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Nov 26 2009

Wonderful Life

You might have noticed I’m happy to go off into areas of science which are not my own with gleeful abandon, applying a physicist’s mind to what I find. This week I’m off to see the biologists, Mrs SomeBeans is a zoologist – so in a sense, I sleep with the enemy. Twitter puts me in touch with so many more of the biological persuasion.

Animals are very well covered in TV documentaries but their view is somewhat partial, favouring the furry and cute, but there’s so much more.

I think my appreciation for the wonders of the tree of life was first stimulated by “Wonderful Life” by Stephen Jay Gould. This book describes the fossils uncovered in the Burgess Shale a collection of fossils, from almost the very earliest life in the record – 500 million years ago. Gould is making two key points in his book, the first is that the so-called Cambrian explosion of species threw up a very diverse range of body plans; his second point is that the ones that survived did so almost by chance, there wasn’t anything obviously superior about them. Richard Dawkin’s book, “The Ancestor’s Tale” is also well worth a read.

I should explain body plan: this is the overall layout of the animal. So for the tetrapods (including mammals, reptiles, amphibians, birds) you get four appendages. Insects: the plan is six legs, three body parts and an exoskeleton. I’m reliably informed that snakes are tetrapods, although I’m struggling with this particularly since one of my advisers previously tried to persuade me that mohair came from mo’s. Basic rule seems to be that you can lose bones, fuse bones but not gain bones.

Once you appreciate this body plan stuff you start to get offended by representations of mythical beasts like angels and centaurs: they are clearly mammalian so angels can either have wings or they can have arms, they can’t have both! Similarly centaurs can either have two pairs of horsey legs and no arms or a pair of horsey legs and a pair of arms, what they can’t have is two pairs of horsey legs and two arms. The only reason I’m letting off the fairies is that I suspect they might be insects.

As a physicist, I really like this approach. Physicists basically have poor memories which shapes their approach to science, they like nice simple rules that encapsulate as much information as possible. You also spot them looking for “the simplest possible” model. So being freed from the requirement to learn lots of animal names by the simple expedient of calling them all ‘tetrapods’ is great. It’s true that the tree of life is more complicated than that but the principle is there. If you want to go into this in more depth, the technical name for this study is phylogenetics… *time passes as I get completely distracted*

At this point I originally made my usual error by referring to other living animals as being further down the tree of life: they’re not, we’re all leaves on the surface of the tree. A good way to wind up a biologist is to refer to another currently living species as ‘primitive’. They don’t like this because, as they point out, they’ve been evolving for just as long as us! So, rather than referring to them as ‘primitive’ here are a couple of distant leaves: Hagfish are the only vertebrates to have a skull, but not a spinal column. They evade capture by covering themselves in slime. And as for tunicates, they start with a notochord (a precursor to a spinal cord) as larva but give it up as adults which indicates a certain bloody-mindedness that I admire. (it’s a tunicate that decorates this post, at the top).

Rather more interesting than yet another furry animal…

7 comments

  1. Jim

    "And as for tunicates, they start with a notochord (a precursor to a spinal cord) as larva but give it up as adults which indicates a certain bloody-mindedness that I admire."

    I enjoy the notion that they're being bloody-minded ;-)

  2. Billy No-Job

    Colin Tudge's book, "The Variety of Life" is a great overview of evolutionary taxonomy which has useful things to say about "body plans".

  3. SomeBeans

    @Jim – well, they have the valued notochord, obvious sign of superiority, in their grasp and then they throw it all away!

    @Bill Gotta-Job – ta for the recommendation. I'm reading Mrs SomeBean's JZ Young 'The Life of Vertebrates', which is a little dated and focuses on the vertebrates.

  4. paolov

    Nice venture into the terrifying realms of fuzz-free zoology! The whole not-gaining-bones thing is a bit misleading, since bones must have been gained at some point and it still occurs occasionally (look at giraffes' ossicones or the dermal bony plates of armadillos).

    Nonetheless skeletal bone has tended to remain remarkably conservative in the tetrapod bodyplan and there is a strong tendency to reduction rather than expansion in a number of groups (cecaelians, snakes, whales, birds, mosasaurs, ichthyosaurs, and so on).

    The trouble with biology is that although it is constrained by natural laws, most of what happens is described by rules, which tend to have plenty of exceptions. That's the beauty of biology – there's always something that puts a spanner in the works of most broad theories and concepts.

  5. SomeBeans

    @paolov – I think the fuzziness and not quite following rules is what put me off biology, that and the smell of dissections in O level biology!

  6. Nora Lumiere

    Such a funny and informative post. You do write beautifully. Especially for non-scientists like me.
    Love the idea of a body plan, like biological architecture.
    I too have been perturbed by centaurs’ having arms and four legs
    and have definitely thought about fairies when looking at a dragonfly.
    I wonder though, how a hagfish can be a vertebrate if it has no spine?

  7. SomeBeans

    @Nora_Lumiere thank you for your very kind compliment. I write terribly slowly, but I'm aiming to be comprehensible for the non-scientist – it's a bit easier to do this in biology, because I'm not a biologist!

    I believe the spine is the bony thing, you can be vertebrate without a bony spine but with some sort of rigid structure called a notochord in it's place.

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