The Evolution of Eyes is Convergent

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. | The Evolution of Eyes is Convergent
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/&gt;, 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.

Close up of squid with its eye at the center. | The Evolution of Eyes is Convergent
Look into the loving eyes of the squid. He does not have a blind spot. Atlantic Ocean squid macro photo. Shutterstock asset id: 1859007028 by Rui Palma

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Author: thomasstigwikman

My name is Thomas Wikman. I am a software/robotics engineer with a background in physics. I am currently retired. I took early retirement. I am a dog lover, and especially a Leonberger lover, a home brewer, craft beer enthusiast, I’m learning French, and I am an avid reader. I live in Dallas, Texas, but I am originally from Sweden. I am married to Claudia, and we have three children. I have two blogs. The first feature the crazy adventures of our Leonberger Le Bronco von der Löwenhöhle as well as information on Leonbergers. The second blog, superfactful, feature information and facts I think are very interesting. With this blog I would like to create a list of facts that are accepted as true among the experts of the field and yet disputed amongst the public or highly surprising. These facts are special and in lieu of a better word I call them super-facts.

69 thoughts on “The Evolution of Eyes is Convergent”

    1. Thank you Kevin. Yes it is amazing how different lineages of species can all start out with the basics like a photo receptor and eventually end of with similar complex structures and similar components, just based on what dies off and not.

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    1. Thank you so much Ada. I agree it is amazing how different lineages of species can all start out with the basics like a photo receptor and eventually end of with similar complex structures and similar components, just based on natural selection.

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    1. Thank you so much Robbie. It is amazing that despite starting out with just a eyespot the early cephalopods and the ancestors of vertabra evolved independently into very similar complex eyes with the same basic components. That’s convergent evolution. But you are right, they are not the same. Cephalopods don’t have a blind spot like vertebra. Their eyes are more perfect.

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        1. Thank you Robbie. Since my early 20’s I have been a bit near sighted. Luckily, it has not gotten worse with age. However, I have had to wear glasses since my early 20’s for driving and sitting in class rooms. The same prescription glasses the whole time. However, unlike the cephalopods all us humans have a blind spot.

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  10. Fascinating information, Thomas! Thanks for the education, and for setting the record straight between what Creationists believe and scientific fact. Very cool image of the squid eye!

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        1. Well assuming you said each number once per second it would take 15 years and 10 months. However, you have to say the complicated numbers like 239,145,463 fast. And you need to sleep and eat. You may have spend your entire life counting. Maybe a life goal.

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  14. Thanks for this very interesting super fact that I did not know. This is one of those areas where it would be interesting to encounter truly alien life to see if it also followed this pattern of eye development, if it found a very different path, or even if it developed eyes at all.

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    1. Thank you so much David. Those are very interesting question you bring up. One could imagine that since so many independent evolutionary paths result in similar structures that the same would happen on alien worlds with advanced life, but you never know. Maybe it would be totally different.

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  15. ve evolved independently 40 to 65 times. It really highlights how strong the evolutionary pressure is to develop vision. The comparison between cephalopod and vertebrate eyes is always such a classic example of this convergence!”

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  17. It’s amazing how much information there is here and how much we take for granted with our eyes. It is quite fascinating and incredible how they have evolved! I just found an interesting thing with my eyes when I stopped drinking wine waiting for my grandbaby since I’ll be part of the team at the birth … since I’ve stopped my eyes aren’t dry like usual. Must be that alcohol dehydrates the eyes! Fun find and great post, Thomas! ❤️

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    1. Thank you so much Cindy for your very kind words. It is interesting what you experienced with the wine, so I looked it up. Alcohol can definitely make your eyes drier. Drinking alcohol dehydrates your body and reduces tear production. Luckily for me, I have not noticed that effect with beer, but maybe I will.

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  18. Lulu: “This is a great write-up about the development of eyes and complex vision! And our Dada says the counter-argument isn’t even really an argument since it’s starting from its conclusion and working backwards and not presenting any evidence and making claims that are not falsifiable.”Charlee: “We have no idea what he is talking about but he sounds like a lawyer.”

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  19. Thank you Lulu, and I believe your Dada is right. It is starting from the conclusion and working backwards when it assumes that all the components must have been there from the start to make a useful eye. Charlee, your Dada is very smart.

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    1. That is a great question. Camera like eyes, wings, echolocation, are examples of things that have evolved multiple times independently. Photosynthesis, placental reproduction (mammals), and the vertebrate jaw only evolved once and then followed lineages. I don’t know the difference.

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