Let us reason about this problem by answering a few questions.
- Which part of the eye is most essential? The answer is obvious: the retina. If light cannot be detected, the remainder of the eye is useless.
- Can a retina, on its own, without the rest of the eye, play a useful role? Obviously, yes. Many groups of not too complex animals have ocelli, photo-receptor cells that just react to the presence of light, but cannot form images. Of course, perceiving the presence of light offers advantage against being totally blind. The proof of this: ocelli have appeared independently in at least 40 different animal groups.
- What is the next step? We also have traces among current animals. The Planaria is a Platyhelminth (flatworm) whose ocelli are located at the bottom of a concavity in its body. Thanks to this, the Planaria not only detects the presence of light, but also, to some extent, the direction it comes from. It is also obvious that being able to perceive the direction of light provides advantage to those who can, against those who cannot.
- And the next step? We can see it in the Nautilus, a very old cephalopod that appeared in the Cambrian period and has barely changed in 500 million years. At present there are only six species, although they were quite abundant in the Paleozoic and the Mesozoic. It happens that the eye of the Nautilus is equivalent to what is called a pinhole camera, a lens-less camera that focuses the image by making light pass through a narrow hole, equivalent to a pupil. In the history of mankind, the pinhole camera was the first technological development after the invention of the camera obscura, and dates back to the fifth century b.C.e. in China. Both in the history of human technology and in the history of life, it plays the role of an intermediate step in evolution, since its structure is similar to the vertebrate eye, although it lacks the lens.
- And the lens? It seems logical to assume that this was the last step in the formation of the eye, and that in turn it went through two successive stages: first a fixed focus lens would appear, which would just focus at a certain distance. Even so, it would provide advantages, since natural selection would adapt the focal length to the needs of each species. For long distance predators, it would focus infinity, while those that captured small prey at close range would focus much closer. In the second stage, the lens would have a variable focal length, the same as our crystalline lens.
The adaptive importance of vision is so evident, that it is not surprising that the problem has been solved in many different ways. Among animals, there are few totally blind, almost all have developed some form of vision. A few of them are spectacular, as the compound eyes of insects, or the eyes of scallops. The vertebrate eye stands out among all of them. As Darwin pointed out, we cannot follow its evolution in detail, because even the most primitive fish had eyes essentially identical to ours. Its evolution during the Cambrian period must have been quite fast, in primitive chordates. Probably the presence of such a perfect eye was one of the causes why vertebrates quickly reached dominance in the evolution of the animal kingdom.
But there is another group of animals with eyes very similar to ours: the cephalopods. And in them, as we have seen, intermediate stages are preserved. The fact that the structure of our eyes arose twice independently in the history of life, lets us deduce that its evolution was not so difficult as the supporters of intelligent design would have us believe.
And now one last question: Why has the structure of the vertebrate eye remained stable for 500 million years? The history of technology gives us the answer. The configuration space of inventions is not regular, there are niches that can be represented as dips in said space. The bottom of the dip can be reached by many different paths, but once there, it is not possible to leave it, for any change in design will always be negative, causing a loss in efficiency. Then we say that a stable equilibrium has been reached. It is evident that the eye of vertebrates and cephalopods is in that situation.
Does this tell us anything on whether evolution is a consequence of pure chance, or a design tool? We’ll get into that in the next post.