New Limbs from Old Fins, Part 1

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September 8, 2011 Tags: History of Life

Today's entry was written by Stephen Matheson. Please note the views expressed here are those of the author, not necessarily of The BioLogos Foundation. You can read more about what we believe here.

Picture an animal – any animal, maybe your favorite animal. Then ask a nearby kid to name her or his favorite animal. I think it's a pretty safe bet that neither of you chose a sponge or a sea squirt, or a planarian or a sea pen, or a moth or a mosquito. And let's hope that neither of you chose a tapeworm or a trombiculid mite. Those unlikely choices are all animals. But it's more likely that you both chose a vertebrate, and I think it's highly likely that you both chose a tetrapod vertebrate – an animal with legs and/or wings, a skull and a backbone. Maybe we prefer these creatures because they're a lot like us, or because they make good pets (or food), or because they're big enough to make an impression, or because they were the animal representatives pictured on the ark in board books. (Or maybe you chose a butterfly, and now you feel a little left out.) What matters is that there is something extra interesting about tetrapod vertebrates.

As you might have guessed, tetrapods are vertebrate animals that have four limbs. The group includes reptiles, amphibians, birds, mammals... you know, the usual suspects. (Snakes and whales, which don't have those limbs, are nonetheless classified as tetrapods, and we'll come back to that.) At first, this might look like a wildly diverse crowd of animals with almost nothing in common: tiny hummingbirds in the air, gigantic whales in the ocean, frogs that come from tadpoles, salamanders that can regrow severed limbs, cats that eat only other vertebrates, misnamed “bears” that eat only eucalyptus leaves. But on closer inspection, some extraordinary patterns emerge. These animals, in all of their magnificent variety, seem to be built in very similar ways. It's as though some kind of master plan has been tweaked over and over, to make a huge collection of variations on a theme.

This master plan for building tetrapods includes numerous components: plans for building backbones, for making skin, for growing a brain. Some of those components are unique to tetrapods; some are more widely employed in animals. Our focus will be the one that is most clearly associated with the tetrapods. We will explore the building of limbs – arms, legs, wings and flippers; and hands, feet, paws and paddles.

Consider, then, the human forelimb, better known as the arm. You may already be familiar with its skeletal structure, nicely illustrated in the 17th-century chalk drawing below.

The upper arm contains a single large bone, the humerus, which is attached to the shoulder and to the elbow. The lower arm sports two parallel bones: the radius and the ulna. Those two bones link the elbow to the wrist. The wrist is composed of a group of small bones called the carpals (made famous by Carpal Tunnel Syndrome, which is reportedly exacerbated by the typing of blog posts). Attached to the carpals are the metacarpals, which are the bones of the fingers. So, the skeletal components of the human arm are as follows: one bone (humerus) attached to two bones (radius and ulna) attached to a set of small blocky bones (the carpals), which anchor finger bones. It's an interesting pattern, but all by itself it's not necessarily remarkable.

Now let's look at the human hindlimb, or leg. The bones have a different set of names, which you may know all too well. Have a look at the 19-century illustration below.

The upper leg contains a single large bone, the femur, which is attached to the hip and to the knee. The lower leg sports two parallel bones: the tibia and the fibula. Those two bones link the knee to the ankle and foot. The ankle and foot are composed of a group of bones that includes a set of small bones called the tarsals. Attached to the tarsals are the metatarsals, which are the bones of the toes. So, the skeletal components of the human leg are as follows: one bone (femur) attached to two bones (tibia and fibula) attached to a set of smaller blocky bones (tarsals and others), which anchor toe bones. It's an interesting pattern, but all by itself it's not necessarily remarkable.

But wait. The leg pattern is essentially identical to the arm pattern. Why just one pattern? Why that pattern? Is there something special, maybe even somehow universal, about the pattern?

Questions like those were the domain of the great Richard Owen, the British naturalist and contemporary of Darwin. Owen's detailed study of limb structure led him to write one of the more influential works in the history of biology: On the Nature of Limbs, first published in 1849 and most recently reprinted in 2007. In that book, Owen argued that all vertebrate limbs were modifications of a basic pattern or plan, called an archetype.

To see why Owen reached this conclusion, consider the wonderful lithograph below, created in Owen's time (1860) by Benjamin Waterhouse Hawkins, who also contributed illustrations to Darwin's Zoology of the Voyage of the HMS Beagle. The limbs of the horse are constructed in an interesting pattern, depicted in the upper left. One large bone is attached to two parallel bones that have fused over most of their length. Those two bones attach to a collection of bones which then attach to some longer bones that form the ends of the limbs.

The pattern is much more striking when the limbs of diverse vertebrates are compared. Have a look at these two illustrations from On the Nature of Limbs. One is a dugong, a large aquatic mammal, and the other is a mole, a tiny mammal known for burrowing and defacing lawns. Do you see the pattern? One bone attaches to two bones which attach to blocky bones that support digits.

That pattern applies to bat wings and whale flippers and frog legs and chicken feet. It applies to dinosaurs and to newts. It's a universal feature of tetrapod limbs, front and back. Neil Shubin, in his brilliant book Your Inner Fish, summarizes the pattern as a simple chant: one bone, two bones, little blobs, digits. Owen's great insight was this: limbs are built according to a common pattern. One bone, two bones, blobs, digits.

Now, that's a remarkable fact about the animal world, and we curious hominids are itching for an explanation. Why are all tetrapod limbs based on the same underlying pattern?

We can use Owen and Darwin to sketch the two main competing explanations: design and descent. In the simple version of the story, Owen the anti-evolutionist, the design theorist of his day, concluded that the archetype was a design, a basic idea in the mind of the Creator. Darwin, of course, proposed a radically different explanation: the “archetype” is a common ancestor, and the variations on that “theme” are exemplars of descent with modification. There's no design, no Creator, just a lot of gradual tinkering with a setup that worked well enough at some time in the distant past.

That outline is hopelessly simplistic. Owen's views on evolution were complex and malleable; indeed, he got in some trouble for suggesting that tetrapods (even humans) were descended from fish through “slow and stately steps, guided by the archetypal light.” Later in life, in the midst of various nasty disputes with contemporaries (most notably with T.H. Huxley, known affectionately as “Darwin's Bulldog”), Owen did seem to oppose evolutionary ideas. But his writing in 1849 shows that he could see no reason to reject common ancestry while exploring the nature of the archetype. In other words, Owen was, at least earlier in his career, comfortable with common ancestry alongside strong conceptions of design.

And that is one theme that we will explore in this series. We will examine the evolution and development of limbs, to see how evolutionary explanations work and how strong and multidisciplinary the evidence for common descent really is. Common descent, I will argue, is really true, at least because it provides vast explanatory resources to those seeking to understand tetrapod limbs. But what about design? Can design also contribute explanatory resources? It's one thing to assert that the limb-construction blueprint proceeds from the mind of God; it's another thing to propose it as an explanation for why limbs are the way they are. Is there something about that plan – one bone, two bones, blobs, digits – that is superior? Could it have been otherwise? Those questions, I think, are the ones that we must address before we can advance design as an adjunct to – or a replacement for – common descent.

With those ideas in mind, let's explore the evolution of limbs. In the next post, we will explore the origins of those limbs, following the long search for their predecessors in the deep past and culminating in one of the most dramatic fossil finds in scientific history. In the third post, we will look at evidence from anatomy and developmental biology that supports the contention that fish fins and tetrapod limbs are variations on a theme. The fourth post will build on the third, looking at fascinating commonalities in the genetic systems that underlie the development of fins and limbs. In the fifth post, we will look at brand-new findings from developmental genetics that solidify and expand the fin-limb connection. The sixth and final post will look at the surprising links between fins, limbs and all other animal appendages, and will address oddities such as lost limbs in whales.

Bring your curiosity and your questions!

Further Reading

Neil Shubin (2009) Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body. New York: Vintage Books.

Brian K. Hall, editor (2007) Fins into Limbs. Chicago: The University of Chicago Press.

Richard Owen (1849) On the Nature of Limbs. London: John Van Noorst. (Google eBook).

Brian Switek (2008) Richard Owen, the forgotten evolutionist. Blog entry at Laelaps.

Carl Zimmer (1999) At the Water's Edge. New York: Simon and Schuster.

Image Credits

The first three images (human arm, human leg, man on horse) are courtesy of Wellcome Images, Creative Commons license. The last two images are taken from On the Nature of Limbs (1849), free online at Google eBooks.

Stephen Matheson is an author, editor, and developmental cell biologist, formerly at Calvin College in Grand Rapids, Michigan. He writes regularly on his blog “Quintessence of Dust”, which explores issues of science and Christian faith, focusing on genetics, development, evolution, neuroscience, and related topics, regularly discussing intelligent design, creationism, and other scientific issues that worry evangelical Christians.

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JimFisherHome - #64581

September 8th 2011

I would be interested in not only the bone structure, but also the design of other tissue structures in the limb joints—and not just an interspecies structure comparison but also a comparison between our two sets of limbs. Why do our knees have meniscii and our elbows do not. Why does our knee have a patella, but our elbows do not? Why are the bursae in those two joints so different? If our fairly recent ancestors were walking around on all fours, why does this design appear (at least to me) to be geared toward what is needed to stand upright and grasp with our hands. We can rotate our hands almost a full 360 degrees (palm up to palm up again). Try to do that with your foot. Fascinating!

beaglelady - #64608

September 9th 2011

I don’t think our “fairly recent ancestors” were walking around on all fours.  As I understand it, bipedalism has been in our line for quite a while. Even the australopithecines were bipedal.  

JimFisherHome - #64639

September 10th 2011

A mere 3 million years ago, and what ... 4 or 5 species ago?

beaglelady - #64645

September 10th 2011

Ardi dates to 4.4 million years ago and was bipedal.

See Jim’s post from January: 

beaglelady - #64605

September 9th 2011

The creation of the human knee, shoulder and back was part of the very first job stimulus program. After all, have you ever met an unemployed orthopedic surgeon?

Jon Garvey - #64615

September 10th 2011

Hi Beaglelady

I specialised in treating backs for 25 years. Bloody good design job, in my view, if maybe somewhat over-engineered in the neurological department, from which came most of the trouble. Pleased to hear yesterday that a gene responsible for initiating neuropathic pain has been identified in susceptible individuals, which is likely to sort a lot of the issues for future generations, without interfering with the biomechanics at all.

To be honest, most of the problems came from lifestyle issues - the regularly active have fewer problems and get over them better. It’s certainly true that cattle get more disc problems than humans, so sounds like the job-creation sceme was around long before we started walking upright.

Finally, a dispassionate observer might consider that laying off some of the orthopaedic surgeons and replacing them with pain specialists and physiotherapists would be beneficial rather than harmful.

beaglelady - #64617

September 10th 2011

If treating backs made you a good living for 25 years….why knock it? 

btw, I’ve never heard of a cow with disc problems.  Are their backs even worse than ours?

Jon Garvey - #64620

September 10th 2011

“Are their backs even worse than ours?”

Well, I was told that by one of the guys who taught me spinal manipulation, but I’ve never actually asked a vet. My guess is that if they are, it’s for similar reasons that ours are, ie abuse. In their case it’s being bred to do nothing but produce excess milk.

In mine it was doing a sedentary job for several years and then carrying a bass amplifier down two flights of narrow stairs. I can hardly blame the hardware for that.

But the instructive thing (if you’re interested in backs, anyway) is how recurrences of the same pain are not usually caused by re-injury (even disc problems usually sort themselves out over  few weeks or months), but by trivial events that trigger old neuropathic pain pathways. In other words it’s the spinal software that does it, and that is at least partly a genetic predisposition.

beaglelady - #64631

September 10th 2011

I’ve never even heard of a cow with a back problem, let alone disc problems. My grandfather had cattle and my friend had a dairy farm with Holstein milk cows.  Cows are treated for all kinds of minor ailments (such as mastitis) and are vaccinated and wormed, and usually not by a vet .

Since cows are raised for food and are usually not pets (unless you’re a Hindu), a poor lame cow would be hustled off to a slaughterhouse.  It wouldn’t be worth it to a rancher or farmer to have a cow’s back problem diagnosed and treated.  Beloved companion animals and expensive race and show horses are generally the critters who get expensive treatments.  

So how would anyone know that a cow had disc problems without going for expensive veterinary care?

btw the orthopedic guys like to try physical therapy on patients first—they aren’t knife-happy.

glsi - #64618

September 10th 2011

My daughter came home from high school with a Darwinist study sheet purporting the discovery of a fossil fish that could breath and drag itself out onto the shore.  Of course, this was Shubin’s Tiktaalik which is being held up as an important transitionary link between aquatic and land animals. 

When I looked into it (I read Shubin’s book) I soon found out that this specimen was a fish that probably hunted in shallow waters and pushed itself up on powerful front fins to grab food in the air, on the shoreline or on the surface of the water.  That’s basically it.  Any “breathing” it did is inferred and unproven.   That it ever left the water is  speculative and highly doubtful.   You would never know this from reading my daughter’s study sheet which paints a definitive and misleading conclusion.  

I think Shubin is largely drawing the conclusions that he wanted to make before he ever saw the evidence.  It’s far from certain.   What’s certain is a paucity of fossil evidence which demonstrates Darwinist speciation.  

sfmatheson - #64621

September 10th 2011

The next post will talk about fossil evidence for the fin-to-limb transition. It’s a fascinating and provocative pattern of evidence.

The study sheet you describe does sound flawed to me. Tiktaalik could probably haul itself onto shore, in the context of a mudflat, but seemed to be adapted for shallow water. The presence of lungs is inferred from other skeletal features, and perhaps that should have been made clearer.

Whether these conclusions are “far from certain” or whether speciation is poorly documented by evolutionary biologists is a judgment I hope people will make only after looking at the evidence and listening to scientists and their reasoning.

beaglelady - #64632

September 10th 2011

You can’t count on glsi to get anything straight.  I wonder if he even read Your Inner Fish.  What does he think of Shubin’s explanation for hiccups and hernias?

Tiktaalik was a spectacular find, and a great example of a transitional fossil.  I got to hear Shubin speak a while ago at the American Museum of Natural History.  He’s a good speaker, and he answered even children’s questions with great attention and without talking down to them.

I’m looking forward to Stephen Matheson’s future posts in this series!

glsi - #64646

September 10th 2011

Hey, give me a break!   I’ve got an inner fish, a reptilian brain, a mammalian sex drive, a crazy uncle bonobo, an egg yolk production gene that  might decide to switch itself on at any minute and my great, great granddaddy was a stromatolite.  What do you expect!!!!!!!!

Mike Gene - #64636

September 10th 2011

Hi Steve,

I have a couple of questions.

First, are you going to help perpetuate the false dichotomy of design vs. descent?  It is, after all, the popular thing to do (and a place where both creationists and gnus think alike).

Second, you write:

These animals, in all of their magnificent variety, seem to be built in very similar ways.

Are you going to address the fact that neo-Darwinian theory not only failed to anticipate this, but actually denied this would turn out to be true?  Or will that bit of science history get swept under the rug as usual?

sfmatheson - #64659

September 11th 2011

Hi Mike, I was trying to allude to the compatibility between design and descent in my description of Owen’s ideas. As you know from reading my blog, I don’t embrace the false dichotomy. I will emphasize the notion of explanation, and as you know I don’t think design has a lot of explanatory power.

The series is not about science history, and your claims are not relevant to my topic. If you would like to emphasize those ancillary themes, by all means do so in the comments or at your excellent blog. Perhaps consider that my failure to mention your favorite themes is different from “sweeping under the rug.”

Mike Gene - #64665

September 12th 2011


Fair enough.  As to the explanatory angle and design, it depends on how one conceptualizes design.  And as it is, most people conceptualize it, consciously or unconsciously, from a creationist perspective.  They tend to think of design as something akin to an assembly process in a factory, as if a bunch of engineers designed the limbs to be attached to some torso.   But the situation changes when you think of design from an evolutionary perspective.  It’s not about assembling the most efficient limb imaginable, it’s about getting something like a limb to emerge.   It’s not whether the limb plan is somehow superior.  It’s whether life itself was set up to provide a context that made the evolution of limbs more likely to appear.  

Design as an adjunct does not come from looking at patterns in the limb.  It comes from looking at patterns in evolution.

sfmatheson - #64668

September 12th 2011

Hi Mike, your description of “design from an evolutionary perspective” is exactly the view that I take. Design in that context is, to me, very interesting. Owen seemed to be thinking along an analogous line in 1849. But it’s hard to tell.

Design as an adjunct does not come from looking at patterns in the limb.  It comes from looking at patterns in evolution.

That’s true. But looking at the limb patterns raises the pertinent question: why are they all that way? Contingency (because they are descended from a design that just happened to work)? Necessity (because that’s a superior design, perhaps by far)? A combination of both? It seems to me that design as an adjunct is right in the center of that one.

sfmatheson - #64690

September 12th 2011

Mike (and others),

What do you think of Owen’s perspective on design? I’m not an expert on his views or his life, but I sure thought that quote about “advancing with slow and stately steps” while “guided by the archetypal light” was a pretty clear allusion to design-based evolution. I suppose his “archetypal light” was quite a bit different from front-loading, but I don’t know. Anyone else know more?

Mike Gene - #64691

September 12th 2011

I don’t know much about Owen.  Perhaps Ted or another historian might take a crack at this.  It might make for an interesting series.

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