The Loom

Old Fourlegs Revisited

i-b3c004492975df67eed4c8545ecc628f-coelacanth.jpgLast week the world press took note of a fish hauled up off the coast of Zanzibar. (AP, Reuters). Why did they care? Because the animal was one of the most celebrated fish of the sea: it was a coelacanth.

The coelacanth is an ugly, bucket-mouthed creature. At first scientists only knew it from its fossils, the youngest of which was 70 million years old. In 1938, however, a flesh-and-blood coelacanth was dredged up near East London, South Africa. The five-foot long beast had many of the hallmarks of fossil coelacanths, such as hollow spines in their vertebrae, peculiar lobe-shaped fins, and a joint dividing its eye and “nose” from its brain and ears. The coelacanth became a celebrity in the, hailed as a “living fossil.”

Its fame was reinforced by its elusiveness. It was not until 1952 that a biologist found a second coelacanth, caught this time off the Comoros Islands. Scientists chased the coelacanth so doggedly in part because of what it might reveal about ourselves. Fossils of the coelacanth lineage dated back over 300 million years to the Devonian Period. They belonged to the same group of fishes as our own ancestors (known now as lobe-fins). While the ancestors of coelacanths stayed in the water, our own fishy ancestors climbed on land and evolved into mammals, birds, reptiles, and amphibians. (See my book At the Water’s Edge for more on this transition.)

Since the living coelacanths seemed to resemble Devonian fossils of coelacanths, the term “living fossil” seemed to carry a double meaning–a lineage that was not just lost for a long time, but also had changed very little. Scientists became particularly intrigued in the fins of the coelacanths, looking at them as potential models for what the fins our own ancestors looked like. One book on coelacanths was even entitled “Old Fourlegs.”

Scientists have learned a lot about coelacanths since then. They’ve discovered the fish not just in Zanzibar, but all the way across the Indian Ocean, in Indonesia. They’ve filmed it underwater. They’ve analyzed its genome and found new coelacanth-related fossils. The fish remains fascinating. Its DNA confirms what paleontologists saw in fossils: Of the 20,000 species of fish, coelacanths (as well as their fellow lobe-fins, the lungfish), are our closest living relatives.

But these days coelacanths don’t wear the label “living fossil” very well any more. In some ways, it turns out, it is we–not the coelacanths–that are the living fossils.

I wrote about one example of this table-turning last year in an essay for PLOS Computational Biology. Land vertebrates and coelacanths share a piece of virus-like DNA that infected a common ancestor long ago. In our lineage, that DNA became frozen in place, while in the coelacanth it continued to make new copies of itself that were reinserted in the genome. (For more details, read my essay here.)

This week, in the wake of the Zanzibar discovery, a new example has come to light. Instead of virus-like DNA, this example involves our hands and their fins.

The pectoral fin of living coelacanths (corresponding to our arm) is composed of a long chain of bones stretching out from its body. At the end of that chain (sometimes called an axis) is a symmetrical fan of smaller bones known as radials. The radials and axis are encased in a fleshy fin. Lungfish, the other living group of lobe-fin fish, have an axis made of a long chain of bones, with a symmetrical set of radials.

This similarity led some scientists to propose that our own fishy ancestors had fins with a similar anatomy. Only later did our own lineage evolve a different plan. Instead of a symmetrical set of radials, our radials began to branch off of one side. Eventually, those radials evolved into fingers and other parts of the arm. (To illustrate all this, I’ve cribbed a diagram from a new paper below. Latimeria are living coelacanths, Glyptoliepis and Neoceratodus are lungfish. Our own lineage is represented here by Tiktaalik, a Devonian lobe-fin that had evolved some of the key features of land vertebrates.)

Paleontologists have been digging up coelacanth fossils for many decades, but until now they have not found good remains of an early coelacanth fin. In the latest issue of Evolution and Development scientists at the University of Chicago describe a fin fossil they found in Wyoming. They dubbed the 360-million year old fish Shoshonia arctoperyx. While they only found part of the Shoshonia fin, it was enough for them to draw some surprising conclusions. Most surprisingly of all is that the fins of living coelacanths are probably different in some important ways from the fins of the first coelacanths.

As the illustration below demonstrates, Shoshonia had the same one-sided anatomy that we have in our arms. When the Chicago scientists drew a tree of lobe fins and noted the evolutionary steps along each branch, they found that the common ancestor of all lobe fins probably had this one-sided anatomy. Our own ancestors retained a primitive one-sided plan, which was elaborated into hands and feet. Only later did lungfish and the ancestors of living coelacanths evolve more symmetrical fins, along parallel evolutionary tracks.

To borrow from Pogo, we have met Old Fourlegs, and it is us.

[For more on our primitive hands, see this post from May on the development of paddlefish embryos.]

i-f52bee775af7f2df1c07674be1a9f2d9-fin evolution.jpg

Comments

  1. #1 matthew
    July 23, 2007

    I love the coelacanth, and I’m not ashamed to admit it.

  2. #2 SG
    July 24, 2007

    Great post. Nova ran a great special on the coelacanth a few years back (see http://www.pbs.org/wgbh/nova/fish/ ), and it does a great job of capturing the excitement surrounding the discovery that coelacanths were not mere fossils. One also learns that the coelacanth is not a good “eating fish”, probably due to its oily composition. Somehow it strikes me as funny that folks were catching (and eating) a fish that had long been believed to be extinct. “…and as an entree tonight, we’re offering honey-glazed coelacanth on a bed of couscous with fresh greens.”

    Just to echo a question from a previous post- does anyone know what a typical lifespan is for a coelacanth? It doesn’t appear to have any predators (apparently they are all extinct) and it has a very slow metabolism, so it is a good candidate for a long life.

  3. #3 JLT
    July 26, 2007

    “LungFish, the other living group of lobe-fin fish, …”

    A missing link! ;-)

  4. #4 me
    July 26, 2007

    The caught coelacanth seems yummy. Wish I could taste that. :)

  5. #5 mydear
    July 26, 2007

    “Lungish, the other living group of lobe-fin fish, have an axis made of a long chain of bones, with a symmetrical set of radials.”

    Shouldn’t that be Lungfish? This is probably a spelling error.

  6. #6 Heleen
    July 26, 2007

    Great.
    Dahn et al, Nature 18 Jan 2007, on fin development in sharks, show a very interesting dorsal fin skeleton that looks fairly comparable.

    Does anyone know when the very first fins appeared, and what their skeleton was?

  7. #7 jotetamu
    July 26, 2007

    [nitpick]
    “… became a celebrity in the, hailed as …”
    Something got lost here, apparently.
    [/nitpick]

    Is it not possible to delete posts like this, which have no merit once the typo or whatever is fixed, or simply have no merit if the nit is not in fact there to pick?

    Jim Roberts

  8. #8 Carl Zimmer
    July 26, 2007

    Joe–Thanks for catching the typo, and thanks to mydear and JLT for the same. My policy, such as it is, is to let comments about my typos and other errors stand. I also use strike marks to indicate all but the most minor fixes to the original post. If I publish a mistake, small or large, I correct it, but I also leave a record behind. I don’t want people to get the feeling that I’m secretly revising posts to hide my mistakes. I just try to do the best I can.

  9. #9 David B. Benson
    July 26, 2007

    Carl Zimmer — Which is d**n good!

  10. #10 Kristjan Kannike
    July 27, 2007

    “It was not until 1952 that a biologist found a second coelacanth, caught this time off the Comoros Islands.” The biologist was James Leonard Brierley Smith (http://en.wikipedia.org/wiki/James_Leonard_Brierley_Smith). He was also the author of “Old Fourlegs”, describing his dogged odyssey of 13 years in pursuit of coelacanths.

    I love the book for his honest description of hardship-defying obsession without which a second coelacanth would no doubt be found much later.

  11. #11 Darren Naish
    July 27, 2007

    On the 1952 coelacanth, rarely mentioned is the interesting fact that it was (apparently) a one-off oddity, lacking one of its dorsal fins. Smith wasn’t able to appreciate this right away, and assumed that he had a second living coelacanth taxon. In 1953, he dubbed it Malania anjouanae after Daniel Malan, the South African prime minister. Finding one extant coelacanth was obviously a huge personal discovery for Smith, so he must have been stunned to think that he’d documented two.

    On correcting mistakes – I think we’re all now used to the idea that nothing in the world of web-publishing is set in stone, and indeed one of its advantages over paper publishing is that we can add updates and corrections. When the changes concerned are minor errors concerning facts, typos and so on, I don’t see any harm in just erasing the old problem and pretending it never happened. After all, readability is what matters when it comes to things like blogs. If an author were to re-write their text after discovering that a published idea was fundamentally wrong.. that wouldn’t be so excusable.

  12. #12 RoaldFalcon
    July 31, 2007

    The caught coelacanth seems yummy. Wish I could taste that.

    That’s almost…cannibalistic.

    Well, compared to other fish.

  13. #13 Michael Stillman
    August 1, 2007

    Merrily, Marjorie
    Courtenay-Latimer
    noted her specimen’s
    widening fame;

    not that the issues were
    ichthyological –
    which of them had the more
    cumbersome name?

  14. #14 John B.
    August 1, 2007

    Is Shoshonia thought to be one of the first land relatives of all ensuing land animals or was it a “dead end”?

  15. #15 Carl Zimmer
    August 1, 2007

    John B. [14]: Shoshonia was a fish, using fins to swim in water. Tetrapods with full-blown digits already existed when Shoshonia was swimming around 360-million years ago. They were probably almost entirely aquatic, though.

  16. #16 John B.
    August 1, 2007

    What then are the first land-dwelling, fish-like ancestors for which there is a fossil record? Were they closely related to Shoshonia?

  17. #17 Carl Zimmer
    August 1, 2007

    John B [16]: The question is a bit trickier than it may appear. Many amphibians are not what one would call “land-dwelling,” since they spend most of their time in water, and their fossils might well be found alongside fossils of fresh-water fish. Yet it is obviously a full-blown tetrapod. The common ancestor of all *living* tetrapods (amphibians and amniotes) lived an estimated 350 million years ago, based on both fossil evidence and DNA clocks. See here.

    These early tetrapods were on the Tiktaalik branch in the tree I showed in this post.

  18. #18 John B.
    August 2, 2007

    I see what you mean. It’s a straight forward question, but there isn’t always a straight forward answer. Interesting stuff. Thanks for the link.

  19. #19 Graculus
    August 4, 2007

    It doesn’t appear to have any predators (apparently they are all extinct)

    Sharks. Which are older than coelacanths by about 100 million years, IIRC. Old Four Legs doesn’t live anywhere that is popular with large sharks, I assume there is a good reason for this.

    Of course, coelacanths happily dine on small sharks, and so ecological karma is complete ;-)