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evolution

Sonic the Hedgehog and Evolution

When I talk to Creationists, I generally run into a lot of ignorance about how we “prove” evolution to be true.  I think a lot of them really do believe that we invented the whole thing because we found some rocks that look like fish.  Young Earth Creationists are a particularly funny bunch because they get so hung up on things like carbon dating.  By raising the objections they do, they prove their own ignorance of not only evolution, but the scientific method itself.  Today, I want to share with you what I think is one of the neatest mechanisms in embryonic development, and show you how it points squarely to shared evolution between humans and other animals.

In the 1950s and 60s, biologists John Saunders and Edgar Zwilling discovered something extraordinary.  In species as diverse as whales, birds, and humans, developing embryos contained two tiny patches of tissue that seem to control the development of the appendages, whether they were flippers or wings or hands.  Remove the tissue and development stops.  Take tissue from one side of a growing wing and put it on the other and you get a “double wing” that mirrored itself on each side.

This “zone of polarizing activity” — or ZPA — remained a mystery until the 1990s, when three separate laboratories worked collaboratively to discover a gene in flies that made one end of the body develop differently from the other.  They named this gene “hedgehog.”  Before long, a version of the same gene was found in chickens.  The scientists named this gene “Sonic hedgehog,” after the video game character.

Once the chicken gene had been found, it became a rather easy matter to look for it in other creatures.  And guess what…  All animals with limbs have it.  In the developing embryo of every limbed creature on earth, the Sonic hedgehog gene literally controls what goes where and how it goes there.  The ZPA is actually a patch of tissue in which this particular gene is active, regardless of what kind of animal embryo it is.

We have since discovered that Sonic is one of several dozen genes that work in conjunction to make limbs, and in every creature that has limbs, the DNA recipe is virtually identical.   This points squarely to a common ancestor for all limbed creatures, but even more than that, it points to a common ancestor even farther back linking limbed animals to flies.  This genetic mechanism is truly ancient.

But it goes even farther.  As we all know, Evolutionary theory states that life began in the water and moved onto land.  And as it turns out, experiments with sharks and skates have backed up that claim.  Obviously, fins are very dissimilar to limbs.  Other than the fact that they protrude from the torso, there really isn’t much we can find to suggest that limbs and fins have anything at all in common.

Shark fins have tiny cartilage skeletal rods, and all of them look essentially alike.  Just from looking at them, it’s very hard to think of any reason we should expect them to form the same way as bony human hands.  But sometimes nature surprises us.  When scientists placed a tiny bead of mouse Sonic hedgehog in between several of the embryonic skeletal rods, guess what happened.

Yep.  Shark fingers.

What a crazy thing, don’t you think?  A warm-blooded, four legged live-birthing mammal with fingers and toes, and a shark with not a single bone in its body (remember, sharks have cartilaginous skeletons!), and yet, the genetic recipe for embryonic development is so similar that you can literally plug one into the other and it will still work!

Flies, chickens, humans, and sharks.  It would be hard to find four more dissimilar animals, and yet, all of them use the same biological mechanism to become what they are.  Once evolution discovered that it could build appendages, it didn’t need to invent them again.  Instead, descent with modification worked on the core process for billions of years, creating staggering diversity, from insects to birds to mammals.  But it never needed to change the formula that worked.  Sonic hedgehog is the proof.

SOURCE: Your Inner Fish, by Neil Shubin, paleontologist and professor of anatomy.

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Discussion

6 thoughts on “Sonic the Hedgehog and Evolution

  1. Very interesting article. Patterns of early development are really amazing, and everything is better understood with evolution.

    With creationists I came to a point where I don’t want to debate with them anymore. I have a somewhat well researched theory for the origin of creationism, and I think it is a regional phenomenom with concrete historical causes:

    The Origins of Creationism
    Pablo

    Posted by Pablo | February 11, 2010, 9:44 am
  2. Wow. Sometimes I read things like this and am truly in awe of the power of nature. How amazing it is that a blueprint like this can be traced back over millions of years and is still identical today. Absolutely fascinating.

    But even though it really is irrefutable proof of evolution those who believe in creation still won’t accept it. They still won’t be swayed form the ridiculous ideal that everything was created in 6 days and 6 nights because of course on the 7th day he rested. Bless.

    Posted by Emy | March 5, 2010, 1:24 pm
  3. I came here in a totally agnostic frame of mind, following a link at the bottom of one of my blogs. All was going well until I came to……

    “Remove the tissue and development stops. Take tissue from one side of a growing wing and put it on the other and you get a “double wing” that mirrored itself on each side…….”

    So you are saying that, in order to prove a point, it’s ok to mutilate various creatures and create Frankenstein-ish monsters out of them? I think I’d rather stick to the creationists, they may have some odd ideas, but mindless cruelty doesn’t seem to be one of them!

    Posted by Dave Howes | April 22, 2010, 8:54 pm
  4. So you are saying that, in order to prove a point, it’s ok to mutilate various creatures and create Frankenstein-ish monsters out of them? I think I’d rather stick to the creationists, they may have some odd ideas, but mindless cruelty doesn’t seem to be one of them!

    Dave, thanks for the comment. I’d like to point out that your argument doesn’t follow. That is, the truth or falsehood of evolution or creationism doesn’t have anything to do with the morality of the scientists who discovered it.

    While I’m on that subject, you might be jumping the gun a bit on your moral judgment of embryologists. The embryos they were working on were very early in development, and in general, it’s safe to say that they had not yet developed the complex neural pathways necessary to feel pain. The deformed embryos were not allowed to develop to term, so the scientists were not keeping hideous monsters alive. But even if they were, does that mean that what they discovered wasn’t true?

    Here’s something else for you to chew on. I don’t know if you’re aware of this, but in the early days of scientific inquiry into anatomy, the CHRISTIAN point of view was that animals were automata, incapable of feeling pain, emotion, or stress. They were simply mimicking human reactions. God had given humans a soul, and the soul was what gave us true life.

    Because of this CHRISTIAN belief, CHRISTIAN doctors nailed living dogs to planks and cut them open to see how they worked. All the while, the poor animals were yelping and wailing, and the CHRISTIAN doctors worked happily away, content in the knowledge that the animals weren’t really feeling any pain at all.

    So, by your logic, I should believe that bones, arteries, and organs don’t exist, right? I mean, the people who discovered them were immoral, so their conclusions must be wrong.

    Obviously, we can judge people’s morality separately from their discoveries. The most hideously immoral person might also be the most brilliant scientist alive, and while that’s regrettable from a human standpoint, it doesn’t make any sense to proceed as if their objective scientific work was all wrong.

    Incidentally, if you feel as strongly as you do about the suffering of animal fetuses, there are organizations in which you might feel right at home. Maybe you could channel some of that righteous indignation into protesting the testing of animals for cosmetics or something like that. Check out PETA’s homepage. But be warned. Most PETA members know that evolution is true.

    Posted by hambydammit | April 23, 2010, 12:50 pm
  5. Hello
    Don’t get me wrong, I’m not a Creationist, and I’m quite convinced about evolution too, it’s just that I have one niggling question that question in the back of my mind – not only do you have to have something to evolve to, you also have to have something to evolve from.
    My specific point was on the morality of the experiments, the premise being that science KNOWS there is no chance of pain or distress. Science has been getting it wrong for several millennia now, and I am as disinclined to believe blindly in absolute scientific truths as I am Creationism. I know that Christianity has always seen the animal kingdom as a well stocked larder and not much else, (although we are talking about post Holy Roman Empire Christianity here), but surely our ways of thinking should have evolved over the past thousand years or so, along with our little toes?
    I always feel a bit sorry for hard-line scientists, and Creationists for that matter, as I feel they are both missing out on a major aspect of what it is to be human.

    On the more general subject of your blog, any engineer or designer knows the value of designing multi-purpose components that can be used over again for different purposes ( or porpoises 🙂 ). Surely any Deity worth his/her/it’s salt would know the same thing?

    Posted by Dave Howes | April 25, 2010, 7:13 am
  6. not only do you have to have something to evolve to, you also have to have something to evolve from.

    Actually, that’s not a very good way of putting it. Evolution is not a journey to a destination. It’s more like a treadmill. If you’ve never read “The Red Queen,” by Matt Ridley, I’d highly recommend it. It’s one of the most accessible books for non-scientists to understand the mechanisms of evolution through the development of human nature. You can get a used copy on Amazon for just a few bucks.

    My specific point was on the morality of the experiments, the premise being that science KNOWS there is no chance of pain or distress. Science has been getting it wrong for several millennia now, and I am as disinclined to believe blindly in absolute scientific truths as I am Creationism.

    Actually, you’re a little backwards on this, too. Science is more like the graphs from beginning calculus, where one variable continually approaches, but never arrives at zero. Science never claims absolute certainty. At least not good science. Instead, it works on completeness of the evidence and probability of accuracy. Probability math, as you should know, is certain. That is, IF all of my information is correct, and the math says there’s a 33% probability of something, then it’s 100% certain that there’s a 33% probability. (I know that’s a little convoluted, but it’s very important.)

    Some things are so overwhelmingly documented that it’s just philosophical pedantry to say that they are not certain. For instance, regardless of the accuracy of the theory that describes the behavior of gravity, we can say that unless something is universally and monumentally wrong, gravity exists. And by something wrong, I mean like… the whole universe is a giant illusion and we’re part of some gigantic experiment in virtual reality, or something equally preposterous.

    Similarly, evolution exists, and ironically, we actually have much more corroborative evidence for the behavior and nature of evolution than we do for gravity! So, we don’t have to bow to the altar of scientific certainty, but we’re perfectly justified in saying that as far as it’s possible to know something at the present, evolution is a fact. If we find out tomorrow that aliens are controlling our minds and we really live in vats, well… we’ll change the theory. But for now, 100% of the evidence points squarely at evolution. There is literally no other scientifically reasonable option.

    I know that Christianity has always seen the animal kingdom as a well stocked larder and not much else, (although we are talking about post Holy Roman Empire Christianity here), but surely our ways of thinking should have evolved over the past thousand years or so, along with our little toes?

    Not really. Evolution takes thousands of generations. At 30 years per generation, we’re only a couple hundred generations away from rubbing two sticks together to make fire. There hasn’t been a stable enough environment or enough generations to exert evolutionary pressure on our brains for any significant evolutionary adaptations since the Holy Roman Empire.

    The one thing that has changed significantly happened more or less during the Enlightenment, when reason and science became the trusted method for acquiring knowledge. But that wasn’t evolutionary. Our brains were perfectly capable of understanding it before. We just hadn’t stumbled onto the truth of it.

    By the way, if you haven’t read THIS ARTICLE about exactly what science is, you should.

    I always feel a bit sorry for hard-line scientists, and Creationists for that matter, as I feel they are both missing out on a major aspect of what it is to be human.

    You shouldn’t. You misunderstand science, and that’s why you think there is such a thing as “hard-line scientists.” The whole point of science is that truth is only as good as the data, and that the data is always subject to change. Every good scientist knows this. It’s the fundamental upon which the entire philosophy of science is built.

    On the more general subject of your blog, any engineer or designer knows the value of designing multi-purpose components that can be used over again for different purposes ( or porpoises ). Surely any Deity worth his/her/it’s salt would know the same thing?

    Well, with regard to the Sonic gene, you’re suggesting that multi-use genes could point to a creator as easily as they could point to evolution, and in a vacuum, that’s true. Engineers can invent multi-purpose tools. Of course, in the world of evidence, the argument doesn’t work nearly as well. The problem is that the theory of evolution parsimoniously and elegantly explains why genes like Sonic exist, and there is simply no need to suppose a creator. Literally all of the evidence — 100% of all the evidence ever gleaned from the study of life points squarely towards evolution. So while it’s true that multi-use genes don’t disprove a creator, they don’t lend any support to the idea at all.

    Posted by hambydammit | April 25, 2010, 2:01 pm

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