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April 07, 2006

Tiktaalik bridges the gap

As you can imagine, the world of science has been abuzz with the news yesterday of the release of a paper in the prestigious science journal Nature heralding the discovery of a 375-million year old transitional fossil between fish and land animal, which has been named Tiktaalik. The fossil seems to provide evidence of a key evolutionary idea that land animals evolved from fish.

Several well-preserved skeletons of the fossil fish were uncovered in sediments of former stream beds in the Canadian Arctic, 600 miles from the North Pole, it is being reported on Thursday in the journal Nature. The skeletons have the fins and scales and other attributes of a giant fish, four to nine feet long.

But on closer examination, scientists found telling anatomical traits of a transitional creature, a fish that is still a fish but exhibiting changes that anticipate the emergence of land animals - a predecessor thus of amphibians, reptiles and dinosaurs, mammals and eventually humans. . .

The scientists described evidence in the forward fins of limbs in the making. There are the beginnings of digits, proto-wrists, elbows and shoulders. The fish also had a flat skull resembling a crocodile's, a neck, ribs and other parts that were similar to four-legged land animals known as tetrapods. . .

Embedded in the pectoral fins were bones that compare to the upper arm, forearm and primitive parts of the hand of land-living animals. The scientists said the joints of the fins appeared to be capable of functioning for movement on land, a case of a fish improvising with its evolved anatomy. In all likelihood, they said, Tiktaalik flexed its proto-limbs primarily on the floor of streams and may have pulled itself up on the shore for brief stretches.

In their journal report, the scientists concluded that Tiktaalik is an intermediate between the fish Panderichthys, which lived 385 million years ago, and early tetrapods.

For those of us who have long accepted natural selection and evolution as the theoretical prism through which to understand how the diversity of life came about, this discovery comes as a welcome, but not revolutionary, development since it seems to be one more confirmation of a major theory.

But what of those people who reject evolution and think that each species was an act of special creation? Should they treat this new discovery as a counter-example to their model and thus lead to its rejection?

Early ('naïve') versions of falsificationist theories of scientific development would argue that they should. In that model, advocated by philosopher of science Karl Popper, while no number of confirming instances can prove a theory right, a single counterexample can prove a theory wrong and warrant its elimination from the scientific canon.

Some people will argue that Tiktaalik is just that kind of counterexample and that it should serve as the death knell of creationism. Michael J. Novacek, a paleontologist at the American Museum of Natural History in Manhattan is quoted as saying: "We've got Archaeopteryx, an early whale that lived on land and now this animal showing the transition from fish to tetrapod. What more do we need from the fossil record to show that the creationists are flatly wrong?"

But he misunderstands how these arguments work because the naïve falsificationist model, while having an appealing intellectual simplicity, was soon shown to not really describe how science actually progresses. It turns out that there are many ways in which a theory can survive a single, or even several, counter-examples. I expect that the reaction to the Tektaalik discovery will provide us with a ringside seat in real time to see these defenses brought out.

Committed creationists will take one of two tacks. One argument is to assert that this new fossil is "really" just a fish or "really" just an animal, thus forcing it into an existing category, leaving the 'gap' unfilled. For example, this is how the creationist website Answers in Genesis dismisses the claim that the earlier Archaeopteryx was a transitional form between reptile and bird, saying: "Archaeopteryx was genuine. . . as shown by anatomical studies and close analysis of the fossil slab. It was a true bird, not a ‘missing link’."

This problem is inevitable because of the way we classify things, requiring that they fit into discrete and identifiable boxes that can be labeled and treated as if they were distinct categories. But in reality we are really dealing with a continuum of items, and have to make choices of how to label it, and pressure exists to put it into a pre-existing box rather than create a new box. For example, is the new fossil that was discovered a fish? Or a land animal? It is actually neither but the way we structure our evolutionary scheme and our language seems to put pressure on us to make that kind of choice.

If the attempt to put the new discovery into a pre-existing category does not work and a fossil is widely accepted as being transitional, creationists can take a different tack and argue that now there are new gaps that require new features and no fossils exist that have them. This is what has been done in the past with previous finds. As the New York Times article points out:

One creationist Web site (emporium.turnpike.net/C/cs/evid1.htm) declares that "there are no transitional forms," adding: "For example, not a single fossil with part fins part feet has been found. And this is true between every major plant and animal kind."

I think it was Ernst Mayr who said that trying to satisfy people who demand that missing links be provided to convince them of evolution had occurred was to pursue a chimera. Because when you find a link to fill the 'gap' between two species, your opponents now have two new 'gaps' that they can ask you to fill, where they only had one before. After all, if your a theory predicts that species A evolved from species Z, and you discover a transitional fossil M, now critics can ask where the transitional fossils are between A and M and between M and Z. And so on.

The new fossil Tiktaalik is called a fishapod because it is both fish and tetrapod. But creationists can soon begin to ask "What about the gap between fish and fishapod? Or between fishapod and tetrapod?" So paradoxically, the more intermediate fossils that are found, the more 'gaps' in the fossil record that will be created.

It is a little like colors. We know that the colors of pigments form a continuum, going smoothly from one to another depending on how the primary pigments are mixed. But historically and out of a need to be able to communicate with one another, we have classified them into distinct colors: red, green, blue, yellow, brown, black, etc, suggesting that colors are discrete and separable. When we encounter colors that do not fit into existing categories, we have 'gaps.' We sometimes invent new names like magenta, cyan, taupe, beige, etc to describe these transitional colors. But that simply raises new and more gaps. What is the shade of color between magenta and red? Between magenta and blue? We can never eliminate all the gaps in a continuum. Trying to do so only creates an increasing number of gaps.

Scientific evidence alone can never prove which theory is true. But what it can do is convince some people that one side is more plausible or fruitful or useful than the other. Thomas Kuhn in his book The Structure of Scientific Revolutions argues that to switch allegiances from one scientific theory to another is often not a reasoned decision but to have something similar to a conversion experience. But the dramatic conversion experience, unlike Paul's conversion to Christianity on the road to Damascus, may not have a dramatic cause.

Scientific conversions often occur because of incremental changes in the available evidence. As new evidence comes to light, holding on to an old theory becomes more difficult. At some point, one reaches a tipping point that causes one to completely switch one's perspective. It is like the way a see-saw or teeter-totter works when it is near balance. Even a small change can cause it to change its orientation completely. While the process leading up the change may be gradual, the change itself is sudden.

It is thus for the individual and scientific theories. A single new element added to the mix can cause the switch. Suddenly, you see things in a new light and cannot imagine why the old idea ever appealed to you, and defending it seems pointless. And when that happens, you go back and re-evaluate all that you had believed before in the light of this new viewpoint.

Tiktaalik will not sway those who are deeply committed to creationist ideas because they have many ways with which to justify retaining their beliefs. But somewhere, there are people who are reading about it and saying to themselves, "Hmmm. . . You know, maybe I should look into this evolution business more closely." And it is those people who will eventually switch.

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