Nathan Lents: How Did The Eye Evolve?

Who said it would be ten steps?


No one at all. I was kidding! You know that right?? I even said “easy steps” to make it obvious that I was being silly!

But, as Joshua mentioned, I am very interested in the subject, from your perspective. I am very much open to learning how something like this can occur, and my own personal stumbling block (beyond a lack of technical knowledge) is that I have a need to picture or visualize how such a thing can occur, in order to believe that it can occur. (I don’t believe I’m unique in this regard.)

My serious obstacle to this is not any single stepwise change (mutation or other) that can occur along the way, but rather how many stepwise changes can occur, undirected, in concert with so many other physiological changes that must also take place. For eyes, there is not just the myriad disparate cells that have different functions, but their construction order, connections to other kinds of cells, and the like. And then, the eyeball is not an island unto itself, but rather there are muscles that move them, lids that cover them, glands that water them, vessels that feed them, ducts that drain them, lashes that shade them, bones that contain them, and so on and so on. This is why I struggle to accept an unguided methodological process having caused them to exist, and this does not even begin to consider the chemical message that is transcribed by the brain to convey the optical image that the eye sees.

I share this with you because I’m not against what you say or teach or evangelize. It’s just that, for me and others like me, it is much easier to accept that a perceived design flaw might be a misunderstood advantage, than to see how an unknown number of events came together, just so, and resulted in vision. That said, I’m open minded and willing to learn what is known!

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Okay, well that’s great! I don’t mean to dodge, but a full explanation of all the steps in vertebrate eye evolution would be a really long post for a forum like this and, more importantly, these long explanations have already been written. Here’s one:

Hell, even the Wiki article looks pretty good (I’ve only skimmed):

The article I am working on is about the Tapetum Lucidum and doesn’t cover evolution of the eye or even the retina, per se, but of that specific part only.


Very true.

@Michael_Callen, I suggest you read that article carefully, and then ask some questions here.

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Could use a tiny font. Just saying.


Thanks, I don’t take it as a dodge at all… I’ve been waiting to ask and I wanted to let you know my question and how I’m considering it. I’ll read the links that you’ve provided and thanks for them!


A post was merged into an existing topic: Robert Byers’ Goals Here

A post was merged into an existing topic: Greg on the Forum

The first step is in realizing that all of these features you list aren’t needed for a functioning eye, and there are species with eyes that lack many of these features. A great example are the Planarians, a group of flatworms:


All they have is an eye cup. It is a pit with light sensitive cells at the bottom. The indentation allows for very crude directionality, but it doesn’t form a focused image and it certainly doesn’t have bones, ducts, and the other stuff you are talking about.


Thank you. I do understand, and we’ve had a similar discussion before. What I’m describing is my own process that I go through in order to process, assimilate and comprehend (which, presumably, will lead to belief.) So I’m trying to be transparent and show that I’m neither unwilling to consider this, nor am I presenting some debating strategy.

So, to use your example, one could explain to me (as you did a few months back) how an “eye cup with light sensitive cells” may have evolved in the first place, how that animal was a precursor to all modern seeing animals, and then how, in a stepwise process, the eye cup with light sensitive cells developed into an eye, complete with the other supporting systems in place. Again, I’m trying to be honest and transparent, but this is where I really struggle. To see how evolutionary change as an unguided process can go from the light sensitive, stand-alone eye cup, to the eyeball with all the items I’ve listed–which are obviously intended to be a part of the optics system as a whole–in a slow, stepwise manner is the thing that I cannot comprehend.

And I did not ask last time either, but is the flatworm with its light-sensing eye cup thought to be a precursor to all other seeing animals, such that it is supposed to have evolved in a manner in which I’m suggesting? Or did the eye, as we know it now, in all its glory, evolve many times in many different branches, independently?

Hopefully you see (and I believe that you know me well enough) that I’m not trying to be obstinate, but rather, I’m trying to be practical.


24 posts were split to a new topic: Side Comments on Evolution of the Eye

  1. A splotch of cells that react to light and give off a nerve impulse. This is an obvious advantage for many purposes, such as detecting predators or finding shaded spots to hide.

  2. The body indents where the photosensitive cells are found. This gives crude directionality to sight letting the organism know which direction the light or shadows are coming from.

  3. The opening of the pit starts to close in. This increases directionality. As the opening becomes very small you can start to form a focused image just as anyone can do with a pinhole camera.

  4. A transparent covering forms over the eye. This protects the cells inside the eye.

  5. Muscles attach to the covering. This allows for deformation of the covering over the eye which allows for a better focused image.

  6. Muscles form around the pinhole. Since a small hole is no longer needed to form a focused image the pinhole can increase in size. Muscles around the opening allow for an adjustable opening that can adapt to different amounts of light.

  7. Muscles around the base of the photosensitive cells allow for the eyes to move independently of the body. This allows for better coverage of the environment. This also allows for features like the fovea which is an area with densely packed photoreceptor cells. Instead of lining the entire retina with tons of photoreceptors you can concentrate them at one spot and let the eye move around to keep the image on a small area of the retina. You can sense this feature in your own eye by trying to read the text in your peripheral vision. Anything just a little bit outside the center of your vision can’t be read because it is too blurry. This is because most of your photoreceptor cells are packed into one tiny portion of your retina called the fovea.

There are probably some small steps I left out, but the main part to notice is that each step offers an advantage. Unguided mutations that land on one of these advantageous steps can be selected for in the right environments.

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I should probably let the biologists answer, but it’s going to be some of each. The last common ancestor (LCA) of flatworms and mammals probably had a precursor eye not too different from modern flatworms, so they have that much in common. There has been branching branching since that LCA which is independent.

You are understandable concerned about how all the different parts of the eye evolve, and these parts are not independent. A change in one part can make another part more effective, and these parts co-evolve to work together.

We also see that eyes may co-evolve similar forms independently between different species (parallel evolution). Basically, eyes are important (to fitness), but there are only so many ways for nature to form eyes that work well.

edit: Oh good, I’ve been rescued! :slight_smile:


The earliest cephalochordates (vertebrates with a head) found in the Cambrian already have eyes, although I don’t think we can determine from fossils exactly what those eyes were like. The most primitive living cephalochordate is the amphioxus (aka lancelet). The amphioxus does not have a true eye or a true brain, but it still has functioning photoreceptors:

The amphioxus also appears to have homologous features in its photoreceptors that are shared by other vertebrate eyes:

We should be careful in projecting the amphioxus “eye” backwards in time since modifications and possible losses of features have occurred in the amphioxus lineage, but there seems to be evidence of a common ancestor who lacked many of the features found in more derived vertebrate lineages.

I’ve always see the variety of extant eyes offered as a rebuttal to the silly YEC argument “what good is half an eye”. It shows there can be whole, functional eyes with simpler make-ups and less capability going all the way back to the simplest light detecting patch. So it’s not “half an eye”, it’s always been a whole eye with less capability at every evolutionary stage.

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A post was split to a new topic: The Patience Required to Deal With The Public

This is a really important point and probably helps to explain my frustration. I thought this was a place to discuss science, scientific evidence, scientific explanations, for those that are curious including those that aren’t in agreement. I did not think this was a place to debate whether or not evolution is real. If this is a forum for debating evolution versus ID or evolution vs YEC, or whatever, I’m out. That debate is over. Has been for a very long time. By continuing to hold debates, we give the impression that there is still serious scientific disagreement here when there isn’t. There are a few pariahs, and they are certainly free to hold their own forums and write their books, and I would fight for the right to do so. But my time is too precious to spend arguing with those determined not to hear. (And all of your time is at least as precious as mine.) But my time is NEVER too precious to explain science to the genuinely curious. There’s a reason I chose academia over industry, government, or research institute - I love teaching and find value in it.


i agree completely… People have the right to believe what they may, and to discuss what they wish. The problem is that there are implications (consequences) when things go from discussion, where learning is taking place, to arguing, where no one is listening. I do respect your time and am thankful for your being here and participating. I’ve got no questions for you now, as I’ve not yet had time to read the links posted earlier. But I do hope that there will be a chance to learn more about what you know.


That is exactly what this is. I’ve shuttled side comments into another thread, and moved this into the Office Hours category. This topic should be moderated more tightly now.

One strategy we have for protecting conversations is by having different rules from certain categories. Scholar driven threads, such as this one, are moderated much more tightly. Just ignore off topic or argumentative posts, and notify the @moderators. They should move them to the side comments thread.

@NLENTS I respect this and thinks this is exemplary. Scientists should be servants of the public. I believe that @Michael_Callen really wants to learn about this, and he will be a thoughtful interlocure. @T_aquaticus and other biologists here will jump in an help when they can, so the burden of explaining this will not fall entirely on you.

Right now, it is clear the rules of this thread, and you should no longer have problems here.


Just a minor correction - cephalochordates aren’t vertebrates, and cephalochordates actually lack a clearly defined head. Craniates are the chordates with an obvious head (skull), and vertebrates arise from within the craniates, so all vertebrates have heads.
Cephalochordates are better defined as “chordates without a head (that aren’t tunicates)”.