Evolution Icon Evolution

An “Ulnare” and an “Intermedium” a Wrist Do Not Make: A Response to Carl Zimmer

Over the past couple years, Tiktaalik, a fossil allegedly documenting parts of the transition from fish to tetrapods, has become a new celebrated icon of evolution. PBS’s “Judgment Day: Intelligent Design on Trial” documentary featured Tiktaalik as their premier transitional fossil, an anachronism since it wasn’t even reported until months AFTER the Dover trial concluded. The NAS’s recent “Science, Evolution, and Creationism” booklet also prominently pushes Tiktaalik, calling it “a notable transitional form.” Darwinists have a lot of rhetorical capital invested in this fossil, and it thus comes as no surprise that they are quick to defend it with the “zero-concession policy” vehemence we’ve come to expect from internet Darwinists. As William Dembski writes regarding this policy:

Our critics have, in effect, adopted a zero-concession policy toward intelligent design. According to this policy, absolutely nothing is to be conceded to intelligent design and its proponents. It is therefore futile to hope for concessions from critics. This is especially difficult for novices to accept. A bright young novice to this debate comes along, makes an otherwise persuasive argument, and finds it immediately shot down. Substantive objections are bypassed. Irrelevancies are stressed. Tables are turned. Misrepresentations abound. One’s competence and expertise are belittled. The novice comes back, reframes the argument, clarifies key points, attempts to answer objections, and encounters the same treatment. The problem is not with the argument but with the context of discourse in which the argument is made. The solution, therefore, is to change the context of discourse.

Hardcore critics who’ve adopted a zero-concession policy toward intelligent design are still worth engaging, but we need to control the terms of engagement. Whenever I engage them, the farthest thing from my mind is to convert them, to win them over, to appeal to their good will, to make my cause seem reasonable in their eyes. We need to set wishful thinking firmly to one side. The point is not to induce a cognitive shift in our critics, but instead to clarify our arguments, to address weaknesses in our own position, to identify areas requiring further work and study, and, perhaps most significantly, to appeal to the undecided middle that is watching this debate and trying to sort through the issues.

(William Dembski, “Dealing with the Backlash Against Intelligent Design” (April 14, 2004).)

Traipsing Into Tetrapod Evolution
Darwinists haven’t always been so eager to walk into the field of the fish-to-tetrapod transition. Barbara Stahl’s 1985 treatise Vertebrate History: Problems in Evolution admits that “the fossil material provides no evidence of other aspects of the transformation from fish to tetrapod.” (pg. 195) 21 years later, Daeschler, Shubin, and Jenkins admitted in Nature that “the origin of major tetrapod features has remained obscure for lack of fossils that document the sequence of evolutionary changes.” (Daeschler et al., “A Devonian tetrapod-like fish and the evolution of the tetrapod body plan,” Nature, Vol. 440:757-763 (April 6, 2006).) Authority Jennifer Clack even admits that before finding Tiktaalik, the morphological gap between fish and tetrapods was “frustratingly wide”:

It has long been clear that limbed vertebrates (tetrapods) evolved from osteolepiform lobefinned fishes, but until recently the morphological gap between the two groups remained frustratingly wide. The gap was bounded at the top by primitive Devonian tetrapods such as Ichthyostega and Acanthostega from Greenland, and at the bottom by Panderichthys, a tetrapod-like predatory fish…

(Jennifer A. Clack & Per Erik Ahlberg, “A firm step from water to land,” Nature, Vol. 440:747-749 (April 6, 2006), internal citations omitted.)

Again Daeschler et al. reiterate the lack of evidence previous fossils provide for a transition, focusing on deficiencies in what was previously considered to be the closest fish to tetrapods (see the diagram here as well):

Panderichthys possesses relatively few tetrapod synapomorphies, and provides only partial insight into the origin of major features of the skull, limbs and axial skeleton of early tetrapods. In view of the morphological gap between elpistostegalian fish and tetrapods, the phylogenetic framework for the immediate sister group of tetrapods has been incomplete and our understanding of major anatomical transformations at the fish-tetrapod transition has remained limited.

(Edward B. Daeschler, Neil H. Shubin, and Farish A. Jenkins, “A Devonian tetrapod-like fish and the evolution of the tetrapod body plan,” Nature, Vol. 440:757-763 (April 6, 2006).)

Darwinists made these concessions when they discovered Tiktaalik, which is viewed as a bridge over some of the gaps between fish and tetrapods. Did Tiktaalik suddenly solve all of these problems?

I recently wrote a critique of claims from Darwinian paleontologists that Tiktaalik has a “wrist.” In my post, I made two main arguments: (1) First, I argued that Neil Shubin, one of the lead paleontologists who reported on Tiktaalik had failed to identify precisely which bones in Tiktaalik‘s limb were homologous to the bones of the true “wrist” of tetrapods. (2) However, I also argued that the bone structure in Tiktaalik‘s fin so different from the bone structure in a tetrapod wrist that it is inappropriate to claim that Tiktaalik has a “wrist.” Based upon a response by Carl Zimmer, Shubin did apparently identify two bones in Tiktaalik‘s fin that are specifically claimed to be homologous to two bones in the tetrapod wrist: the intermedium and the ulnare. I suppose I was misled because Shubin’s book Your Inner Fish says that Tiktaalik has a “wrist,” yet it describes a tetrapod wrist as having “lotsa blobs,” and thus I was expecting that no one would claim that Tiktaalik had a “wrist” unless it had a “lotsa blobs” conglomeration of wrist-bones–not only two “wrist” bones, which is apparently all that is claimed for Tiktaalik.

So it turns out that I was wrong about point (1). But I absolutely stand by my substantive claims in point (2) that the bone structure in Tiktaalik‘s fin is highly different from the bone structure in a tetrapod wrist, such that only a overactive evolutionary imagination would claim that Tiktaalik has a “wrist.” I can admit my mistake on point (1) — something that Zimmer is not known for doing: Shubin did try to directly match two of Tiktaalik‘s bones to the wrist-bones of tetrapods. That still leaves the open the fundamental, core question: Is it appropriate to claim that Tiktaalik has a “wrist”?

Is it appropriate to claim that Tiktaalik has a “wrist”?
Zimmer claims that “Shubin and his colleagues offer a detailed analysis in their paper of how the intermedium and ulnare in Tiktaalik are homologous to the bones of the same name in tetrapod wrists.” Upon closer analysis, Shubin’s reasons are obvious enough: in tetrapods with true wrists, the ulna articulates two wrist-bones, the intermedium and the ulnare. In Tiktaalik, the bone that Shubin calls the “ulna” likewise articulates two bones, which he also calls the intermedium and the ulnare. Unfortunately for Zimmer and Shubin, that’s about where the bone-structure similarities between Tiktaalik‘s fin and tetrapod limbs end. The tetrapod wrist is much more than the intermedium and the ulnare.

Zimmer points us to read Kardong’s treatise Vertebrates: Comparative Anatomy, Function, Evolution to learn about the intermedium and ulnare, and tetrapod wrist anatomy. After waiting a week or so to receive my copy, I too would like to refer readers to this textbook’s description of the tetrapod wrist, which makes for a nice comparison to Tiktaalik:

In the manus [the end portion of a tetrapod forelimb], a digit consists of several phalanges with a metacarpal at its base. In turn, each of the five metacarpals articulates with a carpal. The wrist bones that articulate with the radius and ulna are, respectively, the radiale and the ulnare. The intermedium lies between these two wrist bones. Within the middle of the wrist are one to three centrales.

(Kenneth Kardong, Vertebrates: Comparative Anatomy, Function, Evolution, pg. 332 (2001, McGraw-Hill, Third Edition).)

Tiktaalik has no radiale, because the bone that Shubin calls its “radius” articulates absolutely nothing. Tiktaalik also has no centrales. If we follow Kardong’s definition of the intermedium as the bone that “lies between these two wrist bones” then by such a definition, Tiktaalik also has no “intermedium,” because it has no radiale for an intermedium to “li[e] between.” Tiktaalik also has no phalanges or metacarpals, a point that I made last time by quoting Ahlberg & Clack’s commentary on Tiktaalik:

Although these small distal bones bear some resemblance to tetrapod digits in terms of their function and range of movement, they are still very much components of a fin. There remains a large morphological gap between them and digits as seen in, for example, Acanthostega: if the digits evolved from these distal bones, the process must have involved considerable developmental repatterning.

(Ahlberg & Clack, “A firm step from water to land,” Nature, Vol. 440:747-749 (2006).)

Rather than having phalanges or metacarpals, Tiktaalik has the kind of bones common in the distal sections of the fins of lobe-finned fish: radial bones. (At one point, Shubin even tacitly admits that Tiktaalik has radials, writing that there may be even more radials than were found: “It is not known how many radials lie distal to the first, second and fourth in the proximal series.”)

A “wrist” is comprised of the carpal bones. If Kardong is correct to state that “metacarpals articulat[e] with a carpal,” then given that Tiktaalik had no metacarpals, it would seem that only someone with an overactive evolutionary imagination could find carpals without any metacarpals. Nonetheless, Shubin names two bones in Tiktaalik with the names of two tetrapod carpal bones: the intermedium and ulnare. So under the most charitable interpretation, Tiktaalik only has two “carpal” bones–the intermedium and ulnare–even though a typical tetrapod wrist will have many more carpal bones. In fact, according to Kardong, the generalized wrist of a “primitive tetrapod” has no fewer than 13 carpal bones (see pg. 334). That’s a lot different than two.

How did the rest of the wrist-bones of primitive tetrapods evolve?
To give an idea of just how different Tiktaalik‘s fin is from the wrist of a “primitive tetrapod,” here are just a few of the changes that would be necessary to evolve Tiktaalik‘s fin into a real wrist:

  1. Shrink Tiktaalik‘s radius and reposition it so that it articulates other bones further down the limb.
  2. Evolve a radiale.
  3. Dramatically repattern, reposition, and transform the existing radials by lining them up, separating them out to form digits.
  4. Evolve metacarpals and phalanges so that there are real digits extending distally from the radius.
  5. Evolve the “lotsa blobs,” i.e. evolve other carpal bones between the radius, ulna, and the now-aligned digits to form a real wrist. In other words, evolve the bulk of the wrist-bones.

Shubin claims that Tiktaalik is “a fish with a wrist” (Your Inner Fish, pg. 38). But the simple response to Shubin’s book and Zimmer’s post is that Tiktaalik does not have a wrist because an “ulnare” and an “intermedium,” without these other components, do not make a wrist. And given that the intermedium and ulnare are, in tetrapods with real wrists, bones that articulate other carpal bones that then articulate metacarpals and phalanges, i.e. real tetrapod digits, but in Tiktaalik they articulate a mass of jumbled radial bones (the number of which existed we don’t even know), the strongest argument that Tiktaalik has a “wrist” is this: Tiktaalik’s wrist exists in the minds of Darwinists with overactive imaginations.

To be fair to Shubin, at one point he does admit that Tiktaalik‘s so-called “wrist” merely “presages the establishment of a functional proximal carpal joint.” As I noted in my previous post, the dictionary definition of presage is:

1. something that foreshadows or portends a future event : omen
2. an intuition or feeling of what is going to happen in the future

The active imaginations of Shubin and Zimmer must have also prophetic future-seeing abilities that I do not have, because I’m still not seeing a wrist in Tiktaalik.

If Tiktaalik‘s fin functioned like a “wrist,” would that confer much of an advantage?
catfish.jpgSome Darwinists have claimed that Tiktaalik is transitional between because it was a fish that was capable of propping itself up using its “wrist” fin. Is that a uniquely transitional trait? In fact, there are living fish which can prop themselves up by their fins, and no one considers them transitional. As seen in a recent CNN video, “It’s Raining Fish,” catfish are perfectly capable of both propping themselves up by their fins and fish-flopping around on the land. (See the picture above of flop-walking catfish, or watch the video.)

Such observations not only make Tiktaalik‘s “push-up” abilities a bit less impressive, but they show the great difficulty that fish have propping themselves up using fins. There are good anatomical reasons for these difficulties. Barbara Stahl again provides insight about the difficulties faced by a fish that tries to prop itself up for any significant amounts of time:

I. I. Schmalhausen points out that a fish lying on its side on the ground bears the weight of the body wall upon its viscera despite the presence of ventral ribs. When the internal organs are compressed in this way, the fish can force air through the pharynx only with great difficulty. For air to pass easily into the lungs, the trunk has to be propped up so that the respiratory organs hang suspended in the body cavity. A rhipistidian fish could have used its muscular fins to life its body from the surface temporarily, but in the absence of legs the body would have dragged or flopped when the animal moved. Although there might have been a rhipistidian that could expend the large amounts of energy necessary to make the lungs work under these conditions, a form with a fish-like trunk would have been better adapted for living on land if the body was kept clear of the ground.

(Barbara J. Stahl, Vertebrate History: Problems in Evolution, pg. 201 (Dover Publications, 1985).)

The CNN video confirms Stahl’s critique: catfish have a conspicuous “absence of legs” and thus when they are on land, their bodies are “dragged or flopped when the animal move[s].” There is no indication that Tiktaalik would escape any of these problems, because it also does not have legs nor a wrist such that it could lift its body “clear of the ground.” Though Clack, Ahlberg, and others might disagree, I believe that “the morphological gap” between lobe-finned fish and tetrapods should still be considered “frustratingly wide.”


Casey Luskin

Associate Director and Senior Fellow, Center for Science and Culture
Casey Luskin is a geologist and an attorney with graduate degrees in science and law, giving him expertise in both the scientific and legal dimensions of the debate over evolution. He earned his PhD in Geology from the University of Johannesburg, and BS and MS degrees in Earth Sciences from the University of California, San Diego, where he studied evolution extensively at both the graduate and undergraduate levels. His law degree is from the University of San Diego, where he focused his studies on First Amendment law, education law, and environmental law.