“To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree. When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself originated; but I may remark that, as some of the lowest organisms, in which nerves cannot be detected, are capable of perceiving light, it does not seem impossible that certain sensitive elements in their sarcode should become aggregated and developed into nerves, endowed with this special sensibility.”
And in a letter to Asa Gray, professor of natural history at Harvard:
"The eye to this day gives me a cold shudder, but when I think of the fine known gradations, my reason tells me I ought to conquer the cold shudder.”
> "The eye to this day gives me a cold shudder, but when I think of the fine known gradations, my reason tells me I ought to conquer the cold shudder.”
I think Darwin might be, despite everything, underrated. I wish more people had this level of both intellectual and emotional strength and honesty.
The article is about vertebrates, not about animals in general.
The vertebrates originally had at least 3 eyes, 2 lateral eyes and one in the middle of the head.
Most vertebrates of today have lost the median eye, though there are a few species, like tuatara, which still have it.
The research reported in the article has shown evidence that the retina of the lateral eyes of the vertebrates has evolved not from the original retina of those eyes, but from the retina of the middle eye, which had a different structure, i.e. different photoreceptors.
I was looking for that too. I'm sure I've read that single cell animals were sensitive to light (and/or heat). I guess it's a speculation though, because we'd have no physical evidence.
We know that modern flagellates can steer to or away from light. When they started doing that is, as you say, pretty difficult to establish since they haven't left archaeological evidence. Unlike shellfish.
As another poster has said, most unicellular eukaryotes are either attracted or repelled by light and this is very simple to observe.
For some of them the mechanism of photoreception is partially understood, because they use photosensitive molecules related to those used in the eyes of animals.
Moreover, there are many bacteria that can sense light, so they are also attracted or repelled by light. Again, many of them use rhodopsins for sensing light, the same as our eyes.
Some light-sensitive molecules, like rhodopsins, are known to have existed in living beings for at least a few billion years, so the ability to sense light is that old.
Where the animals have innovated is in developing optical systems that can capture 2-dimensional images, instead of just sensing whether light is present or absent, like the other living beings can do.
Speculation 1. The bicameral mind was created as a result.
Speculation 2. The earliest creatures with two eyes may have been conjoined twins -- which were more successful in life than their single-celled/bodied siblings.
The article is not about this, but about an unexpected way of how the original 3 eyes of the vertebrates have evolved into the 2 eyes that most vertebrates have today.
See other comments.
In most cases the evolution of eyes in animals has been from more eyes to fewer (but more complex) eyes, and not the opposite.
A photosensitive patch of cells could be wired directly to motor cells/muscles on the opposite side, which would allow the organism to swim toward the light (maybe useful for feeding or migrating, etc.)
The "wiring to muscles" is derived from the ability of adjacent cells to communicate by chemical signals.
This communication ability has evolved before the multicellular animals, in the colonies of unicellular ancestors of animals (e.g. choanoflagellates).
The intercellular communication is a prerequisite for the development of multicellularity, like a common language is a prerequisite for a group of humans to be able to work as a team.
In an unicellular organism, a part of the cell senses light and another part, like flagella or contractile filaments reacts, moving the cell. In a multicellular organism, a division of labor appears, the cells from the dorsal side of the animal sense first light and other stimuli from the environment, so some of them specialize as sensory cells. Originally, the cells from the ventral side were more effective for locomotion, by using either cilia or propulsive contraction waves, so some of them specialized for locomotion, becoming motor cells, either muscles or ciliary bands (which in many simple animals are more important than muscles).
With this division of labor, the older intercellular communication methods have been improved, resulting in synapses between the sensory cells and the motor cells, which ensure that a chemical message that is sent reaches only the intended recipient, instead of being broadcast into the neighborhood.
For better reactions to external stimuli, the behavior of the sensory cells had to be coordinated, e.g. even when light is sensed only on one end of the animal, for the entire animal to move an appropriate command must be sent to all motor cells, not only to some of them, which has lead to synapses between the sensory cells themselves, not only between sensory cells and motor cells.
Eventually, there was a further division of labor, a part of the sensory cells has specialized to be middlemen, i.e. to relay the sensory information between the cells that have actually received it and the motor cells. These third kind of cells have become neurons. Initially the neurons were in the skin, together with the sensory cells from which they had derived, but later they migrated inside the body, where eventually they formed ganglia instead of a diffuse net, because this minimizes the reaction times, by shortening the connections between neurons, leading to a centralized nervous system.
They didn't need to come about at the same time. Photosensitive proteins (opsins) and cellular motility both predate multicellular life entirely. Even single-celled euglena detect light and swim toward it with no nervous system at all.
In early multicellular animals, cells were already chemically signaling their neighbors. A photosensitive cell releasing a signaling molecule near a contractile cell isn't a coordinated miracle. It is just two pre-existing cell types sitting next to each other in tissue, which is what bodies are. Natural selection then refines that crude coupling because even a tiny, noisy light response is better than none.
Each piece, light-sensitive proteins, cell-to-cell signaling, contractile cells, evolved independently and for other reasons long before being co-opted into anything resembling vision. The question "how could A and B arise simultaneously?" dissolves once neither A nor B was new.
A fairly simple chemical reaction could cause an organism to turn or move toward or away from light in the ocean, with various imaginable benefits.
And note that box jellyfish have 24 eyes, some of them highly complex, but no brain. You can look into their behavior to find out what they do with the information.
Stated clearly (0) has recently started a fantastic series about evolution that aims to explain bacterial flagella. It starts from basic principles and aims to answer questions like yours in evolutionary biology.
my question has always been why (I think most vertebrates) stop at two? It seems that an extra eye here and there could be really helpful. Maybe it's because all verterbrates evolved from an ancestor that had two eyes, and once the template is in place, it was simply too deep a local maximum to evolve out of? Similar to the 5-digit hand design that all vertebrates share.
TFA is about the fact that originally the vertebrates had at least 3 eyes, 2 lateral eyes and 1 median eye (pointing upwards towards the sky, in the middle of the head).
Most vertebrates, with the exception of a few species, like tuatara, have lost the middle eye.
The subject of the parent article is that it was expected that if the third eye was lost, the retinas of the 2 lateral eyes that have been preserved are derived from the retinas of the 2 ancient lateral eyes, but despite this expectation, the retinas of the modern lateral eyes of the vertebrates are derived from the retina of the ancient middle eye.
Spiders have 8 eyes. As with vertebrates, this number doesn't change, but there is variation in what it means.
A "normal" spider doesn't really use its eyes. It just has them.
Some spiders are different and rely on their vision. Those spiders have two primary eyes, which they rely on, and six secondary ones, which they don't.
Moving to insects, they often have compound eyes. Two compound eyes. A mantis has two primary compound eyes and three secondary non-compound eyes.
All this convergence suggests to me that even if you have the option to grow more eyes, the correct number is two.
Gift link:
https://www.nytimes.com/2026/02/23/science/evolution-vertebr...
Thank you. For those interested the paper this article is based on is:
https://www.cell.com/current-biology/fulltext/S0960-9822(25)...?
Darwin was not puzzled.
“To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree. When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself originated; but I may remark that, as some of the lowest organisms, in which nerves cannot be detected, are capable of perceiving light, it does not seem impossible that certain sensitive elements in their sarcode should become aggregated and developed into nerves, endowed with this special sensibility.”
And in a letter to Asa Gray, professor of natural history at Harvard:
"The eye to this day gives me a cold shudder, but when I think of the fine known gradations, my reason tells me I ought to conquer the cold shudder.”
> "The eye to this day gives me a cold shudder, but when I think of the fine known gradations, my reason tells me I ought to conquer the cold shudder.”
I think Darwin might be, despite everything, underrated. I wish more people had this level of both intellectual and emotional strength and honesty.
Don't we have single celled organisms with more than one 'eye' now?
We've been able to sense light and shadow even before we became multicellular, didn't we? And this article seems to be implying rather otherwise.
Jellyfish (cnidarians) do phototaxis - https://journals.biologists.com/jeb/article/227/18/jeb247503...
The article is about vertebrates, not about animals in general.
The vertebrates originally had at least 3 eyes, 2 lateral eyes and one in the middle of the head.
Most vertebrates of today have lost the median eye, though there are a few species, like tuatara, which still have it.
The research reported in the article has shown evidence that the retina of the lateral eyes of the vertebrates has evolved not from the original retina of those eyes, but from the retina of the middle eye, which had a different structure, i.e. different photoreceptors.
I was looking for that too. I'm sure I've read that single cell animals were sensitive to light (and/or heat). I guess it's a speculation though, because we'd have no physical evidence.
We know that modern flagellates can steer to or away from light. When they started doing that is, as you say, pretty difficult to establish since they haven't left archaeological evidence. Unlike shellfish.
There is plenty of evidence.
As another poster has said, most unicellular eukaryotes are either attracted or repelled by light and this is very simple to observe.
For some of them the mechanism of photoreception is partially understood, because they use photosensitive molecules related to those used in the eyes of animals.
Moreover, there are many bacteria that can sense light, so they are also attracted or repelled by light. Again, many of them use rhodopsins for sensing light, the same as our eyes.
Some light-sensitive molecules, like rhodopsins, are known to have existed in living beings for at least a few billion years, so the ability to sense light is that old.
Where the animals have innovated is in developing optical systems that can capture 2-dimensional images, instead of just sensing whether light is present or absent, like the other living beings can do.
Speculation 1. The bicameral mind was created as a result.
Speculation 2. The earliest creatures with two eyes may have been conjoined twins -- which were more successful in life than their single-celled/bodied siblings.
The article is not about this, but about an unexpected way of how the original 3 eyes of the vertebrates have evolved into the 2 eyes that most vertebrates have today.
See other comments.
In most cases the evolution of eyes in animals has been from more eyes to fewer (but more complex) eyes, and not the opposite.
What benefit is an eye unless there is also the capability of processing and using the information? How would both evolve simultaneously?
A photosensitive patch of cells could be wired directly to motor cells/muscles on the opposite side, which would allow the organism to swim toward the light (maybe useful for feeding or migrating, etc.)
How would the photosensitivity and wiring to muscles come about at the same time?
The "wiring to muscles" is derived from the ability of adjacent cells to communicate by chemical signals.
This communication ability has evolved before the multicellular animals, in the colonies of unicellular ancestors of animals (e.g. choanoflagellates).
The intercellular communication is a prerequisite for the development of multicellularity, like a common language is a prerequisite for a group of humans to be able to work as a team.
In an unicellular organism, a part of the cell senses light and another part, like flagella or contractile filaments reacts, moving the cell. In a multicellular organism, a division of labor appears, the cells from the dorsal side of the animal sense first light and other stimuli from the environment, so some of them specialize as sensory cells. Originally, the cells from the ventral side were more effective for locomotion, by using either cilia or propulsive contraction waves, so some of them specialized for locomotion, becoming motor cells, either muscles or ciliary bands (which in many simple animals are more important than muscles).
With this division of labor, the older intercellular communication methods have been improved, resulting in synapses between the sensory cells and the motor cells, which ensure that a chemical message that is sent reaches only the intended recipient, instead of being broadcast into the neighborhood.
For better reactions to external stimuli, the behavior of the sensory cells had to be coordinated, e.g. even when light is sensed only on one end of the animal, for the entire animal to move an appropriate command must be sent to all motor cells, not only to some of them, which has lead to synapses between the sensory cells themselves, not only between sensory cells and motor cells.
Eventually, there was a further division of labor, a part of the sensory cells has specialized to be middlemen, i.e. to relay the sensory information between the cells that have actually received it and the motor cells. These third kind of cells have become neurons. Initially the neurons were in the skin, together with the sensory cells from which they had derived, but later they migrated inside the body, where eventually they formed ganglia instead of a diffuse net, because this minimizes the reaction times, by shortening the connections between neurons, leading to a centralized nervous system.
As long as a mutation isn't strongly maladaptive, it can evolve prior to its being useful.
They didn't need to come about at the same time. Photosensitive proteins (opsins) and cellular motility both predate multicellular life entirely. Even single-celled euglena detect light and swim toward it with no nervous system at all. In early multicellular animals, cells were already chemically signaling their neighbors. A photosensitive cell releasing a signaling molecule near a contractile cell isn't a coordinated miracle. It is just two pre-existing cell types sitting next to each other in tissue, which is what bodies are. Natural selection then refines that crude coupling because even a tiny, noisy light response is better than none.
Each piece, light-sensitive proteins, cell-to-cell signaling, contractile cells, evolved independently and for other reasons long before being co-opted into anything resembling vision. The question "how could A and B arise simultaneously?" dissolves once neither A nor B was new.
A fairly simple chemical reaction could cause an organism to turn or move toward or away from light in the ocean, with various imaginable benefits.
And note that box jellyfish have 24 eyes, some of them highly complex, but no brain. You can look into their behavior to find out what they do with the information.
Stated clearly (0) has recently started a fantastic series about evolution that aims to explain bacterial flagella. It starts from basic principles and aims to answer questions like yours in evolutionary biology.
(0) https://m.youtube.com/watch?v=eFC9VzexRUk
See also https://en.wikipedia.org/wiki/Parietal_eye, a remnant "third eye" on the top of the head in some species.
my question has always been why (I think most vertebrates) stop at two? It seems that an extra eye here and there could be really helpful. Maybe it's because all verterbrates evolved from an ancestor that had two eyes, and once the template is in place, it was simply too deep a local maximum to evolve out of? Similar to the 5-digit hand design that all vertebrates share.
TFA is about the fact that originally the vertebrates had at least 3 eyes, 2 lateral eyes and 1 median eye (pointing upwards towards the sky, in the middle of the head).
Most vertebrates, with the exception of a few species, like tuatara, have lost the middle eye.
The subject of the parent article is that it was expected that if the third eye was lost, the retinas of the 2 lateral eyes that have been preserved are derived from the retinas of the 2 ancient lateral eyes, but despite this expectation, the retinas of the modern lateral eyes of the vertebrates are derived from the retina of the ancient middle eye.
[dead]
Well, from what I've read...
Spiders have 8 eyes. As with vertebrates, this number doesn't change, but there is variation in what it means.
A "normal" spider doesn't really use its eyes. It just has them.
Some spiders are different and rely on their vision. Those spiders have two primary eyes, which they rely on, and six secondary ones, which they don't.
Moving to insects, they often have compound eyes. Two compound eyes. A mantis has two primary compound eyes and three secondary non-compound eyes.
All this convergence suggests to me that even if you have the option to grow more eyes, the correct number is two.
There are also spider species with 6, 4, 2, and 0 eyes.
That “cyclopean” eye is described as a patch of light-sensitive cells.