Medicine and Evolution, part 6: Ivan Schwab on eye evolution

It occurred to me over the weekend that I hadn’t updated my Medicine and Evolution series in a while. A couple of weeks ago, I wrote about a couple of clueless creationists who had wildly misinterpreted a recent paper about how the cornea prevents blood vessels from growing into it from the surrounding sclera as “evidence” against evolution, basing his conclusion on, in essence, a single sentence from the abstract of the paper. It turns out that my humble efforts attracted the attention of an editor for American Academy of Ophthalmology, who was kind enough to forward to me a fascinating interview with Dr. Ivan Schwab, an ophthalmologist from UC-Davis with a great interest in the evolution of the eye. It’s a great followup to the deserved smackdown administered to the “intelligent design” creationists who think that the existence of the eye is somehow evidence of “irreducible complexity” that somehow casts grave doubt upon evolution.

Dr. Schwab also happens to do cutting edge research about the cornea. In 2000, he made a splash by growing corneal stem cells on amniotic membrane and then transplanted them into patients with corneal disease or damage. Not only that, but he writes a regular feature in the British Journal of Ophthalmology discussing eye evolution. (Why don’t we in surgery and surgical oncology have such a feature? Maybe I should contact some editors about starting one, although, as junior as I am, I don’t have the clout of an Ivan Schwab.)

In the interview, Dr. Schwab explains why evolution is important in ophthamology:

Why should ophthalmologists be interested in the evolution of eyes or the theory of evolution?

Because it is our heritage. There is much to be learned from where “we” have been. For example, evolution explains why we get retinal detachments and the octopus doesn’t. I often quiz my students on some eye feature: Tell me when the Silurian period was and how it’s important to vision. Tell me why a lamprey is important to color vision processing.

Octopus vision? I’m sure PZ would be interested in that one. But he (and I) would also like Dr. Schwab’s straightforward response to the claims of ID advocates that eyes are evidence against evolution:

What is your reply to anti-evolutionists who point to the eye as an example of irreducible complexity?

Nonsense. Each step of evolutionary complexity, from visual pigment in the membrane to an eyespot to a primitive eyecup to a compound or camera eye, is useful to the creature that has it, and it fits that creature’s niche quite well. Incidentally, all of these steps are with us today in some living species. The eye becomes more complex depending on the animal and its niche’s requirements for success. Unfortunately, the creationists use the quarrelsome nature of some paleontologists to say that no one agrees how evolution works, so it must be wrong. But those are quarrels over the fine points of evolution, not the overriding concept that Darwin proposed.

So nothing that paleontologists are saying throws into question the principle of natural selection?
No. Paleontologists band together on that.

Creationists may get creation “science” in the school curriculum, saying that there are too many holes in the theory of evolution. But to do so they must disavow physics, biology, mathematics, geology, chemistry, all the sciences, really, because each one supports the age of the earth and the theory of evolution.Most of the rest of the educated world accepts evolution. They look at the United States and wonder “What are they thinking?”

Yes, it’s true; the rest of the world is puzzled by the hostiility towards evolution among so many in the U.S. But that’s a topic that’s been well trod over before. Dr. Schwab also discusses a number of other aspects of eye evolution that I hadn’t known about. For example:

If a species loses sight altogether, as has happened in some fish and salamanders found in caves, do the ocular structures disappear forever in that species?

No, and that’s a good question. Eyes evolve only to the limit of available photons. As species go deeper into the sea, for example, their eyes get bigger, to capture the diminishing photons. But if there are no photons whatsoever, there is no capturing them, and eyes are useless. From an evolutionary perspective, eyes are very expensive. It takes a lot of effort to form an eye. So if they are not necessary, animals who do not spend energy maintaining an eye can devote that energy elsewhere, perhaps to maximizing their hearing or developing echolocation. But there may remain a vestigial globe, or potential for a globe. The genetic machinery for an eye is still there, it just isn’t switched on. There are curious experiments with blind cavefish in which the lens from a normal fish was placed onto the rudimentary embryonic eye of a cavefish. And that genetically prompted the cavefish to grow a regular eye from a remnant.

That brings up the fascinating case of the box jellyfish, which has no brain, but does have “seeing” mechanisms that help it navigate or catch prey. How do you explain “eyes” that obviously work, but not in tandem with any apparent visual system?

Eyes can function for photoreception without vision just like your gut can digest food without any decision to separate the protein from the fat. It depends on how the information is used. Tripedalia cystophora “recognizes” the light that its “eyes” receive. But it has no brain which might help it assess that information. It’s just a reflex to catch prey, perhaps, or evade predators or navigate. Interestingly, Tripedalia’s “eyes” are nevertheless complex, with a kind of pupil, and an iris that contracts depending on the stimulus. If you look at the eye in section, under a microscope, it looks like a camera-style eye. But the controlling entity is more of a neural net than a brain.

Based on this interview, I had a brief e-mail correspondence with Dr. Schwab, and he pointed out a couple of fascinating examples of how an “incomplete” eye can still be advantageous to an organism, specifically Platynereis dumerilii, an annelid. I tried reading the paper to which Dr. Schwab referred me (Fischer and Dorresteijn, Bioessays 26:314-325, 2004), but parts of it were a bit beyond me; so I ended up relying on his Dr. Schwab’s description to boil it down, This creature has primitive eyes that are precursors to invertebrate eyes. Its eyes contain primitive lenses of perhaps just a couple of layers of epithelium that can concentrate light. Dr. Schwab then was kind enough to give me other examples:

Playnereis has a primitive eye and a very primitive lens of perhaps just a couple of layers of epithelium that concentrates light somewhat–these cells are clear, and I suspect that there never was vascularization of the cornea because the eye started out as a “spot” and gradually developed a lens with a layer or two of overlying epithelium and the successful creatures kept refining the clarity. Some major well developed molluscs do not have corneae at all–an octopus or nautilus both do not have corneae and their eyes are open to sea water. Sea water must flush in and out of their ocular cavity. In water, corneae are of no use (index of refraction of water and the cornea about the same) and hence there is no push to develop one. Primitive fish have thin primitive corneae which probably started out as just a few layers of epithelium and gradually increased in the thickness–extant organisms that have changed little from the Ordovician–the elasmobranchs–have very thin, but tough corneae that have no endothelium for example. These cornea probably developed for protection not refraction. But, I don’t think that the cornea started out vascularized and then became clear. Rather the eye started out as a spot and as it developed a concavity, those successful creatures developed lenses that were clear cobbled together from whatever was around–heat shock proteins, enzymes and so forth. The eye did most of its developing in water. When tetrapods lumbered, crawled and slopped their way into a terrestrial life–they had pretty well formed and surprisingly good eyes (think frog). The first known eye…was probably found in trilobites although there may soon be evidence that there were creatures in the Ediarcaran that had eyes. But, trilobites had no cornea either–just calcite lenses.

All of these examples counter the concept of the “irreducible complexity” of the mammalian eye, because many creatures have light and image sensing systems that work for them that either lack or have rudimentary versions of the structures of the mammalian eye that IDers like to point to as evidence of “design.” More relevant to our creationists, there are many ways that evolution could have produced the clear cornea, and perhaps my speculation that the cornea started out vascularized and became clear over time in organisms that needed sharper vision was incorrect. It may well have been that the cornea began as a protection for the light-sensing spot and then developed from there. Either way, the mere observation that there is a sophisticated mechanism to keep blood vessels from growing into the cornea.

But how can all of this help us treat eye diseases better?

Getting back to the interview, Dr. Schwab suggests at least one way:

Considering how important vision is, especially for primates, how is it that so many of us have relatively poor vision? Why did natural selection permit such an apparent disadvantage?

Well, humans are really good at getting around natural selection. If we were still rural and tribal, low vision would be a tremendous disadvantage. We would not be of much use to our tribe. So from an evolutionary standpoint,myopia is probably not very smart. But if you look at our species in recent times, near vision is crucial: we make tools, we read, we are doing close work. Some would argue that myopia is a consequence of adapting to a near world. This question has come up before, and it’s contested. But the idea of myopia as an evolutionary response to our increasingly near world is worth discussion.

This is, of course, a fascinating debate, about which I have more than a passing interest, mainly because I wear pretty strong glasses. Although it’s a difficult question to study, it is reasonable to hypothesize that, as societies developed, people with myopia, who formerly would have been much more likely not to survive and reproduce, could have valued positions as scribes, toolmakers, and a variety of other positions in which close vision would not be a hindrance and might even be an advantage.

Does the study of eye evolution open avenues for research into ocular disease or injury, or vision deficits?

Lots and lots. For example, dolphins produce a viscous, oily tear to prevent their corneal epithelium from being damaged by the seawater around them No one knows what it is; it probably has antimicrobial compounds, surface wetting agents, compounds new to science. There are many such examples, and we only have to look.

And that’s the real test of a real scientific theory: how useful it is in guiding research and making predictions and connections. I’ve discussed other examples of how evolutionary theory is useful for guiding research in cancer and how studying a trypanosome can provide insights into a variety of human diseases and ciliary function in humans. Thanks to Dr. Schwab, I’m getting a bit more of an idea how evolutionary principles can be applied to the human eye and the diseases that can afflict it.

There was one thing, however, about which I disagreed with Dr. Schwab. He felt that I’m wasting my time rebutting ID advocates, as I’ll never change their minds. I have to admit that he does have a point. However (and I suspect that this is true for PZ Myers, Jason Rosenhouse, Evolgen, John Wilkins, Josh Rosenau, Pat Hayes, and many bloggers who spend far more time than I do rebutting the pseudoscience and misinformation pushed by IDers), I’m directing my rebuttals not at the ID advocates, but rather at those who are either on the fence or just don’t know enough biology to see through the flimsy construct that is ID. If I can educate just a few of them, all the verbiage will have been worthwhile.

But it’s definitely swimming against the tide.