Preserved protein from an 80-million-year old dinosaur support the dinosaurian origin of birds

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

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.


hadriosaur

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.

9 Comments

  1. Hempenstein
    Posted May 6, 2009 at 7:03 am | Permalink

    Peptide bonds are tough – nylon’s made out of them!

  2. Reginald Selkirk
    Posted May 6, 2009 at 7:55 am | Permalink

    Protein sequencing does not suffer from the same contamination problems as PCR-based DNA sequencing. It is also less precise, since it cannot see synonymous mutations (same amino acid residue coded by different DNA codons). The precision in the current case will also be limited by the short lengths of protein involved.

    • Hempenstein
      Posted May 6, 2009 at 11:55 am | Permalink

      Sorry, but, while that might seem to be the case, it isn’t. There’s too much noise in comparing DNA sequences, given that there’s a 25% chance of a match without any relatedness. This is well-established, and there’s even a paper in Nucleic Acids Research (31:3537-9, 2003) to that effect – also available here:
      http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=169015

      (This is just one example that points this out – many more can be found.)

  3. Posted May 6, 2009 at 8:04 am | Permalink

    I enjoy your blog and have a general comment.

    Many of your readers (like me) probably don’t have a subscription to Science and therefore can’t see the articles you are linking to. You might want to excerpt an important paragraph here and there.

    • whyevolutionistrue
      Posted May 6, 2009 at 8:07 am | Permalink

      I think you can access this article without a subscription. Let me know if you can’t.

  4. Jeremy
    Posted May 6, 2009 at 9:50 am | Permalink

    I also can’t access the Science articles from home, since I’m not a subscriber. (I have to access them via my university’s computers.) You can obviously still get the abstracts free of charge, though. This restriction is normal for the top journals though, isn’t it?

  5. CharlesInCharge
    Posted May 6, 2009 at 10:02 am | Permalink

    This proves nothing!

    God put the protein in those dinosaur bones (which he put in the ground to test our faith) in order to test our faith.

    And you can’t prove that those fossils ever had any kids!

    Etc.

  6. Pdiff
    Posted May 6, 2009 at 11:14 am | Permalink

    Dear Mr. Csotonyi,
    I’ve always been a fan of the natural science illustrator. Your work on your WEB site is truly inspiring.

    Pdiff

  7. Posted June 17, 2010 at 5:21 pm | Permalink

    But note the following:
    http://en.wikipedia.org/wiki/Origin_of_birds
    “A second molecular study robustly supported the relationship of birds to dinosaurs, though it did not place birds within Theropoda, as expected. This study utilized eight additional collagen sequences extracted from a femur of Brachylophosaurus canadensis, a hadrosaur.[4]”

    The study shows that birds did not descend from theropod dinosaurs!


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