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For Darwinian Evolution, It’s One Step Forward, Acknowledging Two Steps Back: Taking A Look at Tiktaalik

I love it when new “missing links” are discovered, because it’s then–and only then–that Darwinists admit how precious little evidence had previously existed for the evolutionary transition in question. When reports came out this week of an alleged example of a fossil representative of the stock that might have led from fish to tetrapods — Tiktaalik roseae — evolutionists finally came clean about the previous lack of fossil evidence for such a transition:

“The relationship of limbed vertebrates (tetrapods) to lobe-finned fish (sarcopterygians) is well established, but the origin of major tetrapod features has remained obscure for lack of fossils that document the sequence of evolutionary changes.”

(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))

Authority Jennifer Clack even admits that before finding Tiktaalik, the large morphological gap between fish and true tetrapods was “frustratingly wide”:

“It has long been clear that limbed vertebrates (tetrapods) evolved from osteolepiform lobefinned fishes3, 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 from the latest Middle Devonian of Latvia (Fig. 1).”

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

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 below 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))

Walk Off The Stage, Acanthostega
The previous darling of the “fish-to-tetrapod” transition-representatives was Acanthostega gunnari–a true tetrapod. Acanthostega has extremely tetrapod-like limbs, feet (with a few extra fingers), and a pelvic girdle. This little guy was a star of the PBS Evolution‘s episode II: “Great Transformations,” where Jenny Clack called it a “fish with fingers” (The only problem is that Acanthostega wasn’t a fish–as Daeschler et al. correctly categorize it as a non-fish tetrapod, contrasting “Skull roofs of elpistostegalian fish and the early tetrapod Acanthostega” [Nature 440:759]. Even Clack, quoted above, calls it a “tetrapod” and distinguishes it from fishes, making one wonder what was going on when PBS Evolution showed her calling it a “fish with fingers”.)

But only now that we have Tiktaalik will we hear evolutionists boast about the size of the previously large “gap” in this transition, and how Tiktaalik solves all these previously unanswered questions. I’m super skeptical that this new fossil is good evidence that a transition took place: Acanthostega was truly a tetrapod, but Tiktaalik is a fish. As Clack and Ahlberg write, there’s still a large gap (and any usefulness a fin had for walking was the result of a lucky pre-adaptation):

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. The implication is that function changed in advance of morphology.” (Clack & Ahlberg, Nature 440:748; emphasis added).

I think that Figure 4 from, “The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb” (by Neil H. Shubin, Edward B. Daeschler, & Farish A. Jenkins Jr, Nature, Vol 440:764-771 (April 6, 2006)) says it all:

(Adapted by permission from Macmillan Publishers Ltd: “The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb” (by Neil H. Shubin, Edward B. Daeschler, & Farish A. Jenkins Jr, Nature, Vol 440:764-771 (April 6, 2006); figure resized to fit the page except for the text; click for the full figure)

This figure, which Nature graciously has granted permission to reprint, reveals the massive difference in the ray-finned fish-fin of Tiktaalik and the true tetrapod limbs of Acanthostega and Tulerpeton. Is evidence of a transition missing? This new fish fossil doesn’t seem to add much–if anything–to bridge the gap between fish fins and tetrapod limbs. In fact, if anything, the fin of Panderichthys appears closer to a true tetrapod limb than does the fin of Tiktaalik. I would assume that documenting how fins turned into feet would be one of the more important aspects of the fish-to-tetrapod evolutionary story.

In conclusion, this is a fascinating fossil which I’m sure will stir up much debate. But the next time we dig up some fossil of a fin-bound fish (possibly with a few tetrapod-ish characteristics), we’ll hear again all about the previously existing big gaps and how Tiktaalik didn’t really teach us much after all–but how the new fossil solves all the problems. That’s how it usually works, and that makes me wonder where we’re really left today. Anyone who thinks that we’ve found the “missing link” or clear evidence of an evolutionary transition has either forgotten history, or isn’t looking very carefully at the evidence.

[figure citation corrected 4/14/06]

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.