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.
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.
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.
“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.
Gift link:
https://www.nytimes.com/2026/02/23/science/evolution-vertebr...
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...
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.
See also https://en.wikipedia.org/wiki/Parietal_eye, a remnant "third eye" on the top of the head in some species.
That “cyclopean” eye is described as a patch of light-sensitive cells.
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.
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.)
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.
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.