Of Birds and Thumbs

By Carl Zimmer | June 17, 2009 1:04 pm

limusaurus.jpgMeet Limusaurus. It is not–I repeat NOT–the missing link between anything. And yet it is still an important fossil that may help us understand how birds evolved from dinosaurs.

The recent splash about a certain fossil primate has revealed yet again just how much a lot of people (sadly, including a lot of journalists) want to cling to the notion that paleontologists are only interested in missing links–which, I guess, are supposed to be the direct ancestors of some living group of organisms that are precisely halfway between primitive forerunners and the advanced living creatures.

This notion is wrong in many ways. First of all, the entire body plan shared by, say, living birds did not leap into existence in a single ancestor. In fact, what we today consider the bird body plan actually evolved through a long series of steps. Different parts of bird anatomy evolved at different times. It’s now generally agreed that birds descend from a group of dinosaurs called theropods that includes lots of famous two-legged species like T. rex and Velociraptor. Many studies show that feathers had already evolved in forms seen on birds today on dinosaurs long before they had wings, beaks, or lots of other adaptations that all birds today have. So looking for “the” missing link for birds is ridiculous from the get-go.

The obsession about missing links is wrong-headed for another reason. Paleontologists can learn a lot about the history of a living group of organisms without unbroken chain of direct ancestors (which is fortunate, because the fossil record is far too scrappy to ever uncover such a series). That’s because the evolution of animals is, in many ways, like branches growing from a tree. So paleontologists can look at the branches of a related group of species and note which species have which traits. There are some traits that all the species share, which were already in place in their common ancestor. And there are some traits that are only found among a smaller group of related species within the tree. Those are new traits that evolved later than the earlier traits. Scientists can then mark nodes along the tree to show how new traits evolved, and how old ones got modified into new ones. And as scientists discover new fossils, they can come up with more detailed hypotheses for the pattern of this change.

wagnerbirdhand.jpgLimusaurus, which makes its debut in tomorrow’s issue of Nature, sheds light on one particularly contentious matter about the origin of birds: how the fingers in a dinosaur hand got transformed into the end of a wing. A bird wing starts out as a limb bud, inside of which tiny clusters of cartilage cells start to form. Eventually, these clusters stretch out into wrist and finger bones, which later fuse together to provide a bendable spar that can support flight feathers. (To the left here is a simple diagram of the bones in a chicken’s hand.)

2-3-4.jpgOnly three digits form in a bird limb bud. Many lineages of land vertebrates have lost one or more digits over the course of their evolution. But which digits did the birds lose?

Scientists have generally argued for one of two possibilities. One possibility is that the thumb and pinky were lost, as I’m illustrating with my own hand here. That’s the pattern that has evolved in other land vertebrates, such as in horses. And when developmental biologists look at the clusters of cells that first develop in bird limb buds, they appear where the middle three digits appear in other land vertebrates. This alternative goes by the name 2-3-4. (Scientists name the five digits of the hand or foot starting from the thumb [1] and going out to the pinky [5].)

1-2-3.jpgwagnerarchhand.jpgalligatorhand.jpg

Yet a number of paleontologists have argued that the fingers in a bird’s wing are actually 1-2-3, as I’m showing in my second self-portrait. In early birds, such as Archaeopteryx, the fingers were not yet fused. As a result, they can give a clearer look at the anatomical connections between the bones and how they compare to other land vertebrates. I’ve lined up Archeopteryx‘s three-fingered hand with the five-fingered hand of an alligator, the closest living relative of birds. That top white digit looks a lot more like a stout thumb than a slender index finger. More evidence offered in favor for the 1-2-3 hypothesis comes from how the digits of early birds and related dinosaurs make contact with wrist bones as digits 1, 2, and 3 do in other land vertebrates.

Last year scientists at Yale decided to investigate this intriguing paradox by investigating the genes that build bird wings. In all land vertebrates, the same set of genes help set the identities of the digits. They found that in both mammals and alligators, there’s a key difference in the genes that are active in the thumb and in the four other digits. In the four other digits, a gene called HoxD-11 is active late in development. In the thumb, it’s silent. That difference may be a crucial reason why thumbs are so different from other fingers.

But in birds, the scientists found, something odd happens. In digit 2 (corresponding to our index finger), HoxD-11 is silent. One way to interpret this result is as follows: birds really do have 2-3-4 hands. Their dinosaur ancestors lost their thumb and pinky. And they also evolved a shift in the pattern of gene activity in their hands, so that HoxD-11 stopped switching on in the index finger. As a result, it became thumbish.

limusaurushand220.jpgEnter Limusaurus. This 1.7-meter-long dinosaur was recently discovered in China. It’s interesting for a lot of reasons, such as the fact that it appears to be one of several examples of a carnivorous theropods giving up meat and becoming a plant-eater. (The simple feathery covering is inferred from discovery of feathers on other dinosaurs.) Limusaurus is also interesting for its hands, shown here. After carefully analyzing the different bones that make it up, Limusaurus‘s discoverers have concluded that it has digits 2,3, and 4–plus a tiny digit 1.

This vestigial thumb has only a single bone left, the metacarpal at its base. The scientists conclude that by the time Limusaurus evolved, some features of the bird hand had already evolved–namely, a lost pinky and a vestigial thumb. But the remaining fingers had not yet undergone further changes seen today in birds, such as the thumbiness of the index finger.

To see how the Limusaurus  hand fits into the overall hypothesis about bird evolution, you can take a look at the tree the authors publish in their paper, reproduced below. Limusaurus belongs to the branch marked Ceratosauria. All living birds belong to Neornithes at the top. It will take future fossil discoveries to put this hypothesis to the test. But it’s already a fascinating synthesis, showing how an ordinary five-fingered hand evolved over millions of years into many new forms, including one three-fingered arrangement that you can see soaring overhead today.

Update: Co-author Jim Clark gets into some of the details of the research in his comment on my post. Thanks, Jim.

Reference: Xing Xu et al, “A Jurassic ceratosaur from China helps clarify avian digital homologies,” Nature 459:940 doi:10.1038/nature08124

Images: Limusaurus hand and tree from Nature paper. Hand diagrams from Vargas et al, PLOS One 2008. Limusaurus reconstruction by Portia Clark Sloan.

birdhandtree.jpg

CATEGORIZED UNDER: Darwinius, Evolution

Comments (21)

  1. I saw this at SVP last year and I’ve been waiting waiting waiting for it to come out! I’m actually kind of wierded out by the finger transformation, because fingers 2-3-4, which have different phalangeal counts than 1-2-3, would then develop the phalangeal formula for 1-2-3? That strikes me as odd, and roundabout…why not just keep 1-2-3?!

    I’m not saying I disagree with it, I just need to read the paper. :-)

    I’m MORE interested in the fact that Limusaurus is a toothless, beaked, stubby-armed, long-legged CERATOSAUR.

  2. Great blog! Teachers working on lesson plans for next school year’s science fair modules can use the information in this blog to design lessons that provide students with an updated view of how birds and dinosaurs are related.

  3. Extra awesome post, Carl…even if you’re, ahem, giving us the fingers.

  4. Darren Garrison

    On having googled for more information on this find (before finding the link to here) I found this:

    http://www.eurekalert.org/pub_releases/2009-06/osu-drn060809.php

    Anyone read the original paper this refers to and have comments?

  5. Darren, that paper was discussed by Darren Naish recently as being rubbish. Interestingly, the anatomical implications are quite interesting–the paper suggests that in modern birds, knee-driven walking is intricately tied to respiration, and were that mode of leg movement interrupted, some of the pulmonary air sacs would collapse. That’s great, but by suggesting then that such a finding means that birds could not have evolved from dinosaurs, the authors accidentally assert the following:

    1) Birds could not have evolved from crurotarsians, either;
    2) Birds could not have evolved from basal archosaurs;
    3) Birds could not have evolved from basal birds (Archaeopteryx, Confuciusornis, etc.).

    So there you have it.

  6. Darren Garrison

    Zach, thanks for that reply. When you search for Limusaurus inextricabilis on Google News, one of the top hits is a piece by an AIG creationist about the “knee walking” paper. Which is odd because neither of the search words seems to actually be on that page.

  7. cromercrox

    Did you say M*****g L**k? Come over here and say that, punk.

  8. When the dinosaurs became extinct 65 million years (+ or -) ago, some must have survived for the birds to evolve from. When we think of the extinction of dinosaurs, we think of total extinction. Since some must have survived to continue the line, any idea what they were? As a non-scientist, I have wondered about this. Thanks and I really enjoy your blog. Gavin Craig

    Carl: Bird were already flying for perhaps 80 million years when the KT mass extinctions occurred.

  9. subramanyam

    very interesting. I have read that young ‘HOATZIN’ have claws in their wings,used for climbing trees. Is that a relic or a featured which evolved later on?

  10. Rather than repeat what I just posted at Pharyngula, I’ve said my piece about homology and theropod digits there.

    For Subramanyam: hoatzin chicks have claws on their wings, but they are deeply nested among other birds that do not. So they have reversed the situation. Note that evolution doesn’t always “lose” features, though: genetic controls evolve that suppress the development of features whose genes are still hanging around in the genome. (Hence, among other things, the occasional dolphin with hindlimbs.)

  11. John Kwok

    @ Gavin -

    To be more precise, you should have said non-avian dinosaurs, since there was one monophyletic theropod dinosaur lineage that not only did survive, but indeed, has flourished; birds.

    @ Carl -

    This may be one of the best posts I’ve seen from you pertaining to paleobiology. Too often both the general public – and unfortunately, even some publicity-hungry scientists too – tend to be a bit “obsessed” with “missing links”, when it’s really more interestitng to look at certain lineages just to note how “bushy” they are, as for example, the terrestrial “primitive”
    placental mammal – seagoing cetacean transition, or the for more classic one – I suppose – showing the gradual transition from lobefinned fishes like Eustenopteron to the “fishapod” Tiktaalik, through “primitive” tetrapods like Acanthostega and Icthyostega.

    BTW I believe I heard Alan Feduccia making a big fuss over which digits had fused, claiming that he had some important “evo – devo” evidence refuting work by Ostrom, Gauthier et al. with regards to the dinosaurian ancestry of modern birds. I wonder what he’ll say after he reads the Xing Xu et al. paper in Nature.

  12. Darren G:
    There is some discussion of the Ruben paper here:
    http://dml.cmnh.org/2009Jun/threads.html
    much of it quite rational. What was said in the actual paper is rather different from what the authors said in their press release, but that’s probably another story…

  13. Kilian Hekhuis

    Oh no! The Loom has been Kwok-infected

  14. Gerdien de Jong

    I hope it’s not my browser, but the text to the right of the figures totally disappears in all right margin stuff.

  15. John Kwok

    @ 13 -

    Beg your pardon? I’ve dropped by here before, and with good reason. Consistently Carl has had the best entries I have seen with regards to evolutionary biology. His latest on Limusaurus is a classic example for two reasons:

    1) It is quite succinct and therefore gets to the point.

    2) Carl does a much better job than, for example, a certain “evo-devo” biologist blogger in explaining why this discovery is important to the average, interested layperson.

    As for my previous comment, Alan Feduccia is an ornithologist at, if I’m not mistaken, University of North Carolina, Chapel Hill. He is also among the very few who still doubt the dinosaurian ancestry of birds. A few years ago I heard Feduccia give a talk in which he claimed he had “evo – devo” evidence refuting the now robust hypothesis first proposed by the late John Ostrom (although, to be historically correct, the very first one was none other than Thomas Henry Huxley) that birds are living descendants of small theropod dinosaurs.

  16. Jim Clark

    Thanks for the great post, Carl, it is nice to see our work explained in longer than one paragraph and in a way that doesn’t depend on the word “dinosaur” to dazzle readers. I agree wholeheartedly with the comments about missing links, although I’d like to expand on your post and bring it around to a related concept. The really intriguing thing for us in doing this paper was that it all revolves around identifying the positions from which a phylogenetic analysis starts, what are called primary homology statements.

    In a typical phylogenetic analysis one simply identifies characters that differ among species, more or less assuming that you know what it is you should be comparing. But here, obviously, the question was where to start in making comparisons. We couldn’t simply have a character, “Manus digit I, with no phalanges (0) one phalange (1) or two phalanges (2)” without being sure we knew which digit is the first one in each tetanuran.

    This is a problem that is largely ignored by morphologists, with a few exceptions such as Martin Ramirez, Olivier Rieppel, and my former student Maureen Kearney. The approach we took is one that is more common in molecular sequence analysis, such as the program POY by Ward Wheeler, where alternative primary homology statements are compared against one another, to see how congruent different primary homology hypotheses are with other characters. We therefore coded ceratosaurs and each major group of tetanurans as either 1-2-3 or 2-3-4 and compared the results. If birds are indeed 2-3-4 then it is far simpler for all tetanurans to be 2-3-4 than the alternative.

    Now to bring it around to missing links – at the level of character analysis there are certain taxa, such as Limusaurus, that change primary homology statements by showing some kind of intermediate condition. In some sense these forms can be considered to provide missing links between homologies.

    In closing I need to add that Xu Xing was the primary author and the person who realized first what was going on with this animal, but because he is in the field right now he isn’t available for interviews, so I have been the spokesman for the paper this week.

  17. John Kwok

    Jim -

    Thanks for stopping by. I remember Feduccia was claiming that the evo-devo research he had cited showed a 2-3-4 digit arrangement, and for him, that clearly disproved dinosaurian ancestry (In the interest of full disclosure, I am a former invertebrate paleobiologist, so it’s not really clear to me why Feduccia thought this was relevant.).

    Appreciatively yours,

    John

  18. Peter Dodson

    Excellent essay Carl. I am certain that Jim And Xu Xing are delighted with it! Especially glad to see you trashing the concept of the missing link. I will have to listen at next SVP at how many times I hear the term “missing link.” My bet is somewhere between zero and none. Ted Daeschler and Neil Shubin are too too shrwed to claim this status for Tiktaalik.From my perspective it entirely a press concept, a holdover from high school science class.

    Keep up the great work!

    Peter from Gansu, China

  19. Alexander Vargas

    Vargas, AO, Wagner GP, and Gauthier, JA. Limusaurus and bird digit identity. hdl.handle.net/10101/npre.2009.3828.1

    Here is our response to the Limusaurus paper. It was recently rejected by nature, not for any technical reason but because it was considered not to be of sufficient interest/importance.

    We have uploaded it at the nature precedings citable archive, because we think it is important there is a quick and citable reply that unlike Xu’s proposal, is consistent with the view of the larger community of theropod paleontologists, namely, that tetanuran digits still are I, II, III. We are preparing a longer paper on this topic.

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The Loom

A blog about life, past and future. Written by DISCOVER contributing editor and columnist Carl Zimmer.

About Carl Zimmer

Carl Zimmer writes about science regularly for The New York Times and magazines such as DISCOVER, which also hosts his blog, The LoomHe is the author of 12 books, the most recent of which is Science Ink: Tattoos of the Science Obsessed.

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