, that’s almost impossible,” Schweitzer says.
The three protein fragments originally recovered most closely resembled the collagen found in living alligators and other reptiles. But the new protein fragment data show that B. canadensis collagen was a better match to that of birds. That’s just what paleontologists, who consider birds to be descendants of extinct dinosaurs, would predict.
Just how those collagen sequences survived tens of millions of years is not clear. Schweitzer suggests that as red blood cells decay after an animal dies, iron liberated from blood cells may react with nearby proteins, linking them together. This crosslinking, she says, causes proteins to precipitate out of solution, drying them out in a way that helps preserve them. That’s possible, Collins says. But he doesn’t think the process could arrest protein degradation for tens of millions of years, so he, for one, remains skeptical of Schweitzer’s claim. “Proteins decay in an orderly fashion. We can slow it down, but not by a lot,” Collins says.
The second paper, published this week in Nature Communications, goes back even further in time but offers weaker evidence, Cappellini says. In this work, researchers led by paleontologist Robert Reisz at the University of Toronto in Canada reported finding what they believe is collagen in a 195-million-year-old fossil rib from a large plant-eating dinosaur called Lufengosaurus that lived in what is now southwestern China. Reisz says his team’s methods, called Raman spectroscopy and synchrotron radiation Fourier transform infrared microspectroscopy (FTIRM), can probe the chemical makeup of a sample without the need to purify it first, which lowers the risk of contamination. The rib, he and his colleagues report, absorbed infrared signals that match those of collagen from modern animals.
Schweitzer and Cappellini caution that while FTIRM is good at spotting the so-called amide chemical bonds that link successive amino acids in proteins, it can’t pin down exactly what protein is giving the signal. It also can’t rule out that the bonds are in other compounds, such as the epoxy used to assemble microscope slides. The method also can’t reveal a protein fragment’s sequence, and thus isn’t useful for phylogenetic analyses. “Synchrotron data is very powerful, but it’s limited,” Schweitzer says. “I would like to have seen confirmatory evidence,” such as exposing the fossilized material to an antibody that binds solely to collagen to see whether it targeted the fossilized material. Reisz agrees “that certainly would be the next step.” But he’ll have to team up with other specialists to carry that out.
Still, his work, too, suggests that collagen fragments can survive for astonishing periods of time. Meanwhile, Schweitzer’s team is going beyond collagen. In a 2015 paper in Analytical Chemistry, her group reported isolating fragments of eight other proteins from fossils of dinosaurs and extinct birds, including hemoglobin in blood, the cytoskeletal protein actin, and histones that help package DNA. Comparing those sequences from many different species could reveal evolution’s handiwork over geological time, much as studies of ancient DNA do today.
Her team won’t be the only ones exploring these methods. Now that it seems that very ancient protein fragments can, in fact, be isolated and examined, it’s a safe bet that many new collaborations will soon take shape to pin down the evolutionary relationships among different dinosaurs, as well as among ancient mammals and other extinct creatures.
Source: Science Mag