Dec 13 2007

Another Gap Filled – More Evidence for Eye Evolution

“If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.”
– Charles Darwin, On The Origin of Species

The champions of Intelligent Design (ID) and its core argument, that of irreducible complexity, love quoting this line from the seminal work that launched the modern theory of evolution. One of their favorite targets for this line of argument is the modern vertebrate eye – a highly complex and sophisticated organ.

The classical creationist argument is the logical fallacy known as the argument from personal incredulity – they say, how could something so complex have evolved. ID (or neocreationism, as some call it) makes essentially the same argument but phrased in new lingo, calling the eye “irreducibly complex” – meaning it is too complex to have evolved, but more specifically that the eye could not work if it were any simpler, if any of its parts were missing.

It is astounding that this argument is still often heard, even though Darwin himself put it to rest in Origin of the Species when he pointed to the numerous examples in nature of animals with simpler, yet functional, eyes. Of course, using extant species as examples does not provide an actual evolutionary pathway, but rather it addresses directly the question of irreducible complexity, which is a functional question. The notion is that simpler anatomical forms cannot function – examples from nature show that they do. In fact there are examples in nature that demonstrate the functionality of every step that would need to take place in eye evolution.

But creationists are famous for moving the goalpost, so they then challenge evolutionists to prove not only that the eye can evolve (can pass through functional stages every step of the way) but that it in fact did evolve – show the actual evolutionary path. There is genetic and fossil evidence for much of eye evolution, but answers to historical questions within evolutionary theory – what actually happened in the evolutionary past – are always incomplete. There are always gaps, and this opens the door for another of creationists’ favorite tactics, the god of the gaps arguments. In fact ID is nothing but an extension of the the god of the gaps logical fallacy – any gaps not currently explained by science were miracles performed by God (Oh, wait. I’m sorry. Ahem – the “Intelligent Designer.”)

But the theory of evolution does not predict nor should it be reasonably expected that all gaps will be filled. History is imperfectly preserved. It does predict that gaps will be filled over time – that as we gather new information, like pieces to a puzzle, they will fit into an evolutionary pattern. So far all the evidence fits exquisitely into an evolutionary pattern.

But it is always nice when a new piece of evidence comes to light that fills in a fossil gap, enriching our understanding of evolutionary history. Recently Dr Gavin Young from the Department of Earth and Marine Sciences at ANU published his analysis of a 400-million-year-old Devonian placoderm fossil. The placoderms were jawed ancestors of modern fish whose bodies were protected by thick bony armour – so they are transitional between jawless fish and jawed fish. Evolutionary theory predicts, therefore, that their eye structure is likely to be transitional between the eye structure of jawless and modern jawed fish, and that is exactly what Dr. Young found. (I always wonder if creationists squirm a little when this kind of evidence comes out.)

The article states:

“Part of the trouble in tracing the evolution of the eye is that soft tissues don’t tend to fossilise. But the eye cavities in the braincase of these 400 million-year-old fossil fish were lined with a delicate layer of very thin bone. All the details of the nerve canals and muscle insertions inside the eye socket are preserved – the first definite fossil evidence demonstrating an intermediate stage in the evolution of our most complex sensory organ.

“These extinct placoderms had the eyeball still connected to the braincase by cartilage, as in modern sharks, and a primitive eye muscle arrangement as in living jawless fish.”

Dr Young said that this anatomical arrangement is different from all modern vertebrates, in which there is a consistent pattern of tiny muscles for rotating each eyeball.

The history of the science of evolution and the denial of that science by creationists of all types (including ID’ers) is one of constant retreat by creationists from the evidence. First they argue that the eye could not function if it were anatomically simpler, so scientists give evidence of simpler functioning eyes. Then they say that we have not demonstrated the actual path of evolution, but it was only a matter of time before fossils of sufficient quality to preserve information about soft body parts emerged to show the actual path.

What ID truly represents for creationists is a strategic maneuver. In addition to replacing the Christian God with the more neutral sounding intelligent designer, ID proponents have largely retreated from macroscopic anatomy (which has a frustrating habit of revealing itself in the fossil record – like the current example) to microscopic biochemistry.

Here is a perfect example from Michael Behe, the father of this strategy. He writes:

Darwin succeeded brilliantly, not by actually describing a real pathway that evolution might have used in constructing the eye, but rather by pointing to a variety of animals that were known to have eyes of various constructions, ranging from a simple light sensitive spot to the complex vertebrate camera eye, and suggesting that the evolution of the human eye might have involved similar organs as intermediates.

But the question remains, how do we see? Although Darwin was able to persuade much of the world that a modern eye could be produced gradually from a much simpler structure, he did not even attempt to explain how the simple light sensitive spot that was his starting point actually worked. When discussing the eye Darwin dismissed the question of its ultimate mechanism.

Behe goes on to discuss the complex biochemical process of how the retina produces a neuronal signal from a photon of light. He is retreating to where the fossils cannot reach him – to biochemistry. But he is just starting the “god of the gaps” cycle all over again. Just as the complexity of the modern human eye does not imply that simpler anatomies cannot function, the complexity of the modern retinal neuron and the biochemistry involved in seeing does not imply that simpler biochemical arrangements could not function.

Also, Behe’s strategy of retreat to biochemistry is ultimately a failed one. Just as with the bacterial flagellum, that Behe argued was irreducibly complex but was shown not to be, the evolution of retinal biochemistry is advancing quite nicely. Here is a review article from the same time as Behe’s article referenced above (1996-7), showing, for example, the branching pattern of relatedness in the amino acid structure of the opsin group of proteins used in vision. Here is a slightly more recent article from 2002 which reviews the phylogeny of visual protein systems.

The evidence is there, and growing, but creationists have never been bothered much by the evidence. Behe’s strategy is to just show how gee-whiz-complex the biochemistry of vision is then make the classic argument from personal incredulity. He fails to acknowledge, however, that such complexity evolves not so that the system can work at all but to make the system more sensitive, more specific, or more finely tuned. For example, a protein cascade can amplify the signal, allowing greater sensitivity to light. A less light-sensitive system can still work and provide a survival advantage.

Also, Behe and the ID’ers do not acknowledge what Darwin himself figured out – that of coaptation. Structures may develop for one purpose and then be coopted for another. For example, the bacterial flagellum likely evolved from a simpler structure that was not a motor but was used to inject proteins into prey.

As the evidence for the fact of evolution continues to pile up, the creationists are running out of places to hide. But then again, they are not genuinely interested in scientific inquiry. They are only interested in sowing confusion. What they are really doing is exploiting those areas where it is easiest to confuse the public. The fossil record is just getting too obvious, so for now they will seek refuge in the microscopic.

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