In this week’s Science we find a paper by Schweitzer et al. (total of 16 authors!) that has a quite remarkable result. (See the one page summary by Robert Service here.) The upshot is that protein-sequence data from an 80-million-year old duckbilled dinosaur supports the dinosaurian origin of birds.
This story has a bit of a tortuous background. First of all, we’ve known for a long time that birds evolved from gracile theropod dinosaurs; the fossil and anatomical evidence is given in WEIT. But for some folks, DNA-based data is more convincing than is the fossil record. And molecular evidence is what Schweitzer et al. provided.
In 2007, her group published a paper purporting to show that fragments of the protein collagen (a structural protein found in blood vessels and connective tissue), taken from a 68-million-year old fossil of T. rex, showed that the protein sequence (which reflects the DNA sequence) was more similar to that of birds than to that of modern reptiles. This strongly suggests that birds evolved from a lineage of dinosaurs that had already branched off of the lineage that gave rise to modern reptiles. This meant that birds and dinosaurs are each other’s closest relatives compared to say, turtles or iguanas. In fact, many systematists say that birds are dinosaurs, which is the conclusion you’re forced to if you’re a hidebound cladist. I mentioned this paper in WEIT in footnote 11 on p. 237 (this was probably the last thing I put in the book). Of course this result was no surprise to paleontologists.
As recounted by Service, this result was sharply questioned by other researchers, who claimed that the protein sequence was due to contamination; many others thought it was hard to believe that any protein could survive for so many millions of years. I was a bit depressed about this, because I had already put the result in my book and there was no opportunity to change it before publication. Also, it was just such a cute result — the kind of new finding that gets our juices flowing as scientists.
Well, Schweitzer and her team persisted, and her new result supports the old one. This time the group extracted protein fragments from the femur of a duck-billed dinosaur, Brachylophosaurus canadensis, collected in Montana. Great care was taken to prevent contamination. They found that some of the elements in the demineralized bone bound to antibodies made against bird collagen, indicating that collagen fragments similar in sequence to those of birds were present in the bone. The same was found with antibodies for hemoglobin and two other structural proteins.
Some biochemical wizardry on bone extracts identified eight fragments of collagen, and their sequences (also determined in a separate lab to preclude contamination) were determined. Sure enough, they were similar to the T. rex fragments that were questioned earlier, and were more similar to collagen sequences of modern birds than to those of modern reptiles. (See the phylogeny below.) Note that there’s a small disparity between the fossil and biochemical evidence: T. rex, as a theropod (the group that gave rise to modern birds), should be more closely related to modern birds than is B. canadensis. This discrepancy might be an artifact of not having a complete protein sequence.
Let’s be clear: the phylogeny that we get doesn’t really tell us anything we didn’t know before. Birds are highly-evolved dinosaurs, and that was already confirmed by the fossil record. Still, it’s nice to have this molecular confirmation, and perhaps the most surprising result is the ability to determine the sequences of protein fragments that have survived for millions of years. If this can be done with other fossils, we’ve suddenly gained the ability to solve many long-standing puzzles about ancestry and evolutionary relatedness.
Phylogeny showing closer relationship between dinosaurs and birds (Gallus = chicken, Struthio = ostrich) than between either of these groups and modern reptiles (alligators and Anolis lizards). Ergo, birds are dinosaurs.
Brachylophosaurus canadensis, the duck-billed dinosaur whose proteins were sequenced. Illustration by Julius T. Csotony from Science article.
Schweitzer, M. H. et al. 2009. Biomolecular Characterization and protein sequences of the campanian hadrosaur B. canadensis. Science 324:626-631.