“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.
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.)
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.
marojejian|6 days ago
https://www.nytimes.com/2026/02/23/science/evolution-vertebr...
Herodotus38|6 days ago
https://www.cell.com/current-biology/fulltext/S0960-9822(25)...?
jibal|6 days ago
“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.”
andrewflnr|6 days ago
I think Darwin might be, despite everything, underrated. I wish more people had this level of both intellectual and emotional strength and honesty.
hinkley|6 days ago
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.
ckemere|6 days ago
adrian_b|6 days ago
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.
emmelaich|6 days ago
unknown|6 days ago
[deleted]
emmelaich|6 days ago
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.
adrian_b|6 days ago
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.
theodorejb|6 days ago
doctoboggan|6 days ago
jibal|6 days ago
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.
Azrael3000|6 days ago
(0) https://m.youtube.com/watch?v=eFC9VzexRUk
mkl|6 days ago
fhe|6 days ago
adrian_b|6 days ago
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.
thaumasiotes|6 days ago
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.
unknown|6 days ago
[deleted]
jimmytucson|6 days ago