The evolution of the eye was one of the big issues for those that sought to disprove evolution.
(Picture from here.)
The rationale (which echoed Darwin somewhat) was that the eye was too complex to have evolved since it would not work without all of the working parts. Since evolution requires a successful adaptation to be passed down to offspring, the reasoning went the eye could not have evolved. It had to have been made.
Darwin, himself, recognized the issue of evolving very complex structures from simple origins. Any sufficiently complex biological mechanism has exactly the same issue. Which is why the anti-evolutionists pounced on it and why scientists concentrated on it. The evolution of the eye is fairly well worked out to the point that it no longer shows up in creationist arguments.
That said, it is still instructive to discuss.
For one thing, eyes—as opposed to photoreceptors—have evolved several times. It’s possible that the biochemistry of photoreception evolved very early on. Certainly, it’s comparatively common in single celled organisms such as Euglena. But these are not eyes. They cannot sense shape. They cannot focus. They detect light from darkness and not much else.
The earliest eyes seem to be always associated with some kind of nervous system. One likely scenario was that the integration mechanism afforded by a nervous system evolved in organisms that had some kind of photoreceptor and the two became associated. Photoreception is a sense just like tactile perception. The association of nerve and sense happened very early on in the form of eyespots: collections of photoreceptor molecules bound to a membrane. It should be noted that eyespots have evolved many times but out of the thirty-plus phyla of animals, actual devices that can discriminate merely the direction of light has only arisen in six of them. Those six, however, comprise better than 90% of known animal species.
Which brings us to the evolutionary mechanism itself.
Evolution requires two things to operate: variation and selection. If you have an organism that has no variation in a given system, no amount of selection will select out winners and losers. Similarly, if there is no selection pressure, no variant populations will compete against one another. (There is such a thing called genetic drift that doesn’t require selection but we’ll talk about that another time.)
Variations are not benign. There are not just positive variations. Most are negative. Some are lethal. Some have mixed effects—the sickle-cell anaemia mutation is a good example of this. It occurs when an individual inherits two abnormal copies of the β-globin gene. However, if an individual has a single copy of the abnormal β-globin gene, there some resistance to malaria. Thus, the trait remains in the population because enough people carrying the single gene are sufficiently resistant to malaria to survive and reproduce and enough of those that get both copies fail to reproduce.
This illustrates evolutionary two points: 1) success or failure is a numbers game. 2) Variation does not have to come directly from mutation. The β-globin gene mutation happened in the past but its propagation and selection is done by sexual variation.
Which brings us back to the eye. We’re going to stick with the descent of the vertebrate eye first found in chordates and echinoderms. The other kind of eye found in molluscs, annelid worms, and arthropods, have a somewhat different path. We’ll talk about that another time.
The standard path (shown above) is:
- Region of photosensitive cells associated with some kind of nervous system.
- Depressed or folded area containing photosensitive cells, allowing directional sensitivity
- Pinhole eye—the folded area partially encloses at the top. Retina forms.
- Transparent humor in an enclosed chamber. This requires contained liquid capped by a transparent membrane.
- Development of a distinct lens from the cornea split into two layers. One layer goes on to become the lens.
- Iris, cornea, pupil diameter control—all those finishing touches.
Now, let’s look at this from the point of view of variation and selection.
There is no particular reason that the photosensitive region ends up next to the skin. It could arise anywhere in the organism. However, if the region develops without light, it presents no selective advantage. Transparent organisms might get some use out of it. But photo regions are fairly common—we mentioned that before—so some organisms won’t be transparent. Let’s say, randomly, a photo region ends up next to the skin. Close enough for light to penetrate.
This is enough for selection. There’s variation in skin thickness. Those organisms that have photo regions below thin skin have a better use of that region than others: positive selection. Let’s say, some organisms have the photo region on the skin. Best selection yet. Note that only the initial conditions are random. There is nothing random about selection.
Let’s say are candidate organism has variation in skin shape: some areas are bumpy. Some have divots. Some times the photo region lands at the bottom of a divot. This gives a little directionality to the light. The organism can now tell where the light is coming from. Again, positive selection from variation.
Remember, our organism has variation in skin texture. Animals that have a little higher walls around the photo region work a little better. Now, we are on our way to a cup eye. Many invertebrates have cup eyes.
On the more is better principle, some of those eyes have variants that bring the cup eye opening to a small hole. This becomes the pinhole eye using the same principle as the pinhole camera. An image can now be formed. Again, on the more is better principle, the photoreceptors proliferate and we now have a retina. This requires no more variation than brought us the cup eye regarding the shape but does require variation in number of photoreceptors so more photoreceptors can be selected for. The nautilus has a pinhole eye.
Step 4, to me, is the hard one to model. We need: 1) an outer covering, 2) a maintained liquid interior.
The outer covering, it seems to me, is a natural outgrowth of the same variation that caused the pinhole. Cups can grow over—this is a negative selection. Likely, most cup and pinhole eyes would have mechanisms to prevent blindness.
However, some lucky few don’t have that. And some go blind, get eaten, and don’t reproduce. Some might grow over a transparent cell. They get a protected eye.
The enclosed liquid might have actually come first. Perhaps making a pinhole pocket created a place for bacteria to grow. The organism secreted a slime to protect itself. It would have to be transparent to avoid negative selection—just like before. Thus, when the transparent membrane grew over the eye, it might have already had a humor.
Bringing us to step 5: the lens.
Going back to step 4 for a moment, let’s look at the transparent outer covering: the cornea. It’s unlikely the cornea was a single layer—not much in the way of protection. Likely, it was multilayered skin. In some organisms, there might be liquid between the layers. This happens in other organisms. Let’s say, there’s variation in layer development. There might be more liquid between some layers than others.
Step 5 is adding a liquid between the layers of the cornea creates. This creates a focusing mechanism greater than a mere pinhole camera. It is a true—though primitive—eye.
Step 6, putting muscles around the eye, pupil, etc., is more straightforward. There is muscle in skin. We are now repurposing it. The cornea layers diverge from the lens. There’s selection for variations in liquid and so on.
Each one of these steps is functional. Each transition further involves successful organisms and—this is important—unsuccessful ones. The idea that evolution is a ladder from animal to creation is false. There are lots of dead ends, misturns, failures.
In fact, the eye is littered with problems, variation missteps, and outright failures. There are functional and physical design failures. There is variation in eyeball length: too long causes myopia. Too short causes hyperopia. The fluid in the lens gets harder to adjust as we get older. Then, we get glaucoma, cataracts, and retinal detachment. These are major design flaws. The eye is far from perfect.
We have a primate eye. Bird eyes are better. They have better acuity, an extra transparent layer, and, in some birds, perceive magnetic fields.
Primate eyes weren’t selected for that kind of sight. We have as good an eye as the selection pressure and variation could create. More would have been either too expensive or we had insufficient variation to allow it.
But bird eyes—and primate eyes—have major common physical flaws. The vertebrate retina is backwards: the wiring of the retina is in front of the photoreceptors. That means that we—and every other vertebrate—have to see through the nerve cells, connecting cells, and blood vessels.
Cephalopods—octopuses and cuttlefish—have a camera structure very similar to ours and derived completely independently. But with the photoreceptors in front of the connecting cells. They did it right.
Evolution always works with what it has, even if the result is clunky and problematic. As long as its successful, that’s enough.