Cephalopod Eye – Super Facts

Superfact 104: The evolution of eyes is convergent, meaning different, unrelated animal species independently evolved similar types of eyes. Biologists estimate that eyes have evolved independently between 40 to over 65 different times across various lineages. An example is the evolution of Cephalopod eyes (like squid and octopus) and vertebra eyes.

Esther’s writing prompt: May 20, 2026: Eyes

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First of all, eyes evolved. Creationists often say that eyes are too complex too have evolved. That is because the eye is composed of many interdependent, finely tuned parts, for example, the cornea, iris, retina and lens. And that it cannot function without all those components already evolved. This is referred to as the argument for irreducible complexity.

The problem with that argument is that evolution is not linear. The various intermediate steps may not have functioned as the final product but could still have provided evolutionary advantage. There are many intermediate “eyes” existing today in nature. As you can see in the picture below the evolution of the vertebra eye did not start with all the current parts.

The picture shows six stages of the evolution of the vertebra eye. First a region of photosensitive cells and nerve fibers. Second a depressed/folded area that allows limited directional sensitivity. Thirdly, a “Pinhole” eye that allows finer directional sensitivity and limited imaging. Fourth, a transparent humor develops in an enclosed chamber. Fifth, a distinct lens develops. Sixth, Iris and separate cornea develop. — Major stages in the evolution of the eye in vertebrates. Matticus78 at the English-language Wikipedia, CC BY-SA 3.0 http://creativecommons.org/licenses/by-sa/3.0/, via Wikimedia Commons.

In addition, the evolution of eyes is largely convergent. Biologists estimate that eyes have evolved independently between 40 to over 65 different times across various lineages. The cephalopods (like octopuses and squid) and vertebrates (like humans, mammals, birds and fish) evolved their camera-style eyes completely independently. This is one of nature’s most famous examples of convergent evolution, where two unrelated species arrive at the exact same biological solution to survive in their environments.

The fact that eyes evolved and that the irreducible complexity argument does not work comes as a surprise to creationists. That the various kinds of eyes in nature evolved separately but converged on similar complex structures is in general an amazing fact. It is a kind of an important fact that is true. Therefore, it is a super-fact in my opinion.

Eyes Are not an Example of Irreducible Complexity

The evidence that the complexity of eyes is not an example of irreducible complexity is strong. We can trace lineages via DNA and sub-optimality. We can also simulate the evolution of the eye using computers. In a simulation based on mutations and natural selection it took 363,992 generations to evolve an eye from an eyespot (light-sensing organelle) to a complex camera type eye, which probably corresponds to around half a million years. See The Evidence for Evolution by Alan R. Rogers.

I can add a personal anecdote. In my job as a software engineer trying to find better algorithms for sorting mail using the photos of the mail, including the address block, I tried using genetic algorithms. Genetic algorithms is a type of Artificial Intelligence that simulates evolution to create better systems (better algorithms and software). The genes corresponding to the best algorithms were allowed to propagate, recombine and mutate. That was the natural selection component.

What I saw was that the genetic algorithm could evolve the system into a complex and effective system of interdependent complex components that did not exist at the beginning. Several complex components working together did not require that components/parts evolve one after another. They can go through several formats from primitive to advanced and they can have different functions along the way. Some parts might evolve and then disappear and new kinds of parts pop up, as the total algorithm kept evolving. There is no reason to believe that irreducible complexity even exists.

The Vertebra Eye versus the Cephalopod Eye

The cephalopod eye on the right is very similar to the vertebra eye on the left, except it does not have a blind spot. — 1 is the retina and 2 the nerve fibers. 3 is the optic nerve. 4 is the vertebrate blind spot. Caerbannog, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons.

While both eyes share features like a cornea, iris, lens, and retina, they were built from different starting materials and possess some structural differences. In vertebrate eyes, the nerve fibers route before the retina, blocking some light and creating a blind spot where the fibers pass through the retina. In cephalopod eyes, the nerve fibers route behind the retina, and do not block light or disrupt the retina. In other words, the cephalopod eyes not having a blind spot are more perfect than our eyes.